diff --git a/bazel/testing/BUILD b/bazel/testing/BUILD
index 097d80dde97f..f61cfd423856 100644
--- a/bazel/testing/BUILD
+++ b/bazel/testing/BUILD
@@ -2,4 +2,11 @@
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-exports_files(["lit_test.py"])
+package(default_visibility=["//visibility:public"])
+
+exports_files(["lit_test.py", "merge_output.py"])
+
+py_binary(
+  name = "merge_output",
+  srcs = ["merge_output.py"],
+)
diff --git a/bazel/testing/lit.cfg.py b/bazel/testing/lit.cfg.py
index 7bb56f785475..825d1c15e28b 100644
--- a/bazel/testing/lit.cfg.py
+++ b/bazel/testing/lit.cfg.py
@@ -20,13 +20,16 @@ config.name = "lit"
 config.suffixes = [".carbon"]
 config.test_format = lit.formats.ShTest()
 
+_MERGE_OUTPUT = fullpath("carbon/bazel/testing/merge_output")
+
 config.substitutions.append(
     (
         "%{carbon}",
-        fullpath("carbon/toolchain/driver/carbon"),
+        "%s %s" % (_MERGE_OUTPUT, fullpath("carbon/toolchain/driver/carbon")),
     )
 )
-_EXPLORER = "%s --prelude=%s" % (
+_EXPLORER = "%s %s --prelude=%s" % (
+    _MERGE_OUTPUT,
     fullpath("carbon/explorer/explorer"),
     fullpath("carbon/explorer/data/prelude.carbon"),
 )
diff --git a/bazel/testing/lit_autoupdate_base.py b/bazel/testing/lit_autoupdate_base.py
index 4a9e7fca6d4a..7ebf0fd39587 100755
--- a/bazel/testing/lit_autoupdate_base.py
+++ b/bazel/testing/lit_autoupdate_base.py
@@ -23,11 +23,17 @@ AUTOUPDATE_MARKER = "// AUTOUPDATE: "
 # Indicates no autoupdate is requested.
 NOAUTOUPDATE_MARKER = "// NOAUTOUPDATE"
 
+# Standard replacements normally done in lit.cfg.py.
+MERGE_OUTPUT = "./bazel-bin/bazel/testing/merge_output"
+LIT_REPLACEMENTS = [
+    ("%{carbon}", f"{MERGE_OUTPUT} ./bazel-bin/toolchain/driver/carbon"),
+    ("%{explorer}", f"{MERGE_OUTPUT} ./bazel-bin/explorer/explorer"),
+]
+
 
 class ParsedArgs(NamedTuple):
     build_mode: str
     build_target: str
-    cmd_replace: Tuple[str, str]
     extra_check_replacements: List[Tuple[Pattern, Pattern, str]]
     line_number_format: str
     line_number_pattern: Pattern
@@ -51,14 +57,6 @@ def parse_args() -> ParsedArgs:
         required=True,
         help="The target to build.",
     )
-    parser.add_argument(
-        "--cmd_replace",
-        nargs=2,
-        metavar=("BEFORE", "AFTER"),
-        required=True,
-        help="Adds a command replacement of `BEFORE` with `AFTER`. Typically "
-        "`BEFORE` will look like `%{name}`",
-    )
     parser.add_argument(
         "--extra_check_replacement",
         nargs=3,
@@ -96,7 +94,6 @@ def parse_args() -> ParsedArgs:
     return ParsedArgs(
         build_mode=parsed_args.build_mode,
         build_target=parsed_args.build_target,
-        cmd_replace=parsed_args.cmd_replace,
         extra_check_replacements=extra_check_replacements,
         line_number_format=parsed_args.line_number_format,
         line_number_pattern=re.compile(parsed_args.line_number_pattern),
@@ -230,7 +227,7 @@ def get_matchable_test_output(
     # Mirror lit.cfg.py substitutions; bazel runs don't need --prelude.
     # Also replaces `%s` with the test file.
     autoupdate_cmd = replace_all(
-        autoupdate_cmd, [parsed_args.cmd_replace, ("%s", test)]
+        autoupdate_cmd, [("%s", test)] + LIT_REPLACEMENTS
     )
 
     # Run the autoupdate command to generate output.
@@ -404,6 +401,7 @@ def main() -> None:
             "build",
             "-c",
             parsed_args.build_mode,
+            "//bazel/testing:merge_output",
             parsed_args.build_target,
         ]
     )
diff --git a/bazel/testing/lit_test.py b/bazel/testing/lit_test.py
index ddc59461f401..7d7313de7233 100644
--- a/bazel/testing/lit_test.py
+++ b/bazel/testing/lit_test.py
@@ -9,6 +9,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 import argparse
 import os
 from pathlib import Path
+import shutil
 import subprocess
 
 
@@ -33,9 +34,16 @@ def main():
         "-v",
     ]
 
-    # Force tests to be explicit about command paths.
+    # Force tests to be explicit about command paths. Some tools are required by
+    # bazel's py_binary output in order to find python3, so we add these. The
+    # list of tools may prove to be fragile: if so, we may need to change or
+    # remove the PATH hiding.
+    system_tools = Path(os.environ["TEST_TMPDIR"]).joinpath("system_tools")
+    system_tools.mkdir()
+    for tool in ("env", "grep", "python3", "sh", "which"):
+        system_tools.joinpath(tool).symlink_to(shutil.which(tool))
     env = os.environ.copy()
-    del env["PATH"]
+    env["PATH"] = str(system_tools)
 
     # Run lit.
     try:
diff --git a/bazel/testing/merge_output.py b/bazel/testing/merge_output.py
new file mode 100644
index 000000000000..d5520fab7a0f
--- /dev/null
+++ b/bazel/testing/merge_output.py
@@ -0,0 +1,29 @@
+"""Merges stdout and stderr into a single stream with labels."""
+
+__copyright__ = """
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+"""
+
+import subprocess
+import sys
+import textwrap
+
+
+def main() -> None:
+    p = subprocess.run(
+        sys.argv[1:],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        encoding="utf-8",
+    )
+    if p.stdout:
+        print(textwrap.indent(p.stdout, "STDOUT: "), end="")
+    if p.stderr:
+        print(textwrap.indent(p.stderr, "STDERR: "), end="")
+    exit(p.returncode)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/explorer/README.md b/explorer/README.md
index d70bc2cead37..33e893181b07 100644
--- a/explorer/README.md
+++ b/explorer/README.md
@@ -72,8 +72,8 @@ boilerplate at the top:
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 // CHECK:result: 0
 
diff --git a/explorer/lit_autoupdate.py b/explorer/lit_autoupdate.py
index 0886a6f1fcd5..545023510435 100755
--- a/explorer/lit_autoupdate.py
+++ b/explorer/lit_autoupdate.py
@@ -25,9 +25,6 @@ def main() -> None:
         # Flags to configure for explorer testing.
         "--build_target",
         "//explorer",
-        "--cmd_replace",
-        "%{explorer}",
-        "./bazel-bin/explorer/explorer",
         "--testdata",
         "explorer/testdata",
         "--line_number_pattern",
diff --git a/explorer/testdata/BUILD b/explorer/testdata/BUILD
index 73c2d2ed7365..29007bb3b17e 100644
--- a/explorer/testdata/BUILD
+++ b/explorer/testdata/BUILD
@@ -6,6 +6,7 @@ load("//bazel/testing:lit.bzl", "glob_lit_tests")
 
 glob_lit_tests(
     data = [
+        "//bazel/testing:merge_output",
         "//explorer",
         "@llvm-project//llvm:FileCheck",
         "@llvm-project//llvm:not",
diff --git a/explorer/testdata/addr/fail_method_let.carbon b/explorer/testdata/addr/fail_method_let.carbon
index a9db15bac7b6..1114c712357d 100644
--- a/explorer/testdata/addr/fail_method_let.carbon
+++ b/explorer/testdata/addr/fail_method_let.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -26,7 +26,7 @@ class Point {
 
 fn Main() -> i32 {
   let p: Point = Point.Origin();
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail_method_let.carbon:[[@LINE+1]]: method GetSetX requires its receiver to be an lvalue
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail_method_let.carbon:[[@LINE+1]]: method GetSetX requires its receiver to be an lvalue
   var x: auto = p.GetSetX(42);
   if (p.x == 42) {
     return x;
diff --git a/explorer/testdata/addr/fail_method_me_misspelled.carbon b/explorer/testdata/addr/fail_method_me_misspelled.carbon
index 39c13adeabd7..b895fcef9942 100644
--- a/explorer/testdata/addr/fail_method_me_misspelled.carbon
+++ b/explorer/testdata/addr/fail_method_me_misspelled.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 class C {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/addr/fail_method_me_misspelled.carbon:[[@LINE+1]]: illegal binding pattern in implicit parameter list
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/addr/fail_method_me_misspelled.carbon:[[@LINE+1]]: illegal binding pattern in implicit parameter list
   fn F[addr mew: Self*]() {}
 }
 
diff --git a/explorer/testdata/addr/fail_method_me_type.carbon b/explorer/testdata/addr/fail_method_me_type.carbon
index b560b8f9e491..49c2e7d17a6c 100644
--- a/explorer/testdata/addr/fail_method_me_type.carbon
+++ b/explorer/testdata/addr/fail_method_me_type.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -30,7 +30,7 @@ class Point {
 
 fn Main() -> i32 {
   var p: Point = Point.Origin();
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail_method_me_type.carbon:[[@LINE+1]]: type error in method access, receiver type: 'class Point' is not implicitly convertible to 'class Shape'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail_method_me_type.carbon:[[@LINE+1]]: type error in method access, receiver type: 'class Point' is not implicitly convertible to 'class Shape'
   var x: auto = p.GetSetX(42);
   if (p.x == 42) {
     return x;
diff --git a/explorer/testdata/addr/method.carbon b/explorer/testdata/addr/method.carbon
index 93e7c57c2398..7848e5191756 100644
--- a/explorer/testdata/addr/method.carbon
+++ b/explorer/testdata/addr/method.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/addr/nested_method.carbon b/explorer/testdata/addr/nested_method.carbon
index 9aca45ba4599..419d85677a27 100644
--- a/explorer/testdata/addr/nested_method.carbon
+++ b/explorer/testdata/addr/nested_method.carbon
@@ -3,10 +3,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/alias/class_alias.carbon b/explorer/testdata/alias/class_alias.carbon
index 8406b85d83e2..dbf1fc8be0bb 100644
--- a/explorer/testdata/alias/class_alias.carbon
+++ b/explorer/testdata/alias/class_alias.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:b.n: 1
-// CHECK:d.Get(0): 2
-// CHECK:e.Get(1): 3
-// CHECK:result: 0
+// CHECK:STDOUT: b.n: 1
+// CHECK:STDOUT: d.Get(0): 2
+// CHECK:STDOUT: e.Get(1): 3
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/alias/fail_alias_expression.carbon b/explorer/testdata/alias/fail_alias_expression.carbon
index c6e62a13d548..1a0a15a314a8 100644
--- a/explorer/testdata/alias/fail_alias_expression.carbon
+++ b/explorer/testdata/alias/fail_alias_expression.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/alias/fail_alias_expression.carbon:[[@LINE+1]]: invalid target for alias declaration
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/alias/fail_alias_expression.carbon:[[@LINE+1]]: invalid target for alias declaration
 alias A = 5;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/alias/fail_alias_var.carbon b/explorer/testdata/alias/fail_alias_var.carbon
index 7c37b149ab20..6338fe63036d 100644
--- a/explorer/testdata/alias/fail_alias_var.carbon
+++ b/explorer/testdata/alias/fail_alias_var.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 var v: i32;
 // TODO: This should probably be valid if we allow global variables at all.
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/alias/fail_alias_var.carbon:[[@LINE+1]]: invalid target for alias declaration
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/alias/fail_alias_var.carbon:[[@LINE+1]]: invalid target for alias declaration
 alias A = v;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/alias/fail_self_alias.carbon b/explorer/testdata/alias/fail_self_alias.carbon
index 5df90ee6477b..5a00d2d56c3b 100644
--- a/explorer/testdata/alias/fail_self_alias.carbon
+++ b/explorer/testdata/alias/fail_self_alias.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/alias/fail_self_alias.carbon:[[@LINE+1]]: 'a' is not usable until after it has been completely declared
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/alias/fail_self_alias.carbon:[[@LINE+1]]: 'a' is not usable until after it has been completely declared
 alias a = a;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/alias/function_alias.carbon b/explorer/testdata/alias/function_alias.carbon
index faa9f6bbab06..b6e4a19b3067 100644
--- a/explorer/testdata/alias/function_alias.carbon
+++ b/explorer/testdata/alias/function_alias.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 7
+// CHECK:STDOUT: result: 7
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/alias/interface_alias.carbon b/explorer/testdata/alias/interface_alias.carbon
index 6e63f724d527..802593f60f73 100644
--- a/explorer/testdata/alias/interface_alias.carbon
+++ b/explorer/testdata/alias/interface_alias.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 12345
+// CHECK:STDOUT: result: 12345
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/alias/member_name_alias.carbon b/explorer/testdata/alias/member_name_alias.carbon
index f26084840645..a6cd83539eee 100644
--- a/explorer/testdata/alias/member_name_alias.carbon
+++ b/explorer/testdata/alias/member_name_alias.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/alias/struct_alias.carbon b/explorer/testdata/alias/struct_alias.carbon
index 05e799aca617..9ed04b4cbfeb 100644
--- a/explorer/testdata/alias/struct_alias.carbon
+++ b/explorer/testdata/alias/struct_alias.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:ab.a: 2
-// CHECK:ab.b: 1
-// CHECK:ba.a: 2
-// CHECK:ba.b: 1
-// CHECK:result: 0
+// CHECK:STDOUT: ab.a: 2
+// CHECK:STDOUT: ab.b: 1
+// CHECK:STDOUT: ba.a: 2
+// CHECK:STDOUT: ba.b: 1
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/alias/type_alias.carbon b/explorer/testdata/alias/type_alias.carbon
index 01c6eeef4a0e..d40fbf044d44 100644
--- a/explorer/testdata/alias/type_alias.carbon
+++ b/explorer/testdata/alias/type_alias.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 4
+// CHECK:STDOUT: result: 4
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/array/fail_index.carbon b/explorer/testdata/array/fail_index.carbon
index ec6caef395b4..1b86d10479e4 100644
--- a/explorer/testdata/array/fail_index.carbon
+++ b/explorer/testdata/array/fail_index.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: [i32; 2] = (0, 1);
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/array/fail_index.carbon:[[@LINE+1]]: index 2 out of range in (0, 1)
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/array/fail_index.carbon:[[@LINE+1]]: index 2 out of range in (0, 1)
   return x[2];
 }
diff --git a/explorer/testdata/array/fail_negative_size.carbon b/explorer/testdata/array/fail_negative_size.carbon
index 231eb1f60868..192326090947 100644
--- a/explorer/testdata/array/fail_negative_size.carbon
+++ b/explorer/testdata/array/fail_negative_size.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_negative_size.carbon:[[@LINE+1]]: Array size cannot be negative
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_negative_size.carbon:[[@LINE+1]]: Array size cannot be negative
   var x: [i32; -1] = ();
   return x[0];
 }
diff --git a/explorer/testdata/array/fail_size_mismatch.carbon b/explorer/testdata/array/fail_size_mismatch.carbon
index bfa6b75c223c..bbeb767cdb48 100644
--- a/explorer/testdata/array/fail_size_mismatch.carbon
+++ b/explorer/testdata/array/fail_size_mismatch.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_size_mismatch.carbon:[[@LINE+1]]: type error in name binding: '(i32, i32, i32)' is not implicitly convertible to '[i32; 2]'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_size_mismatch.carbon:[[@LINE+1]]: type error in name binding: '(i32, i32, i32)' is not implicitly convertible to '[i32; 2]'
   var x: [i32; 2] = (0, 1, 2);
   return x[0];
 }
diff --git a/explorer/testdata/array/index.carbon b/explorer/testdata/array/index.carbon
index 4f01243a4b08..1a9baa5cf33f 100644
--- a/explorer/testdata/array/index.carbon
+++ b/explorer/testdata/array/index.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/array/nested.carbon b/explorer/testdata/array/nested.carbon
index 9f04efeaa1e3..e2ae42643710 100644
--- a/explorer/testdata/array/nested.carbon
+++ b/explorer/testdata/array/nested.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/as/as_same_type.carbon b/explorer/testdata/as/as_same_type.carbon
index d4adee86d1c5..311b475dc7bf 100644
--- a/explorer/testdata/as/as_same_type.carbon
+++ b/explorer/testdata/as/as_same_type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/as/convert.carbon b/explorer/testdata/as/convert.carbon
index f27b2636b213..0782c1d833a8 100644
--- a/explorer/testdata/as/convert.carbon
+++ b/explorer/testdata/as/convert.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/as/fail_destination_not_type.carbon b/explorer/testdata/as/fail_destination_not_type.carbon
index 5b530828d249..d694a4a9c7bc 100644
--- a/explorer/testdata/as/fail_destination_not_type.carbon
+++ b/explorer/testdata/as/fail_destination_not_type.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/as/fail_destination_not_type.carbon:[[@LINE+1]]: Expected a type, but got 7
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/as/fail_destination_not_type.carbon:[[@LINE+1]]: Expected a type, but got 7
   return 4 as 7;
 }
diff --git a/explorer/testdata/as/fail_no_conversion.carbon b/explorer/testdata/as/fail_no_conversion.carbon
index 651388b9a6b8..e954686522bc 100644
--- a/explorer/testdata/as/fail_no_conversion.carbon
+++ b/explorer/testdata/as/fail_no_conversion.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ class A { var n: i32; }
 
 fn Main() -> i32 {
   var a: A = {.n = 5};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/as/fail_no_conversion.carbon:[[@LINE+2]]: type error in `as`: `class A` is not explicitly convertible to `i32`:
-  // CHECK:could not find implementation of interface As(T = i32) for class A
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/as/fail_no_conversion.carbon:[[@LINE+2]]: type error in `as`: `class A` is not explicitly convertible to `i32`:
+  // CHECK:STDERR: could not find implementation of interface As(T = i32) for class A
   return a as i32;
 }
diff --git a/explorer/testdata/as/implicit_as.carbon b/explorer/testdata/as/implicit_as.carbon
index 1d679b66f60e..af6509084836 100644
--- a/explorer/testdata/as/implicit_as.carbon
+++ b/explorer/testdata/as/implicit_as.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assert/assert.carbon b/explorer/testdata/assert/assert.carbon
index 39ed0c87082b..11837bae89b8 100644
--- a/explorer/testdata/assert/assert.carbon
+++ b/explorer/testdata/assert/assert.carbon
@@ -5,10 +5,10 @@
 // This won't auto-update because it uses a regex for the prelude line number,
 // so that it doesn't need to be updated on every prelude change.
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // NOAUTOUPDATE
-// CHECK:RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: "HALLO WELT"
+// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: "HALLO WELT"
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assign/convert_rhs.carbon b/explorer/testdata/assign/convert_rhs.carbon
index febb9d6d4259..2dd5a7679b5e 100644
--- a/explorer/testdata/assign/convert_rhs.carbon
+++ b/explorer/testdata/assign/convert_rhs.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assign/destruct_original.carbon b/explorer/testdata/assign/destruct_original.carbon
index 9c1a61d022a1..7ab9d7cae3f2 100644
--- a/explorer/testdata/assign/destruct_original.carbon
+++ b/explorer/testdata/assign/destruct_original.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assign/reassign_original.carbon b/explorer/testdata/assign/reassign_original.carbon
index 881a94da90cf..b758cb42c487 100644
--- a/explorer/testdata/assign/reassign_original.carbon
+++ b/explorer/testdata/assign/reassign_original.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assoc_const/fail_anonymous.carbon b/explorer/testdata/assoc_const/fail_anonymous.carbon
index 8a4b5ed58778..e2cb37ece2b5 100644
--- a/explorer/testdata/assoc_const/fail_anonymous.carbon
+++ b/explorer/testdata/assoc_const/fail_anonymous.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface Iface {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/assoc_const/fail_anonymous.carbon:[[@LINE+1]]: syntax error, unexpected UNDERSCORE, expecting identifier
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/assoc_const/fail_anonymous.carbon:[[@LINE+1]]: syntax error, unexpected UNDERSCORE, expecting identifier
   let _:! Type;
 }
 
diff --git a/explorer/testdata/assoc_const/fail_incomplete_impl_1.carbon b/explorer/testdata/assoc_const/fail_incomplete_impl_1.carbon
index aef1e3e6479e..6961b6467784 100644
--- a/explorer/testdata/assoc_const/fail_incomplete_impl_1.carbon
+++ b/explorer/testdata/assoc_const/fail_incomplete_impl_1.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ interface HasThreeTypes {
   let C:! Type;
 }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_incomplete_impl_1.carbon:[[@LINE+1]]: implementation missing (i32).B; have constraint interface HasThreeTypes where i32 is interface HasThreeTypes and (i32).A == i32 and (i32).C == i32
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_incomplete_impl_1.carbon:[[@LINE+1]]: implementation missing (i32).B; have constraint interface HasThreeTypes where i32 is interface HasThreeTypes and (i32).A == i32 and (i32).C == i32
 external impl i32 as HasThreeTypes where .A == i32 and .C == i32 {}
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/assoc_const/fail_incomplete_impl_2.carbon b/explorer/testdata/assoc_const/fail_incomplete_impl_2.carbon
index 6f925bea5008..2cc63a58e3f2 100644
--- a/explorer/testdata/assoc_const/fail_incomplete_impl_2.carbon
+++ b/explorer/testdata/assoc_const/fail_incomplete_impl_2.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ interface HasThreeTypes {
   let C:! Type;
 }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_incomplete_impl_2.carbon:[[@LINE+1]]: implementation doesn't provide a concrete value for (i32).B
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_incomplete_impl_2.carbon:[[@LINE+1]]: implementation doesn't provide a concrete value for (i32).B
 external impl i32 as HasThreeTypes where .A == i32 and .B == .C {}
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/assoc_const/fail_indirectly_equal.carbon b/explorer/testdata/assoc_const/fail_indirectly_equal.carbon
index e19c5cecef22..999b73b0b9c4 100644
--- a/explorer/testdata/assoc_const/fail_indirectly_equal.carbon
+++ b/explorer/testdata/assoc_const/fail_indirectly_equal.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -23,7 +23,7 @@ fn F2[U:! A where .T == i32](x: i32) -> U.T {
 }
 
 fn F3[T:! A where .T == i32, U:! A where .T == i32](x: T.T) -> U.T {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_indirectly_equal.carbon:[[@LINE+1]]: type error in return value: '(T).T' is not implicitly convertible to '(U).T'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_indirectly_equal.carbon:[[@LINE+1]]: type error in return value: '(T).T' is not implicitly convertible to '(U).T'
   return x;
 }
 
diff --git a/explorer/testdata/assoc_const/fail_match_in_deduction.carbon b/explorer/testdata/assoc_const/fail_match_in_deduction.carbon
index 2616d52b9502..b23b0a8f2cc5 100644
--- a/explorer/testdata/assoc_const/fail_match_in_deduction.carbon
+++ b/explorer/testdata/assoc_const/fail_match_in_deduction.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 // TODO: Should this work?
@@ -22,7 +22,7 @@ fn F[Scalar:! Type, V:! Vector where .Dim == 3](p: Point(Scalar, V.Dim), v: V) {
 fn G[Scalar:! Type](p: Point(Scalar, 3)) {}
 fn H[V:! Vector where .Dim == 3](v: V) {
   var p: Point(i32, V.Dim) = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_match_in_deduction.carbon:[[@LINE+1]]: mismatch in non-type values, `(V).Dim` != `3`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_match_in_deduction.carbon:[[@LINE+1]]: mismatch in non-type values, `(V).Dim` != `3`
   G(p);
 }
 
diff --git a/explorer/testdata/assoc_const/fail_multi_impl_scoping.carbon b/explorer/testdata/assoc_const/fail_multi_impl_scoping.carbon
index 59078106d467..61071827db91 100644
--- a/explorer/testdata/assoc_const/fail_multi_impl_scoping.carbon
+++ b/explorer/testdata/assoc_const/fail_multi_impl_scoping.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -38,7 +38,7 @@ class C(T:! Type) {
 
 external impl C(i32) as B where .TB == () {
   fn FB() -> () { return (); }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_multi_impl_scoping.carbon:[[@LINE+1]]: ambiguous implementations of interface B for class C(T = i32)
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_multi_impl_scoping.carbon:[[@LINE+1]]: ambiguous implementations of interface B for class C(T = i32)
 }
 
 fn Main() -> i32 { return C(i32).FB(); }
diff --git a/explorer/testdata/assoc_const/fail_multiple_deduction.carbon b/explorer/testdata/assoc_const/fail_multiple_deduction.carbon
index 30cc7875cb04..498f1f8e365b 100644
--- a/explorer/testdata/assoc_const/fail_multiple_deduction.carbon
+++ b/explorer/testdata/assoc_const/fail_multiple_deduction.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -16,9 +16,9 @@ interface HasThreeTypes {
 }
 fn F[T:! Type](x: (T, T, T));
 fn G[X:! HasThreeTypes where .A == .B and .B == .C and .C == .A](x: X) {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_multiple_deduction.carbon:[[@LINE+3]]: deduced multiple different values for T:! Type:
-  // CHECK:  (X).A
-  // CHECK:  (X).B
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_multiple_deduction.carbon:[[@LINE+3]]: deduced multiple different values for T:! Type:
+  // CHECK:STDERR:   (X).A
+  // CHECK:STDERR:   (X).B
   F(x.Make());
 }
 
diff --git a/explorer/testdata/assoc_const/fail_overspecified_impl.carbon b/explorer/testdata/assoc_const/fail_overspecified_impl.carbon
index a3738d741459..23c5068a8fd1 100644
--- a/explorer/testdata/assoc_const/fail_overspecified_impl.carbon
+++ b/explorer/testdata/assoc_const/fail_overspecified_impl.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ interface HasType {
   let T:! Type;
 }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_overspecified_impl.carbon:[[@LINE+1]]: implementation provides multiple values for (i32).T: i32 and {.a: i32}
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_overspecified_impl.carbon:[[@LINE+1]]: implementation provides multiple values for (i32).T: i32 and {.a: i32}
 external impl i32 as HasType where .T == i32 and .T == {.a: i32} {}
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/assoc_const/fail_redefined.carbon b/explorer/testdata/assoc_const/fail_redefined.carbon
index c2417f13b9f9..67c38a3fbe27 100644
--- a/explorer/testdata/assoc_const/fail_redefined.carbon
+++ b/explorer/testdata/assoc_const/fail_redefined.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface Iface {
   let T:! Type;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_redefined.carbon:[[@LINE+1]]: Duplicate name `T` also found at {{.*}}/explorer/testdata/assoc_const/fail_redefined.carbon:[[@LINE-1]]
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_redefined.carbon:[[@LINE+1]]: Duplicate name `T` also found at {{.*}}/explorer/testdata/assoc_const/fail_redefined.carbon:[[@LINE-1]]
   let T:! Type;
 }
 
diff --git a/explorer/testdata/assoc_const/fail_unknown_value.carbon b/explorer/testdata/assoc_const/fail_unknown_value.carbon
index 5e3ca25591dd..c2dc2874cc8d 100644
--- a/explorer/testdata/assoc_const/fail_unknown_value.carbon
+++ b/explorer/testdata/assoc_const/fail_unknown_value.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,7 +13,7 @@ interface Iface { let N:! i32; }
 fn PickType(N: i32) -> Type { return i32; }
 
 fn F[T:! Iface](x: T) -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_unknown_value.carbon:[[@LINE+1]]: value of associated constant (T).N is not known
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_unknown_value.carbon:[[@LINE+1]]: value of associated constant (T).N is not known
   var x: PickType(T.N) = 0;
   return x;
 }
diff --git a/explorer/testdata/assoc_const/fail_unknown_value_specified_in_constraint.carbon b/explorer/testdata/assoc_const/fail_unknown_value_specified_in_constraint.carbon
index 71262e2cd51d..4ef7b220ca89 100644
--- a/explorer/testdata/assoc_const/fail_unknown_value_specified_in_constraint.carbon
+++ b/explorer/testdata/assoc_const/fail_unknown_value_specified_in_constraint.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ fn PickType(N: i32) -> Type { return i32; }
 
 fn F[T:! Iface where .N == 5](x: T) -> i32 {
   // TODO: This should be valid: the value of T.N is known to be 5 here.
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_unknown_value_specified_in_constraint.carbon:[[@LINE+1]]: value of associated constant (T).N is not known
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/assoc_const/fail_unknown_value_specified_in_constraint.carbon:[[@LINE+1]]: value of associated constant (T).N is not known
   var x: PickType(T.N) = 0;
   return x;
 }
diff --git a/explorer/testdata/assoc_const/impl_lookup.carbon b/explorer/testdata/assoc_const/impl_lookup.carbon
index 168032caaa83..392e19018fd3 100644
--- a/explorer/testdata/assoc_const/impl_lookup.carbon
+++ b/explorer/testdata/assoc_const/impl_lookup.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assoc_const/implement.carbon b/explorer/testdata/assoc_const/implement.carbon
index a598888538fa..db2454e6ef7f 100644
--- a/explorer/testdata/assoc_const/implement.carbon
+++ b/explorer/testdata/assoc_const/implement.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assoc_const/lookup_in_rewrite.carbon b/explorer/testdata/assoc_const/lookup_in_rewrite.carbon
index 21b92280fdfd..6a6a089837bd 100644
--- a/explorer/testdata/assoc_const/lookup_in_rewrite.carbon
+++ b/explorer/testdata/assoc_const/lookup_in_rewrite.carbon
@@ -2,19 +2,19 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:1
-// CHECK:i32.Hash
-// CHECK:0
-// CHECK:Potato.(Hashable.Hash)
-// CHECK:1
-// CHECK:Potato.Hash
-// CHECK:2
-// CHECK:Potato.Hash
-// CHECK:2
-// CHECK:result: 0
+// CHECK:STDOUT: 1
+// CHECK:STDOUT: i32.Hash
+// CHECK:STDOUT: 0
+// CHECK:STDOUT: Potato.(Hashable.Hash)
+// CHECK:STDOUT: 1
+// CHECK:STDOUT: Potato.Hash
+// CHECK:STDOUT: 2
+// CHECK:STDOUT: Potato.Hash
+// CHECK:STDOUT: 2
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assoc_const/member_of_value.carbon b/explorer/testdata/assoc_const/member_of_value.carbon
index 2daf740a1ec4..fb1506356767 100644
--- a/explorer/testdata/assoc_const/member_of_value.carbon
+++ b/explorer/testdata/assoc_const/member_of_value.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assoc_const/simple_constraint.carbon b/explorer/testdata/assoc_const/simple_constraint.carbon
index b370dbebc6b5..72c979695953 100644
--- a/explorer/testdata/assoc_const/simple_constraint.carbon
+++ b/explorer/testdata/assoc_const/simple_constraint.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assoc_const/simple_equality.carbon b/explorer/testdata/assoc_const/simple_equality.carbon
index 61277a82c330..11198f5013d0 100644
--- a/explorer/testdata/assoc_const/simple_equality.carbon
+++ b/explorer/testdata/assoc_const/simple_equality.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/addition.carbon b/explorer/testdata/basic_syntax/addition.carbon
index 4fb6dd1080a9..08c07673a66d 100644
--- a/explorer/testdata/basic_syntax/addition.carbon
+++ b/explorer/testdata/basic_syntax/addition.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 7
+// CHECK:STDOUT: result: 7
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/choice.carbon b/explorer/testdata/basic_syntax/choice.carbon
index fa9f059d5a14..8d6cbd3057e9 100644
--- a/explorer/testdata/basic_syntax/choice.carbon
+++ b/explorer/testdata/basic_syntax/choice.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/fail_alternative_not_type.carbon b/explorer/testdata/basic_syntax/fail_alternative_not_type.carbon
index 9fe4356cbac0..65cc2727e909 100644
--- a/explorer/testdata/basic_syntax/fail_alternative_not_type.carbon
+++ b/explorer/testdata/basic_syntax/fail_alternative_not_type.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_alternative_not_type.carbon:[[@LINE+1]]: Expected a type, but got (42)
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_alternative_not_type.carbon:[[@LINE+1]]: Expected a type, but got (42)
 choice C { X(42) }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/basic_syntax/fail_alternative_uses_choice.carbon b/explorer/testdata/basic_syntax/fail_alternative_uses_choice.carbon
index bad56a3b8617..d5d56ff427fb 100644
--- a/explorer/testdata/basic_syntax/fail_alternative_uses_choice.carbon
+++ b/explorer/testdata/basic_syntax/fail_alternative_uses_choice.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 choice Ch {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_alternative_uses_choice.carbon:[[@LINE+1]]: 'Ch' is not usable until after it has been completely declared
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_alternative_uses_choice.carbon:[[@LINE+1]]: 'Ch' is not usable until after it has been completely declared
   Opt(Ch)
 }
 
diff --git a/explorer/testdata/basic_syntax/fail_assign_to_function.carbon b/explorer/testdata/basic_syntax/fail_assign_to_function.carbon
index 137a317cfb57..965d327f3b4f 100644
--- a/explorer/testdata/basic_syntax/fail_assign_to_function.carbon
+++ b/explorer/testdata/basic_syntax/fail_assign_to_function.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ fn F() {}
 fn G() {}
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_function.carbon:[[@LINE+1]]: Cannot assign to rvalue 'F'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_function.carbon:[[@LINE+1]]: Cannot assign to rvalue 'F'
   F = G;
   return 0;
 }
diff --git a/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon b/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon
index a158b46871ec..2b8f9d1ac921 100644
--- a/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon
+++ b/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon:[[@LINE+1]]: Cannot assign to rvalue '1'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon:[[@LINE+1]]: Cannot assign to rvalue '1'
   1 = 0;
   return 0;
 }
diff --git a/explorer/testdata/basic_syntax/fail_block.carbon b/explorer/testdata/basic_syntax/fail_block.carbon
index b8db4a04df13..9ca341011a42 100644
--- a/explorer/testdata/basic_syntax/fail_block.carbon
+++ b/explorer/testdata/basic_syntax/fail_block.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   {
-    // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_block.carbon:[[@LINE+1]]: syntax error, unexpected RETURN, expecting PERIOD or RIGHT_CURLY_BRACE
+    // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_block.carbon:[[@LINE+1]]: syntax error, unexpected RETURN, expecting PERIOD or RIGHT_CURLY_BRACE
     return 0;
   }
 }
diff --git a/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon b/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon
index cab244465619..d2fbb08c5ecd 100644
--- a/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon
+++ b/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -17,9 +17,9 @@ choice Ints {
 fn Main() -> i32 {
   match (Ints.None()) {
     case Ints.None => { return 0; }
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon:[[@LINE+3]]: type error in `match` pattern type
-    // CHECK:expected: fn () -> choice Ints
-    // CHECK:actual: choice Ints
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon:[[@LINE+3]]: type error in `match` pattern type
+    // CHECK:STDERR: expected: fn () -> choice Ints
+    // CHECK:STDERR: actual: choice Ints
     default => { return 1; }
   }
 }
diff --git a/explorer/testdata/basic_syntax/fail_invalid_char.carbon b/explorer/testdata/basic_syntax/fail_invalid_char.carbon
index 5d7f93f703bd..e1f271321545 100644
--- a/explorer/testdata/basic_syntax/fail_invalid_char.carbon
+++ b/explorer/testdata/basic_syntax/fail_invalid_char.carbon
@@ -2,9 +2,9 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_char.carbon:[[@LINE+1]]: invalid character '\xEF' in source file.
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_char.carbon:[[@LINE+1]]: invalid character '\xEF' in source file.
 �
diff --git a/explorer/testdata/basic_syntax/fail_invalid_integer.carbon b/explorer/testdata/basic_syntax/fail_invalid_integer.carbon
index a41884fdbec3..fa21f8f0a796 100644
--- a/explorer/testdata/basic_syntax/fail_invalid_integer.carbon
+++ b/explorer/testdata/basic_syntax/fail_invalid_integer.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_integer.carbon:[[@LINE+1]]: Invalid integer literal: 11111111111111111111111111
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_integer.carbon:[[@LINE+1]]: Invalid integer literal: 11111111111111111111111111
   return 11111111111111111111111111;
 }
diff --git a/explorer/testdata/basic_syntax/fail_invalid_integer_type.carbon b/explorer/testdata/basic_syntax/fail_invalid_integer_type.carbon
index f5d34f4f8e89..116f89ca4c7f 100644
--- a/explorer/testdata/basic_syntax/fail_invalid_integer_type.carbon
+++ b/explorer/testdata/basic_syntax/fail_invalid_integer_type.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_integer_type.carbon:[[@LINE+1]]: Invalid type literal: i11111111111111111111111111
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_integer_type.carbon:[[@LINE+1]]: Invalid type literal: i11111111111111111111111111
   var x: i11111111111111111111111111 = 1;
   return 0;
 }
diff --git a/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon b/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon
index 4c4aaac9b05d..8bb49b9a50d9 100644
--- a/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon
+++ b/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon:[[@LINE+1]]: Expected expression for variable type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon:[[@LINE+1]]: Expected expression for variable type
 var a: auto;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/basic_syntax/fail_missing_var.carbon b/explorer/testdata/basic_syntax/fail_missing_var.carbon
index 66881e7a2194..6f83db9c33b3 100644
--- a/explorer/testdata/basic_syntax/fail_missing_var.carbon
+++ b/explorer/testdata/basic_syntax/fail_missing_var.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   // error
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_missing_var.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_missing_var.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
   x : i32;
   return 1;
 }
diff --git a/explorer/testdata/basic_syntax/fail_nested_binding.carbon b/explorer/testdata/basic_syntax/fail_nested_binding.carbon
index 114cfb719bc7..10d00f1bb02a 100644
--- a/explorer/testdata/basic_syntax/fail_nested_binding.carbon
+++ b/explorer/testdata/basic_syntax/fail_nested_binding.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_nested_binding.carbon:[[@LINE+1]]: The type of a binding pattern cannot contain bindings.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_nested_binding.carbon:[[@LINE+1]]: The type of a binding pattern cannot contain bindings.
   var x: (T: Type) = 1;
   return 1;
 }
diff --git a/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon b/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon
index 6e2cbc6cf8c5..2e9e1796177c 100644
--- a/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon
+++ b/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon:[[@LINE+1]]: Unimplemented
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon:[[@LINE+1]]: Unimplemented
   return 1 __unimplemented_example_infix 2;
 }
diff --git a/explorer/testdata/basic_syntax/fail_unknown_intrinsic.carbon b/explorer/testdata/basic_syntax/fail_unknown_intrinsic.carbon
index f27effd02bdf..93954d65158c 100644
--- a/explorer/testdata/basic_syntax/fail_unknown_intrinsic.carbon
+++ b/explorer/testdata/basic_syntax/fail_unknown_intrinsic.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unknown_intrinsic.carbon:[[@LINE+1]]: Unknown intrinsic 'nonexistent'
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unknown_intrinsic.carbon:[[@LINE+1]]: Unknown intrinsic 'nonexistent'
   return __intrinsic_nonexistent();
 }
diff --git a/explorer/testdata/basic_syntax/fail_unsupported_integer_type.carbon b/explorer/testdata/basic_syntax/fail_unsupported_integer_type.carbon
index 39ccc4b48fba..c0ff44a69b9c 100644
--- a/explorer/testdata/basic_syntax/fail_unsupported_integer_type.carbon
+++ b/explorer/testdata/basic_syntax/fail_unsupported_integer_type.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unsupported_integer_type.carbon:[[@LINE+1]]: Only i32 is supported for now: i64
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unsupported_integer_type.carbon:[[@LINE+1]]: Only i32 is supported for now: i64
   var x: i64 = 1;
   return 0;
 }
diff --git a/explorer/testdata/basic_syntax/fail_var_named_self.carbon b/explorer/testdata/basic_syntax/fail_var_named_self.carbon
index 474261a6e454..af27f25a9d57 100644
--- a/explorer/testdata/basic_syntax/fail_var_named_self.carbon
+++ b/explorer/testdata/basic_syntax/fail_var_named_self.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ fn Main() -> i32 {
   // Error: can't use keyword `Self` as the name of a variable.
   // TODO: Current error message is unclear, better would be to say
   // something like: unexpected `Self`, expecting identifier
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_named_self.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_named_self.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
   var Self : i32 = 0;
   return Self;
 }
diff --git a/explorer/testdata/basic_syntax/fail_var_type.carbon b/explorer/testdata/basic_syntax/fail_var_type.carbon
index 70c92ec77dbd..44b0ea064b5a 100644
--- a/explorer/testdata/basic_syntax/fail_var_type.carbon
+++ b/explorer/testdata/basic_syntax/fail_var_type.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Main () -> i32
 {
   // 42 cannot be used as the type of a variable.
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_type.carbon:[[@LINE+1]]: Expected a type, but got 42
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_type.carbon:[[@LINE+1]]: Expected a type, but got 42
   var x: 42 = 0;
   return x;
 }
diff --git a/explorer/testdata/basic_syntax/next.carbon b/explorer/testdata/basic_syntax/next.carbon
index e1f190470b1e..5d3d3a1a798f 100644
--- a/explorer/testdata/basic_syntax/next.carbon
+++ b/explorer/testdata/basic_syntax/next.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/not_compare_precedence.carbon b/explorer/testdata/basic_syntax/not_compare_precedence.carbon
index b64af43f52a6..9644525585cd 100644
--- a/explorer/testdata/basic_syntax/not_compare_precedence.carbon
+++ b/explorer/testdata/basic_syntax/not_compare_precedence.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn CompareBools(a: bool, b: bool) -> bool {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/not_compare_precedence.carbon:[[@LINE+1]]: syntax error, unexpected EQUAL_EQUAL, expecting SEMICOLON
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/not_compare_precedence.carbon:[[@LINE+1]]: syntax error, unexpected EQUAL_EQUAL, expecting SEMICOLON
   return not a == b;
 }
diff --git a/explorer/testdata/basic_syntax/placeholder_variable.carbon b/explorer/testdata/basic_syntax/placeholder_variable.carbon
index 160e6f97a75d..c8c8589e01d2 100644
--- a/explorer/testdata/basic_syntax/placeholder_variable.carbon
+++ b/explorer/testdata/basic_syntax/placeholder_variable.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/record.carbon b/explorer/testdata/basic_syntax/record.carbon
index 0e98a6b28082..f62e4ee5a8b3 100644
--- a/explorer/testdata/basic_syntax/record.carbon
+++ b/explorer/testdata/basic_syntax/record.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/star.carbon b/explorer/testdata/basic_syntax/star.carbon
index 561ff07b8116..b6fdfebd8a50 100644
--- a/explorer/testdata/basic_syntax/star.carbon
+++ b/explorer/testdata/basic_syntax/star.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/trace.carbon b/explorer/testdata/basic_syntax/trace.carbon
index 3a68cc0122d7..ee9b413e3870 100644
--- a/explorer/testdata/basic_syntax/trace.carbon
+++ b/explorer/testdata/basic_syntax/trace.carbon
@@ -2,24 +2,24 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 //
 // A lot of output is elided: this is only checking for a few things for simple
 // sanity checking on --parser_debug --trace_file=- output.
 //
 // NOAUTOUPDATE
-// CHECK:********** source program **********
-// CHECK:interface ImplicitAs {
-// CHECK:********** type checking **********
-// CHECK:** declaring interface ImplicitAs
-// CHECK:********** resolving unformed variables **********
-// CHECK:********** printing declarations **********
-// CHECK:interface ImplicitAs {
-// CHECK:********** starting execution **********
-// CHECK:********** initializing globals **********
-// CHECK:********** calling main function **********
-// CHECK:--- step exp Main() .0. (<Main()>:0) --->
-// CHECK:result: 0
+// CHECK:STDOUT: ********** source program **********
+// CHECK:STDOUT: interface ImplicitAs {
+// CHECK:STDOUT: ********** type checking **********
+// CHECK:STDOUT: ** declaring interface ImplicitAs
+// CHECK:STDOUT: ********** resolving unformed variables **********
+// CHECK:STDOUT: ********** printing declarations **********
+// CHECK:STDOUT: interface ImplicitAs {
+// CHECK:STDOUT: ********** starting execution **********
+// CHECK:STDOUT: ********** initializing globals **********
+// CHECK:STDOUT: ********** calling main function **********
+// CHECK:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/var_tuple.carbon b/explorer/testdata/basic_syntax/var_tuple.carbon
index 28101557560f..c48cf58bed1d 100644
--- a/explorer/testdata/basic_syntax/var_tuple.carbon
+++ b/explorer/testdata/basic_syntax/var_tuple.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/basic_syntax/zero.carbon b/explorer/testdata/basic_syntax/zero.carbon
index cadff806651e..abe632ed587d 100644
--- a/explorer/testdata/basic_syntax/zero.carbon
+++ b/explorer/testdata/basic_syntax/zero.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/block/empty.carbon b/explorer/testdata/block/empty.carbon
index 8bddf9538e05..9afdf0ec73a6 100644
--- a/explorer/testdata/block/empty.carbon
+++ b/explorer/testdata/block/empty.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/block/shadowing.carbon b/explorer/testdata/block/shadowing.carbon
index 71c9187d315e..223ce90c5e9c 100644
--- a/explorer/testdata/block/shadowing.carbon
+++ b/explorer/testdata/block/shadowing.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/choice/generic_choice_multiple_template_arguments.carbon b/explorer/testdata/choice/generic_choice_multiple_template_arguments.carbon
index 192d99216d7d..f620b3ab5b74 100644
--- a/explorer/testdata/choice/generic_choice_multiple_template_arguments.carbon
+++ b/explorer/testdata/choice/generic_choice_multiple_template_arguments.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 22
+// CHECK:STDOUT: result: 22
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon b/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon
index 0f213d3534d4..5d04ce0222d2 100644
--- a/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon
+++ b/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:H 22
-// CHECK:result: 0
+// CHECK:STDOUT: H 22
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/choice/generic_choice_simple_assignment.carbon b/explorer/testdata/choice/generic_choice_simple_assignment.carbon
index 8190ae220573..fed4deef0ad8 100644
--- a/explorer/testdata/choice/generic_choice_simple_assignment.carbon
+++ b/explorer/testdata/choice/generic_choice_simple_assignment.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 22
+// CHECK:STDOUT: result: 22
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/assign.carbon b/explorer/testdata/class/assign.carbon
index 4225afa83f3e..da3dde1065a7 100644
--- a/explorer/testdata/class/assign.carbon
+++ b/explorer/testdata/class/assign.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/assign_member.carbon b/explorer/testdata/class/assign_member.carbon
index 36c2ef3ef74b..eebc5220ac34 100644
--- a/explorer/testdata/class/assign_member.carbon
+++ b/explorer/testdata/class/assign_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/bound_method.carbon b/explorer/testdata/class/bound_method.carbon
index 9d18fada7646..59a17bbc3700 100644
--- a/explorer/testdata/class/bound_method.carbon
+++ b/explorer/testdata/class/bound_method.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/class_function.carbon b/explorer/testdata/class/class_function.carbon
index c3866c400e2a..eeb4784325d4 100644
--- a/explorer/testdata/class/class_function.carbon
+++ b/explorer/testdata/class/class_function.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/class_function_from_instance.carbon b/explorer/testdata/class/class_function_from_instance.carbon
index 0a3de685e89a..65e9cc260099 100644
--- a/explorer/testdata/class/class_function_from_instance.carbon
+++ b/explorer/testdata/class/class_function_from_instance.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/class_function_self.carbon b/explorer/testdata/class/class_function_self.carbon
index 757922ba7b60..5bc7368af45e 100644
--- a/explorer/testdata/class/class_function_self.carbon
+++ b/explorer/testdata/class/class_function_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/class_function_value.carbon b/explorer/testdata/class/class_function_value.carbon
index 0fe9ec00f75a..f0f00289bbc4 100644
--- a/explorer/testdata/class/class_function_value.carbon
+++ b/explorer/testdata/class/class_function_value.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/empty_class.carbon b/explorer/testdata/class/empty_class.carbon
index d877358eb5a1..7abfd74bb729 100644
--- a/explorer/testdata/class/empty_class.carbon
+++ b/explorer/testdata/class/empty_class.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/fail_abstract_class.carbon b/explorer/testdata/class/fail_abstract_class.carbon
index 5eb3f03d927c..7a4e026e5487 100644
--- a/explorer/testdata/class/fail_abstract_class.carbon
+++ b/explorer/testdata/class/fail_abstract_class.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 abstract class C {
   fn F() {}
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_abstract_class.carbon:[[@LINE+1]]: Class prefixes `base` and `abstract` are not supported yet
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_abstract_class.carbon:[[@LINE+1]]: Class prefixes `base` and `abstract` are not supported yet
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/class/fail_base_class.carbon b/explorer/testdata/class/fail_base_class.carbon
index 37163632be3e..58c8eff62f07 100644
--- a/explorer/testdata/class/fail_base_class.carbon
+++ b/explorer/testdata/class/fail_base_class.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 base class C {
   fn F() {}
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_base_class.carbon:[[@LINE+1]]: Class prefixes `base` and `abstract` are not supported yet
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_base_class.carbon:[[@LINE+1]]: Class prefixes `base` and `abstract` are not supported yet
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/class/fail_class_extends.carbon b/explorer/testdata/class/fail_class_extends.carbon
index ffcc95b32358..e655c38c13c9 100644
--- a/explorer/testdata/class/fail_class_extends.carbon
+++ b/explorer/testdata/class/fail_class_extends.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ class A {
 
 class B extends A {
   fn F() {}
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_class_extends.carbon:[[@LINE+1]]: Class extension with `extends` is not supported yet
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_class_extends.carbon:[[@LINE+1]]: Class extension with `extends` is not supported yet
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/class/fail_class_named_self.carbon b/explorer/testdata/class/fail_class_named_self.carbon
index 0a3de792cb42..cb26a2dbea86 100644
--- a/explorer/testdata/class/fail_class_named_self.carbon
+++ b/explorer/testdata/class/fail_class_named_self.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // Error: Can't use keyword `Self` as the name of a class.
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/class/fail_class_named_self.carbon:[[@LINE+1]]: syntax error, unexpected SELF, expecting identifier
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/class/fail_class_named_self.carbon:[[@LINE+1]]: syntax error, unexpected SELF, expecting identifier
 class Self {
   var x: i32;
   var y: i32;
diff --git a/explorer/testdata/class/fail_field_access_mismatch.carbon b/explorer/testdata/class/fail_field_access_mismatch.carbon
index 40cb7841e665..609ad570931f 100644
--- a/explorer/testdata/class/fail_field_access_mismatch.carbon
+++ b/explorer/testdata/class/fail_field_access_mismatch.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,6 +15,6 @@ class Point {
 
 fn Main() -> i32 {
   var p: Point = {.x = 1, .y = 2};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_access_mismatch.carbon:[[@LINE+1]]: class Point does not have a field named z
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_access_mismatch.carbon:[[@LINE+1]]: class Point does not have a field named z
   return p.z - 1;
 }
diff --git a/explorer/testdata/class/fail_field_mismatch.carbon b/explorer/testdata/class/fail_field_mismatch.carbon
index 173ad477606b..87842b49a999 100644
--- a/explorer/testdata/class/fail_field_mismatch.carbon
+++ b/explorer/testdata/class/fail_field_mismatch.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ class Point {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_mismatch.carbon:[[@LINE+1]]: type error in name binding: '{.x: i32, .z: i32}' is not implicitly convertible to 'class Point'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_mismatch.carbon:[[@LINE+1]]: type error in name binding: '{.x: i32, .z: i32}' is not implicitly convertible to 'class Point'
   var p: Point = {.x = 1, .z = 2};
   return p.x - 1;
 }
diff --git a/explorer/testdata/class/fail_field_missing.carbon b/explorer/testdata/class/fail_field_missing.carbon
index 7cfa17de9810..2a3a5380e087 100644
--- a/explorer/testdata/class/fail_field_missing.carbon
+++ b/explorer/testdata/class/fail_field_missing.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ class Point {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_missing.carbon:[[@LINE+1]]: type error in name binding: '{.x: i32}' is not implicitly convertible to 'class Point'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_missing.carbon:[[@LINE+1]]: type error in name binding: '{.x: i32}' is not implicitly convertible to 'class Point'
   var p: Point = {.x = 1};
   return p.x - 1;
 }
diff --git a/explorer/testdata/class/fail_method_deduced.carbon b/explorer/testdata/class/fail_method_deduced.carbon
index b7084bccfd93..eb1e1f5b2339 100644
--- a/explorer/testdata/class/fail_method_deduced.carbon
+++ b/explorer/testdata/class/fail_method_deduced.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -17,7 +17,7 @@ fn H[T:! Type](x: T) {}
 
 fn Main() -> i32 {
   H(C.F);
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_deduced.carbon:[[@LINE+1]]: Expected a type, but got member name G
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_deduced.carbon:[[@LINE+1]]: Expected a type, but got member name G
   H(C.G);
   return 0;
 }
diff --git a/explorer/testdata/class/fail_method_from_class.carbon b/explorer/testdata/class/fail_method_from_class.carbon
index 1fe7507da879..5990ba04de9c 100644
--- a/explorer/testdata/class/fail_method_from_class.carbon
+++ b/explorer/testdata/class/fail_method_from_class.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -22,6 +22,6 @@ class Point {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_from_class.carbon:[[@LINE+1]]: in call `Point.GetX()`, expected callee to be a function, found `member name GetX`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_from_class.carbon:[[@LINE+1]]: in call `Point.GetX()`, expected callee to be a function, found `member name GetX`
   return Point.GetX();
 }
diff --git a/explorer/testdata/class/fail_method_in_var.carbon b/explorer/testdata/class/fail_method_in_var.carbon
index 6b8cde19cd30..98a139e404b0 100644
--- a/explorer/testdata/class/fail_method_in_var.carbon
+++ b/explorer/testdata/class/fail_method_in_var.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,7 +15,7 @@ class C {
 
 fn Main() -> i32 {
   var f: auto = C.F;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_in_var.carbon:[[@LINE+1]]: Expected a type, but got member name G
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_in_var.carbon:[[@LINE+1]]: Expected a type, but got member name G
   var g: auto = C.G;
 
   return 0;
diff --git a/explorer/testdata/class/fail_return_method.carbon b/explorer/testdata/class/fail_return_method.carbon
index 147bbe71a20f..5477da64e299 100644
--- a/explorer/testdata/class/fail_return_method.carbon
+++ b/explorer/testdata/class/fail_return_method.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ class C {
 }
 
 fn ReturnF() -> auto { return C.F; }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_return_method.carbon:[[@LINE+1]]: Expected a type, but got member name G
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_return_method.carbon:[[@LINE+1]]: Expected a type, but got member name G
 fn ReturnG() -> auto { return C.G; }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/class/function_param.carbon b/explorer/testdata/class/function_param.carbon
index b210109ce4d5..efa6718d73da 100644
--- a/explorer/testdata/class/function_param.carbon
+++ b/explorer/testdata/class/function_param.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/global_var.carbon b/explorer/testdata/class/global_var.carbon
index a09930d1ed62..835a2a637a2d 100644
--- a/explorer/testdata/class/global_var.carbon
+++ b/explorer/testdata/class/global_var.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/method.carbon b/explorer/testdata/class/method.carbon
index 876cfe2d4995..b357a3253331 100644
--- a/explorer/testdata/class/method.carbon
+++ b/explorer/testdata/class/method.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/method_call_method.carbon b/explorer/testdata/class/method_call_method.carbon
index 1e52aa1681b0..9bc48733ec1d 100644
--- a/explorer/testdata/class/method_call_method.carbon
+++ b/explorer/testdata/class/method_call_method.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/method_self.carbon b/explorer/testdata/class/method_self.carbon
index c079a9b89c5b..ce70599044bb 100644
--- a/explorer/testdata/class/method_self.carbon
+++ b/explorer/testdata/class/method_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/temp.carbon b/explorer/testdata/class/temp.carbon
index 48f83df76f8a..8367bae2c518 100644
--- a/explorer/testdata/class/temp.carbon
+++ b/explorer/testdata/class/temp.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/class/var.carbon b/explorer/testdata/class/var.carbon
index 9acfbd7ccf6d..3404c7fe5e9c 100644
--- a/explorer/testdata/class/var.carbon
+++ b/explorer/testdata/class/var.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/comparison/builtin_comparsion.carbon b/explorer/testdata/comparison/builtin_comparsion.carbon
index 1a70a679b9f4..360850f17d55 100644
--- a/explorer/testdata/comparison/builtin_comparsion.carbon
+++ b/explorer/testdata/comparison/builtin_comparsion.carbon
@@ -2,18 +2,18 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:strings less: 1
-// CHECK:ints less: 0
-// CHECK:strings less eq: 1
-// CHECK:ints less eq: 1
-// CHECK:strings greater: 0
-// CHECK:ints greater: 0
-// CHECK:strings greater eq: 0
-// CHECK:ints greater eq: 1
-// CHECK:result: 0
+// CHECK:STDOUT: strings less: 1
+// CHECK:STDOUT: ints less: 0
+// CHECK:STDOUT: strings less eq: 1
+// CHECK:STDOUT: ints less eq: 1
+// CHECK:STDOUT: strings greater: 0
+// CHECK:STDOUT: ints greater: 0
+// CHECK:STDOUT: strings greater eq: 0
+// CHECK:STDOUT: ints greater eq: 1
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/comparison/builtin_equality.carbon b/explorer/testdata/comparison/builtin_equality.carbon
index e6f4168dba3b..8864b8fb42ac 100644
--- a/explorer/testdata/comparison/builtin_equality.carbon
+++ b/explorer/testdata/comparison/builtin_equality.carbon
@@ -2,34 +2,34 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:bool: 1
-// CHECK:bool: 1
-// CHECK:bool: 1
-// CHECK:bool: 1
-// CHECK:bool: 1
-// CHECK:bool: 1
-// CHECK:bool: 1
-// CHECK:bool: 1
-// CHECK:string: 1
-// CHECK:string: 1
-// CHECK:string: 1
-// CHECK:string: 1
-// CHECK:string: 1
-// CHECK:string: 1
-// CHECK:string: 1
-// CHECK:string: 1
-// CHECK:int: 1
-// CHECK:int: 1
-// CHECK:int: 1
-// CHECK:int: 1
-// CHECK:int: 1
-// CHECK:int: 1
-// CHECK:int: 1
-// CHECK:int: 1
-// CHECK:result: 0
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: bool: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: string: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: int: 1
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/comparison/custom_equality.carbon b/explorer/testdata/comparison/custom_equality.carbon
index 558974d76be7..e61c1c44b1cc 100644
--- a/explorer/testdata/comparison/custom_equality.carbon
+++ b/explorer/testdata/comparison/custom_equality.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:structs equal: 0
-// CHECK:structs not equal: 1
-// CHECK:result: 0
+// CHECK:STDOUT: structs equal: 0
+// CHECK:STDOUT: structs not equal: 1
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/comparison/fail_empty_struct.carbon b/explorer/testdata/comparison/fail_empty_struct.carbon
index be08bc4373f7..d00fae715c48 100644
--- a/explorer/testdata/comparison/fail_empty_struct.carbon
+++ b/explorer/testdata/comparison/fail_empty_struct.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // TODO: This should work
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/comparison/fail_empty_struct.carbon:[[@LINE+1]]: type error in call: '({})' is not implicitly convertible to '(Type)'
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/comparison/fail_empty_struct.carbon:[[@LINE+1]]: type error in call: '({})' is not implicitly convertible to '(Type)'
 external impl {} as EqWith({}) {
   fn Equal[me: Self](other: Self) -> bool {
     return true;
diff --git a/explorer/testdata/comparison/fail_no_impl.carbon b/explorer/testdata/comparison/fail_no_impl.carbon
index 01d51ee48c2c..365df8a965fa 100644
--- a/explorer/testdata/comparison/fail_no_impl.carbon
+++ b/explorer/testdata/comparison/fail_no_impl.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/comparison/fail_no_impl.carbon:[[@LINE+1]]: i32 is not equality comparable with String (could not find implementation of interface EqWith(U = String) for i32)
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/comparison/fail_no_impl.carbon:[[@LINE+1]]: i32 is not equality comparable with String (could not find implementation of interface EqWith(U = String) for i32)
   Print("different types equal: {0}", if 1 == "1" then 1 else 0);
   return 0;
 }
diff --git a/explorer/testdata/constraint/binding_self.carbon b/explorer/testdata/constraint/binding_self.carbon
index 216c84933899..fc45d31c7020 100644
--- a/explorer/testdata/constraint/binding_self.carbon
+++ b/explorer/testdata/constraint/binding_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 12
+// CHECK:STDOUT: result: 12
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/combined_interfaces.carbon b/explorer/testdata/constraint/combined_interfaces.carbon
index 934a2d8f6b14..dbd04ecd3f8e 100644
--- a/explorer/testdata/constraint/combined_interfaces.carbon
+++ b/explorer/testdata/constraint/combined_interfaces.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 526
+// CHECK:STDOUT: result: 526
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/dot_self_is_other.carbon b/explorer/testdata/constraint/dot_self_is_other.carbon
index 9506314c4337..cd5290ac1046 100644
--- a/explorer/testdata/constraint/dot_self_is_other.carbon
+++ b/explorer/testdata/constraint/dot_self_is_other.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 12
+// CHECK:STDOUT: result: 12
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/fail_ambiguous_member.carbon b/explorer/testdata/constraint/fail_ambiguous_member.carbon
index 99af5bcbdbbb..d6c45678adb7 100644
--- a/explorer/testdata/constraint/fail_ambiguous_member.carbon
+++ b/explorer/testdata/constraint/fail_ambiguous_member.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface A { fn F() -> i32; }
 interface B { fn F() -> i32; }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_ambiguous_member.carbon:[[@LINE+1]]: ambiguous member access, F found in interface A and interface B
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_ambiguous_member.carbon:[[@LINE+1]]: ambiguous member access, F found in interface A and interface B
 fn Get[T:! A & B](n: T) -> i32 { return n.F(); }
 
 impl i32 as A {
diff --git a/explorer/testdata/constraint/fail_dot_self_after_scope.carbon b/explorer/testdata/constraint/fail_dot_self_after_scope.carbon
index 12acb7eda8fe..e55752b5527e 100644
--- a/explorer/testdata/constraint/fail_dot_self_after_scope.carbon
+++ b/explorer/testdata/constraint/fail_dot_self_after_scope.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface FrobWith(T:! Type) {}
 
 fn F[T:! FrobWith(.Self)]
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_after_scope.carbon:[[@LINE+1]]: could not resolve '.Self'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_after_scope.carbon:[[@LINE+1]]: could not resolve '.Self'
   (U: .Self) {
 }
 
diff --git a/explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon b/explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon
index 5532df470e91..795036496707 100644
--- a/explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon
+++ b/explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface FrobWith(T:! Type) {}
 
 fn F[T:! FrobWith(.Self),
-     // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `U`
+     // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `U`
      U:! .Self]() {
 }
 
diff --git a/explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon b/explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon
index ddee0b30257f..c13ed86d1a45 100644
--- a/explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon
+++ b/explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon:[[@LINE+1]]: could not resolve '.Self'
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon:[[@LINE+1]]: could not resolve '.Self'
 fn Main() -> .Self {
   return 0;
 }
diff --git a/explorer/testdata/constraint/fail_infinite_self.carbon b/explorer/testdata/constraint/fail_infinite_self.carbon
index cb517b5c9a7a..584d048872b2 100644
--- a/explorer/testdata/constraint/fail_infinite_self.carbon
+++ b/explorer/testdata/constraint/fail_infinite_self.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_infinite_self.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_infinite_self.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
 fn F[T:! .Self]() {}
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/constraint/fail_infinite_self_ptr.carbon b/explorer/testdata/constraint/fail_infinite_self_ptr.carbon
index b35ca237fce7..0ca4dfdf76bd 100644
--- a/explorer/testdata/constraint/fail_infinite_self_ptr.carbon
+++ b/explorer/testdata/constraint/fail_infinite_self_ptr.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_infinite_self_ptr.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_infinite_self_ptr.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
 fn F[T:! .Self*]() {}
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/constraint/fail_non_type_self.carbon b/explorer/testdata/constraint/fail_non_type_self.carbon
index f2d9b678159d..0c06f1d9ff2a 100644
--- a/explorer/testdata/constraint/fail_non_type_self.carbon
+++ b/explorer/testdata/constraint/fail_non_type_self.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 class X(T:! Type) {}
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_non_type_self.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_non_type_self.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
 fn F[T:! X(.Self)](x: T) {}
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/constraint/fail_where_equals_different_types.carbon b/explorer/testdata/constraint/fail_where_equals_different_types.carbon
index e6f6ef07d503..7965651aa3ec 100644
--- a/explorer/testdata/constraint/fail_where_equals_different_types.carbon
+++ b/explorer/testdata/constraint/fail_where_equals_different_types.carbon
@@ -2,17 +2,17 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface A {}
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_equals_different_types.carbon:[[@LINE+3]]: type mismatch between values in `where LHS == RHS`
-// CHECK:  LHS type: i32
-// CHECK:  RHS type: Type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_equals_different_types.carbon:[[@LINE+3]]: type mismatch between values in `where LHS == RHS`
+// CHECK:STDERR:   LHS type: i32
+// CHECK:STDERR:   RHS type: Type
 alias B = A where 4 == i32;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/constraint/fail_where_is_non_constraint.carbon b/explorer/testdata/constraint/fail_where_is_non_constraint.carbon
index 0261b91a3a05..cb3f76f65fe9 100644
--- a/explorer/testdata/constraint/fail_where_is_non_constraint.carbon
+++ b/explorer/testdata/constraint/fail_where_is_non_constraint.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface A {}
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_is_non_constraint.carbon:[[@LINE+1]]: expression after `is` does not resolve to a constraint, found value i32 of type Type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_is_non_constraint.carbon:[[@LINE+1]]: expression after `is` does not resolve to a constraint, found value i32 of type Type
 alias B = A where i32 is i32;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/constraint/fail_where_is_non_type.carbon b/explorer/testdata/constraint/fail_where_is_non_type.carbon
index d1304d6b9500..1986f1ba98b7 100644
--- a/explorer/testdata/constraint/fail_where_is_non_type.carbon
+++ b/explorer/testdata/constraint/fail_where_is_non_type.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface A {}
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_is_non_type.carbon:[[@LINE+1]]: expression after `is` does not resolve to a constraint, found i32
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_is_non_type.carbon:[[@LINE+1]]: expression after `is` does not resolve to a constraint, found i32
 alias B = A where i32 is 5;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/constraint/fail_where_non_type_is.carbon b/explorer/testdata/constraint/fail_where_non_type_is.carbon
index 4c0cdd599e4b..f7a85585f9ef 100644
--- a/explorer/testdata/constraint/fail_where_non_type_is.carbon
+++ b/explorer/testdata/constraint/fail_where_non_type_is.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface A {}
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_non_type_is.carbon:[[@LINE+1]]: Expected a type, but got 4
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_where_non_type_is.carbon:[[@LINE+1]]: Expected a type, but got 4
 alias B = A where 4 is A;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/constraint/missing_member.carbon b/explorer/testdata/constraint/missing_member.carbon
index 1cae2211f55f..be012c2d08e6 100644
--- a/explorer/testdata/constraint/missing_member.carbon
+++ b/explorer/testdata/constraint/missing_member.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface A { fn F() -> i32; }
 interface B { fn G() -> i32; }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/missing_member.carbon:[[@LINE+1]]: member access, H not in constraint interface A & interface B where T is interface A and T is interface B
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/missing_member.carbon:[[@LINE+1]]: member access, H not in constraint interface A & interface B where T is interface A and T is interface B
 fn Get[T:! A & B](n: T) -> i32 { return n.H(); }
 
 impl i32 as A {
diff --git a/explorer/testdata/constraint/no_combine_equality.carbon b/explorer/testdata/constraint/no_combine_equality.carbon
index 19ab601ec7b4..37bc9c640309 100644
--- a/explorer/testdata/constraint/no_combine_equality.carbon
+++ b/explorer/testdata/constraint/no_combine_equality.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,7 +13,7 @@ impl i32 as I {}
 
 fn F(A:! i32, B:! i32, C:! i32, D:! i32, E:! i32,
      T:! I where A == B and C == D and C == E and B == D) {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/no_combine_equality.carbon:[[@LINE+1]]: member access, F not in constraint interface I where T is interface I and A == B and C == D and C == E and B == D
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/no_combine_equality.carbon:[[@LINE+1]]: member access, F not in constraint interface I where T is interface I and A == B and C == D and C == E and B == D
   T.F();
 }
 
diff --git a/explorer/testdata/constraint/nondependent_where.carbon b/explorer/testdata/constraint/nondependent_where.carbon
index b5d9aa23efe0..3d514b1129ae 100644
--- a/explorer/testdata/constraint/nondependent_where.carbon
+++ b/explorer/testdata/constraint/nondependent_where.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/qualified_lookup_in_where.carbon b/explorer/testdata/constraint/qualified_lookup_in_where.carbon
index cd51301b3ab6..3b892a2b3e73 100644
--- a/explorer/testdata/constraint/qualified_lookup_in_where.carbon
+++ b/explorer/testdata/constraint/qualified_lookup_in_where.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 122
+// CHECK:STDOUT: result: 122
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/rewrite.carbon b/explorer/testdata/constraint/rewrite.carbon
index 62d0cf994e70..fe454f76f2ba 100644
--- a/explorer/testdata/constraint/rewrite.carbon
+++ b/explorer/testdata/constraint/rewrite.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 12
+// CHECK:STDOUT: result: 12
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/rewrite_compound.carbon b/explorer/testdata/constraint/rewrite_compound.carbon
index d00ee89a5933..91a918c5cbd8 100644
--- a/explorer/testdata/constraint/rewrite_compound.carbon
+++ b/explorer/testdata/constraint/rewrite_compound.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/rewrite_compound_2.carbon b/explorer/testdata/constraint/rewrite_compound_2.carbon
index 7558caa2f569..413c770f9351 100644
--- a/explorer/testdata/constraint/rewrite_compound_2.carbon
+++ b/explorer/testdata/constraint/rewrite_compound_2.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/rewrite_in_qualifier.carbon b/explorer/testdata/constraint/rewrite_in_qualifier.carbon
index ee5233a1730a..42f9ca5d48dc 100644
--- a/explorer/testdata/constraint/rewrite_in_qualifier.carbon
+++ b/explorer/testdata/constraint/rewrite_in_qualifier.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/rewrite_in_qualifier_and_type.carbon b/explorer/testdata/constraint/rewrite_in_qualifier_and_type.carbon
index 19ce6cd0773f..eac4a8ecac91 100644
--- a/explorer/testdata/constraint/rewrite_in_qualifier_and_type.carbon
+++ b/explorer/testdata/constraint/rewrite_in_qualifier_and_type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/constraint/where_self.carbon b/explorer/testdata/constraint/where_self.carbon
index 467626603b5b..3eeee87416ba 100644
--- a/explorer/testdata/constraint/where_self.carbon
+++ b/explorer/testdata/constraint/where_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 12
+// CHECK:STDOUT: result: 12
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/destructor/call_destructor_from_destructor.carbon b/explorer/testdata/destructor/call_destructor_from_destructor.carbon
index 93df6b9bd692..b252bf6b0828 100644
--- a/explorer/testdata/destructor/call_destructor_from_destructor.carbon
+++ b/explorer/testdata/destructor/call_destructor_from_destructor.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:DESTRUCTOR A 1
-// CHECK:DESTRUCTOR B 2
-// CHECK:DESTRUCTOR A 3
-// CHECK:DESTRUCTOR A 4
-// CHECK:result: 1
+// CHECK:STDOUT: DESTRUCTOR A 1
+// CHECK:STDOUT: DESTRUCTOR B 2
+// CHECK:STDOUT: DESTRUCTOR A 3
+// CHECK:STDOUT: DESTRUCTOR A 4
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/destructor/destructor_with_return.carbon b/explorer/testdata/destructor/destructor_with_return.carbon
index 93d85a61931c..b6a6095b0457 100644
--- a/explorer/testdata/destructor/destructor_with_return.carbon
+++ b/explorer/testdata/destructor/destructor_with_return.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:DESTRUCTOR A 3
-// CHECK:result: 1
+// CHECK:STDOUT: DESTRUCTOR A 3
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon b/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon
index c513999f8089..8f8ce1f66fbd 100644
--- a/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon
+++ b/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ class A{
     }
     destructor[me: Self]{
         Print("DESTRUCTOR A2 {0}",me.n);
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon:[[@LINE+1]]: Duplicate name `destructor` also found at {{.*}}/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon:[[@LINE-3]]
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon:[[@LINE+1]]: Duplicate name `destructor` also found at {{.*}}/explorer/testdata/destructor/fail_multiple_destructor_declaration.carbon:[[@LINE-3]]
     }
     var n: i32;
 }
diff --git a/explorer/testdata/destructor/function_with_return.carbon b/explorer/testdata/destructor/function_with_return.carbon
index 7c0767faf620..a207ddd17266 100644
--- a/explorer/testdata/destructor/function_with_return.carbon
+++ b/explorer/testdata/destructor/function_with_return.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:DESTRUCTOR C 1
-// CHECK:DESTRUCTOR B 2
-// CHECK:DESTRUCTOR A 3
-// CHECK:result: 1
+// CHECK:STDOUT: DESTRUCTOR C 1
+// CHECK:STDOUT: DESTRUCTOR B 2
+// CHECK:STDOUT: DESTRUCTOR A 3
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/destructor/loop_block.carbon b/explorer/testdata/destructor/loop_block.carbon
index 0d2534d7830d..98c578de14d1 100644
--- a/explorer/testdata/destructor/loop_block.carbon
+++ b/explorer/testdata/destructor/loop_block.carbon
@@ -2,16 +2,16 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:DESTRUCTOR A 1
-// CHECK:DESTRUCTOR A 2
-// CHECK:DESTRUCTOR A 3
-// CHECK:DESTRUCTOR A 1
-// CHECK:DESTRUCTOR A 2
-// CHECK:DESTRUCTOR A 3
-// CHECK:result: 1
+// CHECK:STDOUT: DESTRUCTOR A 1
+// CHECK:STDOUT: DESTRUCTOR A 2
+// CHECK:STDOUT: DESTRUCTOR A 3
+// CHECK:STDOUT: DESTRUCTOR A 1
+// CHECK:STDOUT: DESTRUCTOR A 2
+// CHECK:STDOUT: DESTRUCTOR A 3
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/destructor/loop_break_continue.carbon b/explorer/testdata/destructor/loop_break_continue.carbon
index 7b6dddad6ead..6ae7a6420235 100644
--- a/explorer/testdata/destructor/loop_break_continue.carbon
+++ b/explorer/testdata/destructor/loop_break_continue.carbon
@@ -2,21 +2,21 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:DESTRUCTOR A 1
-// CHECK:DESTRUCTOR A 2
-// CHECK:DESTRUCTOR A 3
-// CHECK:DESTRUCTOR A 4
-// CHECK:DESTRUCTOR A 5
-// CHECK:DESTRUCTOR A 6
-// CHECK:DESTRUCTOR A 7
-// CHECK:DESTRUCTOR A 8
-// CHECK:DESTRUCTOR A 6
-// CHECK:DESTRUCTOR A 7
-// CHECK:DESTRUCTOR A 8
-// CHECK:result: 1
+// CHECK:STDOUT: DESTRUCTOR A 1
+// CHECK:STDOUT: DESTRUCTOR A 2
+// CHECK:STDOUT: DESTRUCTOR A 3
+// CHECK:STDOUT: DESTRUCTOR A 4
+// CHECK:STDOUT: DESTRUCTOR A 5
+// CHECK:STDOUT: DESTRUCTOR A 6
+// CHECK:STDOUT: DESTRUCTOR A 7
+// CHECK:STDOUT: DESTRUCTOR A 8
+// CHECK:STDOUT: DESTRUCTOR A 6
+// CHECK:STDOUT: DESTRUCTOR A 7
+// CHECK:STDOUT: DESTRUCTOR A 8
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/destructor/nested_block_with_return.carbon b/explorer/testdata/destructor/nested_block_with_return.carbon
index fe4bcf7370f8..e81b18d97bc1 100644
--- a/explorer/testdata/destructor/nested_block_with_return.carbon
+++ b/explorer/testdata/destructor/nested_block_with_return.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:DESTRUCTOR A 1
-// CHECK:DESTRUCTOR A 2
-// CHECK:DESTRUCTOR A 3
-// CHECK:DESTRUCTOR A 4
-// CHECK:result: 2
+// CHECK:STDOUT: DESTRUCTOR A 1
+// CHECK:STDOUT: DESTRUCTOR A 2
+// CHECK:STDOUT: DESTRUCTOR A 3
+// CHECK:STDOUT: DESTRUCTOR A 4
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/experimental_continuation/await_maintains_scope.carbon b/explorer/testdata/experimental_continuation/await_maintains_scope.carbon
index c7567a910536..36a7610bdc16 100644
--- a/explorer/testdata/experimental_continuation/await_maintains_scope.carbon
+++ b/explorer/testdata/experimental_continuation/await_maintains_scope.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/experimental_continuation/convert_run.carbon b/explorer/testdata/experimental_continuation/convert_run.carbon
index ff45f9516544..b8146e88955d 100644
--- a/explorer/testdata/experimental_continuation/convert_run.carbon
+++ b/explorer/testdata/experimental_continuation/convert_run.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/experimental_continuation/creation_is_noop.carbon b/explorer/testdata/experimental_continuation/creation_is_noop.carbon
index ba8e89c7a5f2..6d16a5c1dcef 100644
--- a/explorer/testdata/experimental_continuation/creation_is_noop.carbon
+++ b/explorer/testdata/experimental_continuation/creation_is_noop.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/experimental_continuation/fail_auto_return_await.carbon b/explorer/testdata/experimental_continuation/fail_auto_return_await.carbon
index 56a5a2153ad0..ce1d3a61c606 100644
--- a/explorer/testdata/experimental_continuation/fail_auto_return_await.carbon
+++ b/explorer/testdata/experimental_continuation/fail_auto_return_await.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn f() -> auto {
-   // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_auto_return_await.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+   // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_auto_return_await.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
    __await;
 }
 
diff --git a/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon b/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon
index 9d14d5160993..8788511c9231 100644
--- a/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon
+++ b/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 0;
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon:[[@LINE+1]]: syntax error, unexpected identifier, expecting LEFT_CURLY_BRACE
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon:[[@LINE+1]]: syntax error, unexpected identifier, expecting LEFT_CURLY_BRACE
   __continuation k x = 3;
   __run k;
   return x;
diff --git a/explorer/testdata/experimental_continuation/fail_lifetime.carbon b/explorer/testdata/experimental_continuation/fail_lifetime.carbon
index 0f95d2546bcb..11825f6bc0f5 100644
--- a/explorer/testdata/experimental_continuation/fail_lifetime.carbon
+++ b/explorer/testdata/experimental_continuation/fail_lifetime.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,7 +15,7 @@ package ExplorerTest api;
 fn capture() -> __Continuation {
   var x: i32 = 1;
   __continuation k {
-    // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_lifetime.carbon:[[@LINE+1]]: undefined behavior: access to dead value 1
+    // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_lifetime.carbon:[[@LINE+1]]: undefined behavior: access to dead value 1
     var y: i32 = x;
   }
   return k;
diff --git a/explorer/testdata/experimental_continuation/fail_recursive_continuation.carbon b/explorer/testdata/experimental_continuation/fail_recursive_continuation.carbon
index f793108f6215..345f1d9dcf33 100644
--- a/explorer/testdata/experimental_continuation/fail_recursive_continuation.carbon
+++ b/explorer/testdata/experimental_continuation/fail_recursive_continuation.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn f() {
   __continuation k {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_recursive_continuation.carbon:[[@LINE+1]]: 'k' is not usable until after it has been completely declared
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_recursive_continuation.carbon:[[@LINE+1]]: 'k' is not usable until after it has been completely declared
   __run k;
   }
 }
diff --git a/explorer/testdata/experimental_continuation/fail_return_in_continuation.carbon b/explorer/testdata/experimental_continuation/fail_return_in_continuation.carbon
index 92e4cfdd37ed..e8ff3b5a45be 100644
--- a/explorer/testdata/experimental_continuation/fail_return_in_continuation.carbon
+++ b/explorer/testdata/experimental_continuation/fail_return_in_continuation.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ fn Main() -> i32 {
   var x: i32 = 0;
   __continuation k {
     x = x + 1;
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_return_in_continuation.carbon:[[@LINE+1]]: return is not within a function body
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_return_in_continuation.carbon:[[@LINE+1]]: return is not within a function body
     return x;
   }
 }
diff --git a/explorer/testdata/experimental_continuation/recursive.carbon b/explorer/testdata/experimental_continuation/recursive.carbon
index acfec0867b9f..50e2ac50b63f 100644
--- a/explorer/testdata/experimental_continuation/recursive.carbon
+++ b/explorer/testdata/experimental_continuation/recursive.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 10
+// CHECK:STDOUT: result: 10
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/experimental_continuation/run.carbon b/explorer/testdata/experimental_continuation/run.carbon
index dcd4bf559da5..9c308016a952 100644
--- a/explorer/testdata/experimental_continuation/run.carbon
+++ b/explorer/testdata/experimental_continuation/run.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/experimental_continuation/run_with_await.carbon b/explorer/testdata/experimental_continuation/run_with_await.carbon
index b2b01b27ca12..d7654b1fc831 100644
--- a/explorer/testdata/experimental_continuation/run_with_await.carbon
+++ b/explorer/testdata/experimental_continuation/run_with_await.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/experimental_continuation/shallow_copy.carbon b/explorer/testdata/experimental_continuation/shallow_copy.carbon
index 24ac6d4bee9e..ffd9e0bc2a59 100644
--- a/explorer/testdata/experimental_continuation/shallow_copy.carbon
+++ b/explorer/testdata/experimental_continuation/shallow_copy.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/for/for_loop.carbon b/explorer/testdata/for/for_loop.carbon
index 0ca88473f6af..8435457cfe3d 100644
--- a/explorer/testdata/for/for_loop.carbon
+++ b/explorer/testdata/for/for_loop.carbon
@@ -2,18 +2,18 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:HALLO WELT 0
-// CHECK:HALLO WELT 1
-// CHECK:HALLO WELT 2
-// CHECK:HALLO WELT 3
-// CHECK:HALLO WELT 0
-// CHECK:HALLO WELT 1
-// CHECK:HALLO WELT 2
-// CHECK:HALLO WELT 3
-// CHECK:result: 8
+// CHECK:STDOUT: HALLO WELT 0
+// CHECK:STDOUT: HALLO WELT 1
+// CHECK:STDOUT: HALLO WELT 2
+// CHECK:STDOUT: HALLO WELT 3
+// CHECK:STDOUT: HALLO WELT 0
+// CHECK:STDOUT: HALLO WELT 1
+// CHECK:STDOUT: HALLO WELT 2
+// CHECK:STDOUT: HALLO WELT 3
+// CHECK:STDOUT: result: 8
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/for/for_loop_auto.carbon b/explorer/testdata/for/for_loop_auto.carbon
index dfe7ddfc9a67..21f5a742d418 100644
--- a/explorer/testdata/for/for_loop_auto.carbon
+++ b/explorer/testdata/for/for_loop_auto.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:HALLO WELT 0
-// CHECK:HALLO WELT 1
-// CHECK:HALLO WELT 2
-// CHECK:HALLO WELT 3
-// CHECK:result: 4
+// CHECK:STDOUT: HALLO WELT 0
+// CHECK:STDOUT: HALLO WELT 1
+// CHECK:STDOUT: HALLO WELT 2
+// CHECK:STDOUT: HALLO WELT 3
+// CHECK:STDOUT: result: 4
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/for/for_loop_break.carbon b/explorer/testdata/for/for_loop_break.carbon
index d1011ea90cec..75dff02d8a4a 100644
--- a/explorer/testdata/for/for_loop_break.carbon
+++ b/explorer/testdata/for/for_loop_break.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/for/for_loop_continue.carbon b/explorer/testdata/for/for_loop_continue.carbon
index 4936a3784f43..2bf2f6232353 100644
--- a/explorer/testdata/for/for_loop_continue.carbon
+++ b/explorer/testdata/for/for_loop_continue.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 4
+// CHECK:STDOUT: result: 4
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/for/for_loop_empty.carbon b/explorer/testdata/for/for_loop_empty.carbon
index 1eeeeeb2ef1c..9363bbbbfa8f 100644
--- a/explorer/testdata/for/for_loop_empty.carbon
+++ b/explorer/testdata/for/for_loop_empty.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/for/for_loop_nested.carbon b/explorer/testdata/for/for_loop_nested.carbon
index f000e39803f7..6d7d19d1f62f 100644
--- a/explorer/testdata/for/for_loop_nested.carbon
+++ b/explorer/testdata/for/for_loop_nested.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 20
+// CHECK:STDOUT: result: 20
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/auto_return/add.carbon b/explorer/testdata/function/auto_return/add.carbon
index 1c4b1aae0140..bb1ca081fe02 100644
--- a/explorer/testdata/function/auto_return/add.carbon
+++ b/explorer/testdata/function/auto_return/add.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/auto_return/fail_direct_recurse.carbon b/explorer/testdata/function/auto_return/fail_direct_recurse.carbon
index 4af82fb2c6b6..637e26bb574b 100644
--- a/explorer/testdata/function/auto_return/fail_direct_recurse.carbon
+++ b/explorer/testdata/function/auto_return/fail_direct_recurse.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ package ExplorerTest api;
 // used.
 fn Recurse(x: i32, do_recurse: bool) -> auto {
   if (do_recurse) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_direct_recurse.carbon:[[@LINE+1]]: Function calls itself, but has a deduced return type
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_direct_recurse.carbon:[[@LINE+1]]: Function calls itself, but has a deduced return type
     Recurse(x, false);
   }
   return x;
diff --git a/explorer/testdata/function/auto_return/fail_multiple_returns.carbon b/explorer/testdata/function/auto_return/fail_multiple_returns.carbon
index 7c01e564701c..81935bf80ac7 100644
--- a/explorer/testdata/function/auto_return/fail_multiple_returns.carbon
+++ b/explorer/testdata/function/auto_return/fail_multiple_returns.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ fn Add(x: i32, y: i32) -> auto {
   if (x == 0) {
     return x;
   } else if (y == 0) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_multiple_returns.carbon:[[@LINE+1]]: Only one return is allowed in a function with an `auto` return type.
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_multiple_returns.carbon:[[@LINE+1]]: Only one return is allowed in a function with an `auto` return type.
     return y;
   } else {
     return x + y;
diff --git a/explorer/testdata/function/auto_return/fail_no_return.carbon b/explorer/testdata/function/auto_return/fail_no_return.carbon
index 76c67a86ff16..2ae26baa9150 100644
--- a/explorer/testdata/function/auto_return/fail_no_return.carbon
+++ b/explorer/testdata/function/auto_return/fail_no_return.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn NoReturn() -> auto {
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 }
 
 fn Main() {
diff --git a/explorer/testdata/function/auto_return/fail_separate_decl.carbon b/explorer/testdata/function/auto_return/fail_separate_decl.carbon
index 77b22af1efa2..e94cf51008a2 100644
--- a/explorer/testdata/function/auto_return/fail_separate_decl.carbon
+++ b/explorer/testdata/function/auto_return/fail_separate_decl.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // This declaration is not allowed.
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_separate_decl.carbon:[[@LINE+1]]: Function declaration has deduced return type but no body
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_separate_decl.carbon:[[@LINE+1]]: Function declaration has deduced return type but no body
 fn Add(x: i32, y: i32) -> auto;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/function/auto_return/modify_arg_type.carbon b/explorer/testdata/function/auto_return/modify_arg_type.carbon
index 4f9cdb9e4867..948344123fbc 100644
--- a/explorer/testdata/function/auto_return/modify_arg_type.carbon
+++ b/explorer/testdata/function/auto_return/modify_arg_type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/auto_return/modify_return_type.carbon b/explorer/testdata/function/auto_return/modify_return_type.carbon
index 05d26c5de5c8..278e01e6fe90 100644
--- a/explorer/testdata/function/auto_return/modify_return_type.carbon
+++ b/explorer/testdata/function/auto_return/modify_return_type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/auto_return/type.carbon b/explorer/testdata/function/auto_return/type.carbon
index d0d4c9a04287..64c62a306700 100644
--- a/explorer/testdata/function/auto_return/type.carbon
+++ b/explorer/testdata/function/auto_return/type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/convert_args.carbon b/explorer/testdata/function/convert_args.carbon
index 342746cd22ff..be7eb4771322 100644
--- a/explorer/testdata/function/convert_args.carbon
+++ b/explorer/testdata/function/convert_args.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 123
+// CHECK:STDOUT: result: 123
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/empty_params.carbon b/explorer/testdata/function/empty_params.carbon
index 79cfa9a30fec..5292590e6483 100644
--- a/explorer/testdata/function/empty_params.carbon
+++ b/explorer/testdata/function/empty_params.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/fail_call_with_tuple.carbon b/explorer/testdata/function/fail_call_with_tuple.carbon
index 263ac56f35e5..860b809330bc 100644
--- a/explorer/testdata/function/fail_call_with_tuple.carbon
+++ b/explorer/testdata/function/fail_call_with_tuple.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,6 +13,6 @@ fn f(x: i32, y: i32) -> i32 { return x + y; }
 fn Main() -> i32 {
   var xy: (i32, i32) = (1, 2);
   // should fail to type-check
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_call_with_tuple.carbon:[[@LINE+1]]: wrong number of arguments in function call, expected 2 but got 1
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_call_with_tuple.carbon:[[@LINE+1]]: wrong number of arguments in function call, expected 2 but got 1
   return f(xy);
 }
diff --git a/explorer/testdata/function/fail_invalid_fnty.carbon b/explorer/testdata/function/fail_invalid_fnty.carbon
index 65c2801fa316..a9a6450770da 100644
--- a/explorer/testdata/function/fail_invalid_fnty.carbon
+++ b/explorer/testdata/function/fail_invalid_fnty.carbon
@@ -2,16 +2,16 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // TODO: We should type-check the prelude before the main input so that we can
 // use it within the types of top-level declarations.
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_invalid_fnty.carbon:[[@LINE+2]]: type error in `-`:
-// CHECK:missing declaration for builtin `Negate`
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_invalid_fnty.carbon:[[@LINE+2]]: type error in `-`:
+// CHECK:STDERR: missing declaration for builtin `Negate`
 fn f(g: __Fn(-true) -> true) {
 }
 
diff --git a/explorer/testdata/function/fail_match_no_return.carbon b/explorer/testdata/function/fail_match_no_return.carbon
index d481d36458ef..87172290ad70 100644
--- a/explorer/testdata/function/fail_match_no_return.carbon
+++ b/explorer/testdata/function/fail_match_no_return.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var x: i32 = 0;
   match(0) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
     case 1 => { x = 1; }
     case _: auto => { x = 2; }
   }
diff --git a/explorer/testdata/function/fail_match_partial_return.carbon b/explorer/testdata/function/fail_match_partial_return.carbon
index d67960293946..89a1bfeea9b9 100644
--- a/explorer/testdata/function/fail_match_partial_return.carbon
+++ b/explorer/testdata/function/fail_match_partial_return.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var x: i32 = 0;
   match (0) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_partial_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_partial_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
     case 1 => { x = 1; }
     case _: auto => { return 0; }
   }
diff --git a/explorer/testdata/function/fail_non_exhaustive_match.carbon b/explorer/testdata/function/fail_non_exhaustive_match.carbon
index 8505f6f8ae35..91a70a8e0d88 100644
--- a/explorer/testdata/function/fail_non_exhaustive_match.carbon
+++ b/explorer/testdata/function/fail_non_exhaustive_match.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,5 +13,5 @@ fn Main() -> i32 {
   match (n) {
     case 1 => { return 0; }
   }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_non_exhaustive_match.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_non_exhaustive_match.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
 }
diff --git a/explorer/testdata/function/fail_parameter_type.carbon b/explorer/testdata/function/fail_parameter_type.carbon
index d99d659624fd..8cd1ee03c259 100644
--- a/explorer/testdata/function/fail_parameter_type.carbon
+++ b/explorer/testdata/function/fail_parameter_type.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // 42 cannot be used as the type of a parameter.
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_parameter_type.carbon:[[@LINE+1]]: Expected a type, but got 42
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_parameter_type.carbon:[[@LINE+1]]: Expected a type, but got 42
 fn f(x: 42) -> i32 {
   return x - 1;
 }
diff --git a/explorer/testdata/function/fnty.carbon b/explorer/testdata/function/fnty.carbon
index 269b44251b2e..88ce979b9651 100644
--- a/explorer/testdata/function/fnty.carbon
+++ b/explorer/testdata/function/fnty.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/ignored_parameter.carbon b/explorer/testdata/function/ignored_parameter.carbon
index 97fe833fe7a3..f9134eb0494d 100644
--- a/explorer/testdata/function/ignored_parameter.carbon
+++ b/explorer/testdata/function/ignored_parameter.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/multiple_args.carbon b/explorer/testdata/function/multiple_args.carbon
index 3cf664c64c63..350bd8dc5c39 100644
--- a/explorer/testdata/function/multiple_args.carbon
+++ b/explorer/testdata/function/multiple_args.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/non_exhaustive_match_no_return_type.carbon b/explorer/testdata/function/non_exhaustive_match_no_return_type.carbon
index bff7bb133819..4a50b4bc300b 100644
--- a/explorer/testdata/function/non_exhaustive_match_no_return_type.carbon
+++ b/explorer/testdata/function/non_exhaustive_match_no_return_type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/non_exhaustive_match_return.carbon b/explorer/testdata/function/non_exhaustive_match_return.carbon
index f55980ca48fa..d330d1ad8fca 100644
--- a/explorer/testdata/function/non_exhaustive_match_return.carbon
+++ b/explorer/testdata/function/non_exhaustive_match_return.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/param_lifetime.carbon b/explorer/testdata/function/param_lifetime.carbon
index 3de28786a3cc..695dc86d0e34 100644
--- a/explorer/testdata/function/param_lifetime.carbon
+++ b/explorer/testdata/function/param_lifetime.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/recursive.carbon b/explorer/testdata/function/recursive.carbon
index 55b7e11fd9da..e513deb1fff7 100644
--- a/explorer/testdata/function/recursive.carbon
+++ b/explorer/testdata/function/recursive.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/return.carbon b/explorer/testdata/function/return.carbon
index 8196b9497fd4..d32058dc923d 100644
--- a/explorer/testdata/function/return.carbon
+++ b/explorer/testdata/function/return.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/return_exhaustive_match.carbon b/explorer/testdata/function/return_exhaustive_match.carbon
index edcc82f08483..0cecf5b0b8a4 100644
--- a/explorer/testdata/function/return_exhaustive_match.carbon
+++ b/explorer/testdata/function/return_exhaustive_match.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/function/type_match.carbon b/explorer/testdata/function/type_match.carbon
index 48fc382b2055..4df72d6350cd 100644
--- a/explorer/testdata/function/type_match.carbon
+++ b/explorer/testdata/function/type_match.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/class_function.carbon b/explorer/testdata/generic_class/class_function.carbon
index 14acc5e4c5e8..237b2455ef78 100644
--- a/explorer/testdata/generic_class/class_function.carbon
+++ b/explorer/testdata/generic_class/class_function.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/convert_from_struct.carbon b/explorer/testdata/generic_class/convert_from_struct.carbon
index 828017ca7f22..4d2e237e8044 100644
--- a/explorer/testdata/generic_class/convert_from_struct.carbon
+++ b/explorer/testdata/generic_class/convert_from_struct.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/fail_args_mismatch.carbon b/explorer/testdata/generic_class/fail_args_mismatch.carbon
index 6d7680f47fb8..602cf33d99eb 100644
--- a/explorer/testdata/generic_class/fail_args_mismatch.carbon
+++ b/explorer/testdata/generic_class/fail_args_mismatch.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 class Point(T:! Type) {
   // Point(T, T) does not match class declaration
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_args_mismatch.carbon:[[@LINE+1]]: wrong number of arguments in function call, expected 1 but got 2
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_args_mismatch.carbon:[[@LINE+1]]: wrong number of arguments in function call, expected 1 but got 2
   fn Origin(zero: T) -> Point(T, T) {
     return {.x = zero, .y = zero};
   }
diff --git a/explorer/testdata/generic_class/fail_argument_deduction.carbon b/explorer/testdata/generic_class/fail_argument_deduction.carbon
index 5f6f7518fbc7..568d64e7aac4 100644
--- a/explorer/testdata/generic_class/fail_argument_deduction.carbon
+++ b/explorer/testdata/generic_class/fail_argument_deduction.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -20,8 +20,8 @@ fn FirstOfTwoPoints[T:! Type](a: Point(T), b: Point(T)) -> Point(T) {
 fn Main() -> i32 {
   var p: Point(i32) = {.x = 0, .y = 1};
   var q: Point(bool) = {.x = true, .y = false};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_argument_deduction.carbon:[[@LINE+3]]: deduced multiple different values for T:! Type:
-  // CHECK:  i32
-  // CHECK:  bool
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_argument_deduction.carbon:[[@LINE+3]]: deduced multiple different values for T:! Type:
+  // CHECK:STDERR:   i32
+  // CHECK:STDERR:   bool
   return FirstOfTwoPoints(p, q).x;
 }
diff --git a/explorer/testdata/generic_class/fail_bad_parameter_type.carbon b/explorer/testdata/generic_class/fail_bad_parameter_type.carbon
index b0be30411e26..12ae4a8b0e4d 100644
--- a/explorer/testdata/generic_class/fail_bad_parameter_type.carbon
+++ b/explorer/testdata/generic_class/fail_bad_parameter_type.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -22,7 +22,7 @@ class Point(T:! i32) {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_bad_parameter_type.carbon:[[@LINE+1]]: type error in call: 'Type' is not implicitly convertible to 'i32'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_bad_parameter_type.carbon:[[@LINE+1]]: type error in call: 'Type' is not implicitly convertible to 'i32'
   var p: Point(i32) = Point(i32).Origin(0);
   return p.GetX();
 }
diff --git a/explorer/testdata/generic_class/fail_field_access_on_generic.carbon b/explorer/testdata/generic_class/fail_field_access_on_generic.carbon
index 751d3536e4ae..dc471e51ba2d 100644
--- a/explorer/testdata/generic_class/fail_field_access_on_generic.carbon
+++ b/explorer/testdata/generic_class/fail_field_access_on_generic.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn BadSimpleMemberAccess[T:! Type](a: T) -> T {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_field_access_on_generic.carbon:[[@LINE+1]]: member access in unconstrained type
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_field_access_on_generic.carbon:[[@LINE+1]]: member access in unconstrained type
   return a.x;
 }
 
diff --git a/explorer/testdata/generic_class/fail_generic_class_arg.carbon b/explorer/testdata/generic_class/fail_generic_class_arg.carbon
index 21526e81dc8c..4d0e52fe37a4 100644
--- a/explorer/testdata/generic_class/fail_generic_class_arg.carbon
+++ b/explorer/testdata/generic_class/fail_generic_class_arg.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -18,7 +18,7 @@ class Point(T:! Type) {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_class_arg.carbon:[[@LINE+1]]: wrong number of arguments in function call, expected 1 but got 2
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_class_arg.carbon:[[@LINE+1]]: wrong number of arguments in function call, expected 1 but got 2
   var p: Point(i32) = Point(i32, i32).Create(0, 1);
   return p.x;
 }
diff --git a/explorer/testdata/generic_class/fail_generic_in_pattern.carbon b/explorer/testdata/generic_class/fail_generic_in_pattern.carbon
index 4545be5dd8ce..5e108db41666 100644
--- a/explorer/testdata/generic_class/fail_generic_in_pattern.carbon
+++ b/explorer/testdata/generic_class/fail_generic_in_pattern.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var t: auto = 5;
   match (t) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_in_pattern.carbon:[[@LINE+1]]: Generic binding may not occur in pattern with expected type: T:! i32
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_in_pattern.carbon:[[@LINE+1]]: Generic binding may not occur in pattern with expected type: T:! i32
     case T:! i32 => { return 0; }
     default => { return 1; }
   }
diff --git a/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon b/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon
index 0d5e78283ff2..54837234a470 100644
--- a/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon
+++ b/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -17,7 +17,7 @@ class Point {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon:[[@LINE+1]]: in call `Point(i32)`, expected callee to be a function, found `typeof(class Point)`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon:[[@LINE+1]]: in call `Point(i32)`, expected callee to be a function, found `typeof(class Point)`
   var p: Point(i32) = Point.Origin();
   return 0;
 }
diff --git a/explorer/testdata/generic_class/fail_no_args.carbon b/explorer/testdata/generic_class/fail_no_args.carbon
index 6114d833fc6f..f2d52778a2ad 100644
--- a/explorer/testdata/generic_class/fail_no_args.carbon
+++ b/explorer/testdata/generic_class/fail_no_args.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ class Point(T:! Type) {
   }
 
   // Error: wrote `Point` instead of `Point(T)`, `Point` by itself is not a type.
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_no_args.carbon:[[@LINE+1]]: Expected a type, but got Point
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_no_args.carbon:[[@LINE+1]]: Expected a type, but got Point
   fn GetX[me: Point]() -> T {
     return me.x;
   }
diff --git a/explorer/testdata/generic_class/fail_point_equal.carbon b/explorer/testdata/generic_class/fail_point_equal.carbon
index 0edf94acfae6..a9663354b9a8 100644
--- a/explorer/testdata/generic_class/fail_point_equal.carbon
+++ b/explorer/testdata/generic_class/fail_point_equal.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,7 +15,7 @@ class Point(T:! Type) {
 
 fn Main() -> i32 {
   var p: Point(i32) = {.x = 0, .y = 0};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_point_equal.carbon:[[@LINE+1]]: type error in name binding: 'class Point(T = i32)' is not implicitly convertible to 'class Point(T = bool)'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_point_equal.carbon:[[@LINE+1]]: type error in name binding: 'class Point(T = i32)' is not implicitly convertible to 'class Point(T = bool)'
   var q: Point(bool) = p;
   return 0;
 }
diff --git a/explorer/testdata/generic_class/fail_return_type_is_type.carbon b/explorer/testdata/generic_class/fail_return_type_is_type.carbon
index f1e08c459d33..0922905a7d2a 100644
--- a/explorer/testdata/generic_class/fail_return_type_is_type.carbon
+++ b/explorer/testdata/generic_class/fail_return_type_is_type.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 class Point(T:! Type) {
   // The return type should be Point(T). Point by itself is not a type.
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_return_type_is_type.carbon:[[@LINE+1]]: Expected a type, but got Point
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_return_type_is_type.carbon:[[@LINE+1]]: Expected a type, but got Point
   fn Create(x: T, y: T) -> Point {
     return {.x = x, .y = y};
   }
diff --git a/explorer/testdata/generic_class/fail_self_with_arg.carbon b/explorer/testdata/generic_class/fail_self_with_arg.carbon
index 34e1b1907ac7..d736ec9d61f8 100644
--- a/explorer/testdata/generic_class/fail_self_with_arg.carbon
+++ b/explorer/testdata/generic_class/fail_self_with_arg.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 class Point(T:! Type) {
 
   // Error: wrote `Self(T)` instead of `Self`.
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_self_with_arg.carbon:[[@LINE+1]]: in call `Self(T)`, expected callee to be a function, found `typeof(class Point(T = T))`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_self_with_arg.carbon:[[@LINE+1]]: in call `Self(T)`, expected callee to be a function, found `typeof(class Point(T = T))`
   fn Origin(zero: T) -> Self(T) {
     return {.x = zero, .y = zero};
   }
diff --git a/explorer/testdata/generic_class/fail_two_arg_lists.carbon b/explorer/testdata/generic_class/fail_two_arg_lists.carbon
index ed7a02599713..fdfad94db270 100644
--- a/explorer/testdata/generic_class/fail_two_arg_lists.carbon
+++ b/explorer/testdata/generic_class/fail_two_arg_lists.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 class Point(T:! Type) {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_two_arg_lists.carbon:[[@LINE+1]]: in call `Point(T)(T)`, expected callee to be a function, found `typeof(class Point(T = T))`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_two_arg_lists.carbon:[[@LINE+1]]: in call `Point(T)(T)`, expected callee to be a function, found `typeof(class Point(T = T))`
   fn Origin(zero: T) -> Point(T)(T) {
     return {.x = zero, .y = zero};
   }
diff --git a/explorer/testdata/generic_class/fail_value_param_mismatch.carbon b/explorer/testdata/generic_class/fail_value_param_mismatch.carbon
index 6d8c7f5dc5f1..fe86420a7947 100644
--- a/explorer/testdata/generic_class/fail_value_param_mismatch.carbon
+++ b/explorer/testdata/generic_class/fail_value_param_mismatch.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ fn F(x: HasI32Param(1)) {}
 
 fn Main() -> i32 {
   var n: HasI32Param(0) = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_value_param_mismatch.carbon:[[@LINE+1]]: mismatch in non-type values, `0` != `1`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_value_param_mismatch.carbon:[[@LINE+1]]: mismatch in non-type values, `0` != `1`
   F(n);
   return 0;
 }
diff --git a/explorer/testdata/generic_class/generic_class_substitution.carbon b/explorer/testdata/generic_class/generic_class_substitution.carbon
index 779d21b00af4..ebdef0955714 100644
--- a/explorer/testdata/generic_class/generic_class_substitution.carbon
+++ b/explorer/testdata/generic_class/generic_class_substitution.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/generic_fun_and_class.carbon b/explorer/testdata/generic_class/generic_fun_and_class.carbon
index e0c1c4d55ef8..9bbe4130996a 100644
--- a/explorer/testdata/generic_class/generic_fun_and_class.carbon
+++ b/explorer/testdata/generic_class/generic_fun_and_class.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/generic_point.carbon b/explorer/testdata/generic_class/generic_point.carbon
index 5c18e6a0c007..3a99c6eadcda 100644
--- a/explorer/testdata/generic_class/generic_point.carbon
+++ b/explorer/testdata/generic_class/generic_point.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/impl_with_argument.carbon b/explorer/testdata/generic_class/impl_with_argument.carbon
index 06b6b49d89dc..c18f6ff8414b 100644
--- a/explorer/testdata/generic_class/impl_with_argument.carbon
+++ b/explorer/testdata/generic_class/impl_with_argument.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/impl_with_self.carbon b/explorer/testdata/generic_class/impl_with_self.carbon
index 34ec8996ccec..dd2699e3b530 100644
--- a/explorer/testdata/generic_class/impl_with_self.carbon
+++ b/explorer/testdata/generic_class/impl_with_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/instantiation_at_compile_time.carbon b/explorer/testdata/generic_class/instantiation_at_compile_time.carbon
index b133d3c5ff01..254855ec34fa 100644
--- a/explorer/testdata/generic_class/instantiation_at_compile_time.carbon
+++ b/explorer/testdata/generic_class/instantiation_at_compile_time.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/param_with_dependent_type.carbon b/explorer/testdata/generic_class/param_with_dependent_type.carbon
index 1a7a4271aae7..ee67e22dbab7 100644
--- a/explorer/testdata/generic_class/param_with_dependent_type.carbon
+++ b/explorer/testdata/generic_class/param_with_dependent_type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/parameter_type_conversion.carbon b/explorer/testdata/generic_class/parameter_type_conversion.carbon
index 306484cad452..c0fa95f5ac0e 100644
--- a/explorer/testdata/generic_class/parameter_type_conversion.carbon
+++ b/explorer/testdata/generic_class/parameter_type_conversion.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 4
+// CHECK:STDOUT: result: 4
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/point_with_interface.carbon b/explorer/testdata/generic_class/point_with_interface.carbon
index d9ad38df7486..192f9581cde9 100644
--- a/explorer/testdata/generic_class/point_with_interface.carbon
+++ b/explorer/testdata/generic_class/point_with_interface.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/use_at_compile_time.carbon b/explorer/testdata/generic_class/use_at_compile_time.carbon
index 113bd83321b2..cea42334e516 100644
--- a/explorer/testdata/generic_class/use_at_compile_time.carbon
+++ b/explorer/testdata/generic_class/use_at_compile_time.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_class/use_self.carbon b/explorer/testdata/generic_class/use_self.carbon
index a5459922fced..65de71b1c481 100644
--- a/explorer/testdata/generic_class/use_self.carbon
+++ b/explorer/testdata/generic_class/use_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/apply.carbon b/explorer/testdata/generic_function/apply.carbon
index c34f6256f3d2..9a6b75390938 100644
--- a/explorer/testdata/generic_function/apply.carbon
+++ b/explorer/testdata/generic_function/apply.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: -2
+// CHECK:STDOUT: result: -2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/call_at_compile_time.carbon b/explorer/testdata/generic_function/call_at_compile_time.carbon
index d8558c870f99..161d0400acc0 100644
--- a/explorer/testdata/generic_function/call_at_compile_time.carbon
+++ b/explorer/testdata/generic_function/call_at_compile_time.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/fail_implicit_conversion_extra_field.carbon b/explorer/testdata/generic_function/fail_implicit_conversion_extra_field.carbon
index 255fe071fa3d..1aae3a384523 100644
--- a/explorer/testdata/generic_function/fail_implicit_conversion_extra_field.carbon
+++ b/explorer/testdata/generic_function/fail_implicit_conversion_extra_field.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Bad[T:! Type](x: {.a: i32, .b: T}) {}
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_implicit_conversion_extra_field.carbon:[[@LINE+1]]: mismatch in field names, source field `c` not in destination type `{.a: i32, .b: T}`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_implicit_conversion_extra_field.carbon:[[@LINE+1]]: mismatch in field names, source field `c` not in destination type `{.a: i32, .b: T}`
   Bad({.b = 5, .a = 7, .c = 2});
   return 0;
 }
diff --git a/explorer/testdata/generic_function/fail_implicit_conversion_missing_field.carbon b/explorer/testdata/generic_function/fail_implicit_conversion_missing_field.carbon
index cb9dc04defa6..b92e2f0519fe 100644
--- a/explorer/testdata/generic_function/fail_implicit_conversion_missing_field.carbon
+++ b/explorer/testdata/generic_function/fail_implicit_conversion_missing_field.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Bad[T:! Type](x: {.a: i32, .b: T}) {}
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_implicit_conversion_missing_field.carbon:[[@LINE+1]]: mismatch in field names, destination field `b` not in source type `{.a: i32}`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_implicit_conversion_missing_field.carbon:[[@LINE+1]]: mismatch in field names, destination field `b` not in source type `{.a: i32}`
   Bad({.a = 5});
   return 0;
 }
diff --git a/explorer/testdata/generic_function/fail_missing_exclam.carbon b/explorer/testdata/generic_function/fail_missing_exclam.carbon
index 28bf96da919c..be0cba6e2860 100644
--- a/explorer/testdata/generic_function/fail_missing_exclam.carbon
+++ b/explorer/testdata/generic_function/fail_missing_exclam.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/generic_function/fail_missing_exclam.carbon:[[@LINE+1]]: illegal binding pattern in implicit parameter list
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/generic_function/fail_missing_exclam.carbon:[[@LINE+1]]: illegal binding pattern in implicit parameter list
 fn F[T: Type]();
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/generic_function/fail_not_addable.carbon b/explorer/testdata/generic_function/fail_not_addable.carbon
index d991f8560ea5..34fc5aede12d 100644
--- a/explorer/testdata/generic_function/fail_not_addable.carbon
+++ b/explorer/testdata/generic_function/fail_not_addable.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn id[T:! Type](x: T) -> T {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_not_addable.carbon:[[@LINE+2]]: type error in `+`:
-  // CHECK:could not find implementation of interface AddWith(U = i32) for T
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_not_addable.carbon:[[@LINE+2]]: type error in `+`:
+  // CHECK:STDERR: could not find implementation of interface AddWith(U = i32) for T
   return x + 0;
 }
 
diff --git a/explorer/testdata/generic_function/fail_not_type.carbon b/explorer/testdata/generic_function/fail_not_type.carbon
index dc9d873a852e..c47b0f862829 100644
--- a/explorer/testdata/generic_function/fail_not_type.carbon
+++ b/explorer/testdata/generic_function/fail_not_type.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_not_type.carbon:[[@LINE+1]]: Expected a type, but got 42
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_not_type.carbon:[[@LINE+1]]: Expected a type, but got 42
 fn F[a:! 42]();
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon b/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon
index 0011131b61ed..8c3932a68602 100644
--- a/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon
+++ b/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,8 +13,8 @@ fn fst[T:! Type](x: T, y: T) -> T {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon:[[@LINE+3]]: deduced multiple different values for T:! Type:
-  // CHECK:  i32
-  // CHECK:  bool
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon:[[@LINE+3]]: deduced multiple different values for T:! Type:
+  // CHECK:STDERR:   i32
+  // CHECK:STDERR:   bool
   return fst(0, true);
 }
diff --git a/explorer/testdata/generic_function/fail_type_deduction_unused.carbon b/explorer/testdata/generic_function/fail_type_deduction_unused.carbon
index 6d71da40791a..60bb6b929d7c 100644
--- a/explorer/testdata/generic_function/fail_type_deduction_unused.carbon
+++ b/explorer/testdata/generic_function/fail_type_deduction_unused.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,6 +13,6 @@ fn id[T:! Type](x: i32) -> i32 {
 }
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_unused.carbon:[[@LINE+1]]: could not deduce type argument for type parameter T in call
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_unused.carbon:[[@LINE+1]]: could not deduce type argument for type parameter T in call
   return id(0);
 }
diff --git a/explorer/testdata/generic_function/fail_wrong_variable_substituation.carbon b/explorer/testdata/generic_function/fail_wrong_variable_substituation.carbon
index 2988fd927f17..5ffeabf064df 100644
--- a/explorer/testdata/generic_function/fail_wrong_variable_substituation.carbon
+++ b/explorer/testdata/generic_function/fail_wrong_variable_substituation.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package Foo api;
@@ -15,7 +15,7 @@ fn Get[T:! ()](n: T) -> i32 {  return 2; }
 
 fn Main() -> i32 {
   var x : X = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_wrong_variable_substituation.carbon:[[@LINE+1]]: cannot convert deduced value class X for T to parameter type ()
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_wrong_variable_substituation.carbon:[[@LINE+1]]: cannot convert deduced value class X for T to parameter type ()
   Get(x);
   return 0;
 }
diff --git a/explorer/testdata/generic_function/generic_method.carbon b/explorer/testdata/generic_function/generic_method.carbon
index 7b2356bcdd27..68ecd2f3455d 100644
--- a/explorer/testdata/generic_function/generic_method.carbon
+++ b/explorer/testdata/generic_function/generic_method.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/implicit_conversion.carbon b/explorer/testdata/generic_function/implicit_conversion.carbon
index dabd9a026957..802cfc4dca65 100644
--- a/explorer/testdata/generic_function/implicit_conversion.carbon
+++ b/explorer/testdata/generic_function/implicit_conversion.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 4213
+// CHECK:STDOUT: result: 4213
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/non_generic_param.carbon b/explorer/testdata/generic_function/non_generic_param.carbon
index 13f736142574..7c02cea2f4df 100644
--- a/explorer/testdata/generic_function/non_generic_param.carbon
+++ b/explorer/testdata/generic_function/non_generic_param.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/nondeduced_generic_param.carbon b/explorer/testdata/generic_function/nondeduced_generic_param.carbon
index 88947c351e64..0791aa7d8d4b 100644
--- a/explorer/testdata/generic_function/nondeduced_generic_param.carbon
+++ b/explorer/testdata/generic_function/nondeduced_generic_param.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 class A(T:! Type) {
diff --git a/explorer/testdata/generic_function/return_val.carbon b/explorer/testdata/generic_function/return_val.carbon
index 6da039103aa4..d2b908d436c1 100644
--- a/explorer/testdata/generic_function/return_val.carbon
+++ b/explorer/testdata/generic_function/return_val.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/swap.carbon b/explorer/testdata/generic_function/swap.carbon
index be89cb1db70a..5920ca60cc67 100644
--- a/explorer/testdata/generic_function/swap.carbon
+++ b/explorer/testdata/generic_function/swap.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/tuple_map.carbon b/explorer/testdata/generic_function/tuple_map.carbon
index 06b82a789f06..4593bafa1d1b 100644
--- a/explorer/testdata/generic_function/tuple_map.carbon
+++ b/explorer/testdata/generic_function/tuple_map.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/generic_function/type_matching.carbon b/explorer/testdata/generic_function/type_matching.carbon
index 7fd2db7c231b..eedac35bce13 100644
--- a/explorer/testdata/generic_function/type_matching.carbon
+++ b/explorer/testdata/generic_function/type_matching.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/global_variable/fail_init_type_mismatch.carbon b/explorer/testdata/global_variable/fail_init_type_mismatch.carbon
index c5fd8d30d0d7..83bf8e39f771 100644
--- a/explorer/testdata/global_variable/fail_init_type_mismatch.carbon
+++ b/explorer/testdata/global_variable/fail_init_type_mismatch.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // Test type checking of global variable. Error expected.
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/global_variable/fail_init_type_mismatch.carbon:[[@LINE+1]]: type error in initializer of variable: 'bool' is not implicitly convertible to 'i32'
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/global_variable/fail_init_type_mismatch.carbon:[[@LINE+1]]: type error in initializer of variable: 'bool' is not implicitly convertible to 'i32'
 var flag: i32 = true;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/global_variable/fail_named_self.carbon b/explorer/testdata/global_variable/fail_named_self.carbon
index f7ab098e6478..94926ac9efd7 100644
--- a/explorer/testdata/global_variable/fail_named_self.carbon
+++ b/explorer/testdata/global_variable/fail_named_self.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // Error: global variable may not be named with keyword `Self`.
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/global_variable/fail_named_self.carbon:[[@LINE+1]]: syntax error, unexpected SELF, expecting identifier
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/global_variable/fail_named_self.carbon:[[@LINE+1]]: syntax error, unexpected SELF, expecting identifier
 var Self: i32 = 0;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/global_variable/implicit_conversion.carbon b/explorer/testdata/global_variable/implicit_conversion.carbon
index d06e55ac9773..b09cbcb41db0 100644
--- a/explorer/testdata/global_variable/implicit_conversion.carbon
+++ b/explorer/testdata/global_variable/implicit_conversion.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/global_variable/init_and_read.carbon b/explorer/testdata/global_variable/init_and_read.carbon
index d684d894aa46..931886db35cb 100644
--- a/explorer/testdata/global_variable/init_and_read.carbon
+++ b/explorer/testdata/global_variable/init_and_read.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/global_variable/init_from_function.carbon b/explorer/testdata/global_variable/init_from_function.carbon
index 0b2072c17b31..36f95791680b 100644
--- a/explorer/testdata/global_variable/init_from_function.carbon
+++ b/explorer/testdata/global_variable/init_from_function.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/global_variable/init_order.carbon b/explorer/testdata/global_variable/init_order.carbon
index 9079bb4e6a6a..5b2caf134b43 100644
--- a/explorer/testdata/global_variable/init_order.carbon
+++ b/explorer/testdata/global_variable/init_order.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/global_variable/shadowing.carbon b/explorer/testdata/global_variable/shadowing.carbon
index b4fc9eb4cdb7..a64c592fd30d 100644
--- a/explorer/testdata/global_variable/shadowing.carbon
+++ b/explorer/testdata/global_variable/shadowing.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/global_variable/write.carbon b/explorer/testdata/global_variable/write.carbon
index db66abd5c68c..6f53ff06365b 100644
--- a/explorer/testdata/global_variable/write.carbon
+++ b/explorer/testdata/global_variable/write.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/global_variable/write_from_function.carbon b/explorer/testdata/global_variable/write_from_function.carbon
index 234896ef2c6a..a15d3213a582 100644
--- a/explorer/testdata/global_variable/write_from_function.carbon
+++ b/explorer/testdata/global_variable/write_from_function.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_else/convert_condition.carbon b/explorer/testdata/if_else/convert_condition.carbon
index 9ce83509ab58..58a696cbffd6 100644
--- a/explorer/testdata/if_else/convert_condition.carbon
+++ b/explorer/testdata/if_else/convert_condition.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_else/if_else.carbon b/explorer/testdata/if_else/if_else.carbon
index d630af13f03e..3cfef84c9db9 100644
--- a/explorer/testdata/if_else/if_else.carbon
+++ b/explorer/testdata/if_else/if_else.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_else/if_else_if.carbon b/explorer/testdata/if_else/if_else_if.carbon
index e632c0c52444..9ca07822880b 100644
--- a/explorer/testdata/if_else/if_else_if.carbon
+++ b/explorer/testdata/if_else/if_else_if.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_else/if_else_if_else.carbon b/explorer/testdata/if_else/if_else_if_else.carbon
index 0070b11bc3c6..8886dbc4a0b1 100644
--- a/explorer/testdata/if_else/if_else_if_else.carbon
+++ b/explorer/testdata/if_else/if_else_if_else.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_else/if_false.carbon b/explorer/testdata/if_else/if_false.carbon
index 3c14a38b50d3..404c908dae25 100644
--- a/explorer/testdata/if_else/if_false.carbon
+++ b/explorer/testdata/if_else/if_false.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_else/if_nesting.carbon b/explorer/testdata/if_else/if_nesting.carbon
index ce8875c22158..c4e3dfc06859 100644
--- a/explorer/testdata/if_else/if_nesting.carbon
+++ b/explorer/testdata/if_else/if_nesting.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_else/if_true.carbon b/explorer/testdata/if_else/if_true.carbon
index 925a1e9ad10e..40a557de3f2c 100644
--- a/explorer/testdata/if_else/if_true.carbon
+++ b/explorer/testdata/if_else/if_true.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_expression/convert_condition.carbon b/explorer/testdata/if_expression/convert_condition.carbon
index 3beb412a2640..f67084405dd0 100644
--- a/explorer/testdata/if_expression/convert_condition.carbon
+++ b/explorer/testdata/if_expression/convert_condition.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/if_expression/if_then_else.carbon b/explorer/testdata/if_expression/if_then_else.carbon
index 5468e5501640..2645c015a194 100644
--- a/explorer/testdata/if_expression/if_then_else.carbon
+++ b/explorer/testdata/if_expression/if_then_else.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 12
+// CHECK:STDOUT: result: 12
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/deducible_parameter.carbon b/explorer/testdata/impl/deducible_parameter.carbon
index f89e6e17ddf1..13fb07e59623 100644
--- a/explorer/testdata/impl/deducible_parameter.carbon
+++ b/explorer/testdata/impl/deducible_parameter.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/fail_ambiguous_impl.carbon b/explorer/testdata/impl/fail_ambiguous_impl.carbon
index 2dace25bcc5c..7f88f375ee07 100644
--- a/explorer/testdata/impl/fail_ambiguous_impl.carbon
+++ b/explorer/testdata/impl/fail_ambiguous_impl.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 
@@ -35,7 +35,7 @@ external impl Point as Vector {
   fn Scale[me: Point](v: i32) -> Point {
       return {.x = me.x * v, .y = me.y * v};
   }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_ambiguous_impl.carbon:[[@LINE+1]]: ambiguous implementations of interface Vector for class Point
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_ambiguous_impl.carbon:[[@LINE+1]]: ambiguous implementations of interface Vector for class Point
 }
 
 fn AddAndScaleGeneric[T:! Vector](a: T, b: T, s: i32) -> T {
diff --git a/explorer/testdata/impl/fail_ambiguous_impl_generic.carbon b/explorer/testdata/impl/fail_ambiguous_impl_generic.carbon
index 938f9c8e9373..e465ea8397a2 100644
--- a/explorer/testdata/impl/fail_ambiguous_impl_generic.carbon
+++ b/explorer/testdata/impl/fail_ambiguous_impl_generic.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -22,7 +22,7 @@ fn Main() -> i32 {
   let b: A(i32) = {};
   F(a);
   G(b);
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_ambiguous_impl_generic.carbon:[[@LINE+1]]: ambiguous implementations of interface B(T = i32) for class A(T = i32)
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_ambiguous_impl_generic.carbon:[[@LINE+1]]: ambiguous implementations of interface B(T = i32) for class A(T = i32)
   F(b);
   return 0;
 }
diff --git a/explorer/testdata/impl/fail_bad_member_kind.carbon b/explorer/testdata/impl/fail_bad_member_kind.carbon
index 4ad500830e44..c30abca90cb9 100644
--- a/explorer/testdata/impl/fail_bad_member_kind.carbon
+++ b/explorer/testdata/impl/fail_bad_member_kind.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ interface A {
 }
 
 external impl i32 as A {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/impl/fail_bad_member_kind.carbon:[[@LINE+1]]: syntax error, unexpected CLASS, expecting ALIAS or FN or RIGHT_CURLY_BRACE
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/impl/fail_bad_member_kind.carbon:[[@LINE+1]]: syntax error, unexpected CLASS, expecting ALIAS or FN or RIGHT_CURLY_BRACE
   class T {}
 }
 
diff --git a/explorer/testdata/impl/fail_impl_as.carbon b/explorer/testdata/impl/fail_impl_as.carbon
index fb34687bff5b..6d5f0695917a 100644
--- a/explorer/testdata/impl/fail_impl_as.carbon
+++ b/explorer/testdata/impl/fail_impl_as.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as.carbon:[[@LINE+1]]: expected a constraint in impl declaration, found i32
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as.carbon:[[@LINE+1]]: expected a constraint in impl declaration, found i32
 external impl i32 as i32 {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/fail_impl_as_non_interface.carbon b/explorer/testdata/impl/fail_impl_as_non_interface.carbon
index d61798155b8a..059fb245f3b0 100644
--- a/explorer/testdata/impl/fail_impl_as_non_interface.carbon
+++ b/explorer/testdata/impl/fail_impl_as_non_interface.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_non_interface.carbon:[[@LINE+1]]: expected a constraint in impl declaration, found String
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_non_interface.carbon:[[@LINE+1]]: expected a constraint in impl declaration, found String
 external impl i32 as String {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/fail_impl_as_not_constraint.carbon b/explorer/testdata/impl/fail_impl_as_not_constraint.carbon
index ee37da4c8144..546ecf185701 100644
--- a/explorer/testdata/impl/fail_impl_as_not_constraint.carbon
+++ b/explorer/testdata/impl/fail_impl_as_not_constraint.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_not_constraint.carbon:[[@LINE+1]]: expected a constraint in impl declaration, found i32
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_not_constraint.carbon:[[@LINE+1]]: expected a constraint in impl declaration, found i32
 external impl i32 as i32 {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/fail_impl_as_parameterized.carbon b/explorer/testdata/impl/fail_impl_as_parameterized.carbon
index 49d1201adc2f..eb5307974b6c 100644
--- a/explorer/testdata/impl/fail_impl_as_parameterized.carbon
+++ b/explorer/testdata/impl/fail_impl_as_parameterized.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface Vector(Scalar:! Type) {
 }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_parameterized.carbon:[[@LINE+1]]: Expected a type, but got Vector
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_parameterized.carbon:[[@LINE+1]]: Expected a type, but got Vector
 external impl i32 as Vector {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/fail_nondeducible_parameter.carbon b/explorer/testdata/impl/fail_nondeducible_parameter.carbon
index c51094d8f039..f0e9ce9e6ff5 100644
--- a/explorer/testdata/impl/fail_nondeducible_parameter.carbon
+++ b/explorer/testdata/impl/fail_nondeducible_parameter.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface A(T:! Type) {}
 interface B {}
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_nondeducible_parameter.carbon:[[@LINE+1]]: parameter `T:! Type` is not deducible from `impl i32 as interface B`
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_nondeducible_parameter.carbon:[[@LINE+1]]: parameter `T:! Type` is not deducible from `impl i32 as interface B`
 external impl forall [T:! Type] i32 as A(T) & B {}
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/impl/fail_param_interface_in_impl.carbon b/explorer/testdata/impl/fail_param_interface_in_impl.carbon
index 9cb15188008f..864ad108e9d1 100644
--- a/explorer/testdata/impl/fail_param_interface_in_impl.carbon
+++ b/explorer/testdata/impl/fail_param_interface_in_impl.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -19,7 +19,7 @@ fn Main() -> i32 {
   let n: i32 = 0;
   CheckSimilar(true, false);
   CheckSimilar(true, n);
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_param_interface_in_impl.carbon:[[@LINE+1]]: could not find implementation of interface Similar(T = i32) for bool
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_param_interface_in_impl.carbon:[[@LINE+1]]: could not find implementation of interface Similar(T = i32) for bool
   CheckSimilar(n, false);
   return 0;
 }
diff --git a/explorer/testdata/impl/fail_unambiguous_impl_generic.carbon b/explorer/testdata/impl/fail_unambiguous_impl_generic.carbon
index c9ab20e996a7..30bf99a8dd88 100644
--- a/explorer/testdata/impl/fail_unambiguous_impl_generic.carbon
+++ b/explorer/testdata/impl/fail_unambiguous_impl_generic.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -44,7 +44,7 @@ fn Main() -> i32 {
   var a: Point(i32) = {.x = 1, .y = 1};
   var b: Point(i32) = {.x = 2, .y = 3};
   // TODO: This shouldn't be considered ambiguous.
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_unambiguous_impl_generic.carbon:[[@LINE+1]]: ambiguous implementations of interface Vector(T = i32) for class Point(T = i32)
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_unambiguous_impl_generic.carbon:[[@LINE+1]]: ambiguous implementations of interface Vector(T = i32) for class Point(T = i32)
   var p: Point(i32) = AddAndScaleGeneric(a, b, 5);
   return p.x - 15;
 }
diff --git a/explorer/testdata/impl/fail_unmet_impl_constraint.carbon b/explorer/testdata/impl/fail_unmet_impl_constraint.carbon
index 3795dc57b7a2..a3f1b379222d 100644
--- a/explorer/testdata/impl/fail_unmet_impl_constraint.carbon
+++ b/explorer/testdata/impl/fail_unmet_impl_constraint.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface A {}
 interface B {}
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_unmet_impl_constraint.carbon:[[@LINE+1]]: could not find implementation of interface B for i32
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_unmet_impl_constraint.carbon:[[@LINE+1]]: could not find implementation of interface B for i32
 external impl i32 as A where i32 is B {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/generic_method_impl.carbon b/explorer/testdata/impl/generic_method_impl.carbon
index cabe33ab7807..c93897d92a42 100644
--- a/explorer/testdata/impl/generic_method_impl.carbon
+++ b/explorer/testdata/impl/generic_method_impl.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/impl_constraint.carbon b/explorer/testdata/impl/impl_constraint.carbon
index 5be7b44f35f4..c12f869a1536 100644
--- a/explorer/testdata/impl/impl_constraint.carbon
+++ b/explorer/testdata/impl/impl_constraint.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1234
+// CHECK:STDOUT: result: 1234
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/impl_in_generic_class.carbon b/explorer/testdata/impl/impl_in_generic_class.carbon
index a858329d37ab..24726bb480e1 100644
--- a/explorer/testdata/impl/impl_in_generic_class.carbon
+++ b/explorer/testdata/impl/impl_in_generic_class.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1133
+// CHECK:STDOUT: result: 1133
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/param_impl.carbon b/explorer/testdata/impl/param_impl.carbon
index 4029d8c897db..af1b9077f1ad 100644
--- a/explorer/testdata/impl/param_impl.carbon
+++ b/explorer/testdata/impl/param_impl.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/param_impl2.carbon b/explorer/testdata/impl/param_impl2.carbon
index f986f00a20da..a5c51dd6f686 100644
--- a/explorer/testdata/impl/param_impl2.carbon
+++ b/explorer/testdata/impl/param_impl2.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/param_impl_with_self.carbon b/explorer/testdata/impl/param_impl_with_self.carbon
index 9eef8beab04a..6d3cb3968e35 100644
--- a/explorer/testdata/impl/param_impl_with_self.carbon
+++ b/explorer/testdata/impl/param_impl_with_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/impl/param_interface_in_impl.carbon b/explorer/testdata/impl/param_interface_in_impl.carbon
index 87fd79405823..1845976cf91f 100644
--- a/explorer/testdata/impl/param_interface_in_impl.carbon
+++ b/explorer/testdata/impl/param_interface_in_impl.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/import/fail_nonexistent_library.carbon b/explorer/testdata/import/fail_nonexistent_library.carbon
index 865a5c18bf2c..9fa8767e9a34 100644
--- a/explorer/testdata/import/fail_nonexistent_library.carbon
+++ b/explorer/testdata/import/fail_nonexistent_library.carbon
@@ -4,10 +4,10 @@
 //
 // TODO: This SHOULD fail but doesn't presently.
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/import/fail_nonexistent_package.carbon b/explorer/testdata/import/fail_nonexistent_package.carbon
index 835f807a9f2f..fc41d610b436 100644
--- a/explorer/testdata/import/fail_nonexistent_package.carbon
+++ b/explorer/testdata/import/fail_nonexistent_package.carbon
@@ -4,10 +4,10 @@
 //
 // TODO: This SHOULD fail but doesn't presently.
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/import/fail_order.carbon b/explorer/testdata/import/fail_order.carbon
index 9ed469348812..888b2990c0f7 100644
--- a/explorer/testdata/import/fail_order.carbon
+++ b/explorer/testdata/import/fail_order.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,5 +12,5 @@ fn Main() -> i32 {
   return 0;
 }
 
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/import/fail_order.carbon:[[@LINE+1]]: syntax error, unexpected IMPORT, expecting END_OF_FILE
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/import/fail_order.carbon:[[@LINE+1]]: syntax error, unexpected IMPORT, expecting END_OF_FILE
 import ExplorerTest library "Nonexistent";
diff --git a/explorer/testdata/interface/assoc_constant_constraints_in_scope.carbon b/explorer/testdata/interface/assoc_constant_constraints_in_scope.carbon
index 7c169ad49068..e76c40ec3c08 100644
--- a/explorer/testdata/interface/assoc_constant_constraints_in_scope.carbon
+++ b/explorer/testdata/interface/assoc_constant_constraints_in_scope.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/class_function.carbon b/explorer/testdata/interface/class_function.carbon
index 4da06400a319..09d87492f98e 100644
--- a/explorer/testdata/interface/class_function.carbon
+++ b/explorer/testdata/interface/class_function.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/constrained_parameter.carbon b/explorer/testdata/interface/constrained_parameter.carbon
index 3e7468fa6d32..3f4875d326b2 100644
--- a/explorer/testdata/interface/constrained_parameter.carbon
+++ b/explorer/testdata/interface/constrained_parameter.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/extends.carbon b/explorer/testdata/interface/extends.carbon
index 15b65cc83fa8..4adb48f8c994 100644
--- a/explorer/testdata/interface/extends.carbon
+++ b/explorer/testdata/interface/extends.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Carrot.G
-// CHECK:result: 5
+// CHECK:STDOUT: Carrot.G
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/external_impl_point_vector.carbon b/explorer/testdata/interface/external_impl_point_vector.carbon
index 2c4ccf13732d..775b4f068589 100644
--- a/explorer/testdata/interface/external_impl_point_vector.carbon
+++ b/explorer/testdata/interface/external_impl_point_vector.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/external_impl_use_self.carbon b/explorer/testdata/interface/external_impl_use_self.carbon
index e9bff8f6f5fb..366ed49891ac 100644
--- a/explorer/testdata/interface/external_impl_use_self.carbon
+++ b/explorer/testdata/interface/external_impl_use_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/fail_bad_member_kind.carbon b/explorer/testdata/interface/fail_bad_member_kind.carbon
index 7e7b709a9e0e..14181240008c 100644
--- a/explorer/testdata/interface/fail_bad_member_kind.carbon
+++ b/explorer/testdata/interface/fail_bad_member_kind.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface Bad {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/interface/fail_bad_member_kind.carbon:[[@LINE+1]]: syntax error, unexpected VAR
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/interface/fail_bad_member_kind.carbon:[[@LINE+1]]: syntax error, unexpected VAR
   var V: i32;
 }
 
diff --git a/explorer/testdata/interface/fail_extends_not_constraint.carbon b/explorer/testdata/interface/fail_extends_not_constraint.carbon
index 605a99d6d1cf..67e624d6a2cb 100644
--- a/explorer/testdata/interface/fail_extends_not_constraint.carbon
+++ b/explorer/testdata/interface/fail_extends_not_constraint.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface Vector {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_extends_not_constraint.carbon:[[@LINE+1]]: expected a constraint in extends declaration, found i32
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_extends_not_constraint.carbon:[[@LINE+1]]: expected a constraint in extends declaration, found i32
   extends i32;
 }
 
diff --git a/explorer/testdata/interface/fail_external_impl_omit_self.carbon b/explorer/testdata/interface/fail_external_impl_omit_self.carbon
index 23faf5cffa77..e0a3d0bb3b2c 100644
--- a/explorer/testdata/interface/fail_external_impl_omit_self.carbon
+++ b/explorer/testdata/interface/fail_external_impl_omit_self.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -19,7 +19,7 @@ class Point {
 }
 
 // Error: need to specify which type implementing `Vector` for.
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_external_impl_omit_self.carbon:[[@LINE+1]]: could not resolve 'Self'
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_external_impl_omit_self.carbon:[[@LINE+1]]: could not resolve 'Self'
 external impl as Vector {
   fn Add[me: Point](b: Point) -> Point {
     return {.x = me.x + b.x, .y = me.y + b.y};
diff --git a/explorer/testdata/interface/fail_external_impl_self.carbon b/explorer/testdata/interface/fail_external_impl_self.carbon
index 6456b59570fd..05f073f43ea0 100644
--- a/explorer/testdata/interface/fail_external_impl_self.carbon
+++ b/explorer/testdata/interface/fail_external_impl_self.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -18,7 +18,7 @@ class Point {
   var y: i32;
 }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_external_impl_self.carbon:[[@LINE+1]]: could not resolve 'Self'
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_external_impl_self.carbon:[[@LINE+1]]: could not resolve 'Self'
 external impl Self as Vector {
   fn Add[me: Point](b: Point) -> Point {
     return {.x = me.x + b.x, .y = me.y + b.y};
diff --git a/explorer/testdata/interface/fail_impl_as_not_constraint.carbon b/explorer/testdata/interface/fail_impl_as_not_constraint.carbon
index 8f8549c2a179..c7e9f49414a5 100644
--- a/explorer/testdata/interface/fail_impl_as_not_constraint.carbon
+++ b/explorer/testdata/interface/fail_impl_as_not_constraint.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 interface Vector {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_as_not_constraint.carbon:[[@LINE+1]]: expected a constraint in impl as declaration, found i32
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_as_not_constraint.carbon:[[@LINE+1]]: expected a constraint in impl as declaration, found i32
   impl Self as i32;
 }
 
diff --git a/explorer/testdata/interface/fail_impl_as_not_type.carbon b/explorer/testdata/interface/fail_impl_as_not_type.carbon
index 7790822647db..3047b1e44bbd 100644
--- a/explorer/testdata/interface/fail_impl_as_not_type.carbon
+++ b/explorer/testdata/interface/fail_impl_as_not_type.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface A {}
 
 interface B {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_as_not_type.carbon:[[@LINE+1]]: Expected a type, but got 5
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_as_not_type.carbon:[[@LINE+1]]: Expected a type, but got 5
   impl 5 as A;
 }
 
diff --git a/explorer/testdata/interface/fail_impl_bad_member.carbon b/explorer/testdata/interface/fail_impl_bad_member.carbon
index de901c067c67..40e22a81105e 100644
--- a/explorer/testdata/interface/fail_impl_bad_member.carbon
+++ b/explorer/testdata/interface/fail_impl_bad_member.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -22,9 +22,9 @@ class Point {
     }
     fn Scale[me: Point](v: i32) -> i32 {
       return 0;
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_bad_member.carbon:[[@LINE+3]]: type error in member of implementation
-    // CHECK:expected: fn (i32) -> class Point
-    // CHECK:actual: fn (i32) -> i32
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_bad_member.carbon:[[@LINE+3]]: type error in member of implementation
+    // CHECK:STDERR: expected: fn (i32) -> class Point
+    // CHECK:STDERR: actual: fn (i32) -> i32
     }
   }
 }
diff --git a/explorer/testdata/interface/fail_impl_missing_member.carbon b/explorer/testdata/interface/fail_impl_missing_member.carbon
index c5b733a812cb..b9d0e01ca98d 100644
--- a/explorer/testdata/interface/fail_impl_missing_member.carbon
+++ b/explorer/testdata/interface/fail_impl_missing_member.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -20,7 +20,7 @@ class Point {
     fn Add[me: Point](b: Point) -> Point {
       return {.x = me.x + b.x, .y = me.y + b.y};
     }
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_missing_member.carbon:[[@LINE+1]]: implementation missing Scale
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_missing_member.carbon:[[@LINE+1]]: implementation missing Scale
   }
 }
 
diff --git a/explorer/testdata/interface/fail_impl_not_type.carbon b/explorer/testdata/interface/fail_impl_not_type.carbon
index 752d10380b4a..bbf5779fcab6 100644
--- a/explorer/testdata/interface/fail_impl_not_type.carbon
+++ b/explorer/testdata/interface/fail_impl_not_type.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 interface Vector {
   fn Zero() -> i32;
 }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_not_type.carbon:[[@LINE+1]]: Expected a type, but got "hello"
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_not_type.carbon:[[@LINE+1]]: Expected a type, but got "hello"
 impl "hello" as Vector {
   fn Zero() -> i32 { return 0; }
 }
diff --git a/explorer/testdata/interface/fail_interface_missing_member.carbon b/explorer/testdata/interface/fail_interface_missing_member.carbon
index c5294fd3203f..37d0e618bec6 100644
--- a/explorer/testdata/interface/fail_interface_missing_member.carbon
+++ b/explorer/testdata/interface/fail_interface_missing_member.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,7 +13,7 @@ interface Vector {
 }
 
 fn ScaleGeneric[T:! Vector](a: T, s: i32) -> T {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_interface_missing_member.carbon:[[@LINE+1]]: member access, Scale not in constraint interface Vector where T is interface Vector
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_interface_missing_member.carbon:[[@LINE+1]]: member access, Scale not in constraint interface Vector where T is interface Vector
   return a.Scale(s);
 }
 
diff --git a/explorer/testdata/interface/fail_no_impl.carbon b/explorer/testdata/interface/fail_no_impl.carbon
index 832d071cb504..21ff0f114ab5 100644
--- a/explorer/testdata/interface/fail_no_impl.carbon
+++ b/explorer/testdata/interface/fail_no_impl.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -25,7 +25,7 @@ fn AddAndScaleGeneric[T:! Vector](a: T, b: T, s: i32) -> T {
 fn Main() -> i32 {
   var a: Point = {.x = 0, .y = 0};
   var b: Point = {.x = 2, .y = 3};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_no_impl.carbon:[[@LINE+1]]: could not find implementation of interface Vector for class Point
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_no_impl.carbon:[[@LINE+1]]: could not find implementation of interface Vector for class Point
   var p: Point = AddAndScaleGeneric(a, b, 3);
   return p.x - 6;
 }
diff --git a/explorer/testdata/interface/fail_use_symbolic_member.carbon b/explorer/testdata/interface/fail_use_symbolic_member.carbon
index 803a696724d0..82c0de478f4b 100644
--- a/explorer/testdata/interface/fail_use_symbolic_member.carbon
+++ b/explorer/testdata/interface/fail_use_symbolic_member.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ interface X {
   fn F() -> Type;
 }
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_use_symbolic_member.carbon:[[@LINE+1]]: member lookup for F in symbolic witness 0 of witness for T:! X
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_use_symbolic_member.carbon:[[@LINE+1]]: member lookup for F in symbolic witness 0 of witness for T:! X
 fn G[T:! X]() -> T.F() {
   return {};
 }
diff --git a/explorer/testdata/interface/generic_call_generic.carbon b/explorer/testdata/interface/generic_call_generic.carbon
index 7102146bbb49..18765a822332 100644
--- a/explorer/testdata/interface/generic_call_generic.carbon
+++ b/explorer/testdata/interface/generic_call_generic.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/generic_with_two_params.carbon b/explorer/testdata/interface/generic_with_two_params.carbon
index 447d459eb068..8b8cd48a8de3 100644
--- a/explorer/testdata/interface/generic_with_two_params.carbon
+++ b/explorer/testdata/interface/generic_with_two_params.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/impl_as.carbon b/explorer/testdata/interface/impl_as.carbon
index 38f55f92a173..4a0c59591e07 100644
--- a/explorer/testdata/interface/impl_as.carbon
+++ b/explorer/testdata/interface/impl_as.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Carrot.G
-// CHECK:result: 5
+// CHECK:STDOUT: Carrot.G
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/impl_as_not_self.carbon b/explorer/testdata/interface/impl_as_not_self.carbon
index a9e76dbdb16a..4d51fe1bace6 100644
--- a/explorer/testdata/interface/impl_as_not_self.carbon
+++ b/explorer/testdata/interface/impl_as_not_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/impl_self_interface_parameter.carbon b/explorer/testdata/interface/impl_self_interface_parameter.carbon
index bb3c339419d0..6b7f08daed59 100644
--- a/explorer/testdata/interface/impl_self_interface_parameter.carbon
+++ b/explorer/testdata/interface/impl_self_interface_parameter.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/omit_self.carbon b/explorer/testdata/interface/omit_self.carbon
index a04cb878e5a4..9a86bebaad10 100644
--- a/explorer/testdata/interface/omit_self.carbon
+++ b/explorer/testdata/interface/omit_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/param_with_dependent_type.carbon b/explorer/testdata/interface/param_with_dependent_type.carbon
index 26e35d62f0d8..b4e25be7bfae 100644
--- a/explorer/testdata/interface/param_with_dependent_type.carbon
+++ b/explorer/testdata/interface/param_with_dependent_type.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 42
+// CHECK:STDOUT: result: 42
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/parameterized.carbon b/explorer/testdata/interface/parameterized.carbon
index 9ffcefc5b80c..7ed5d12f2ef9 100644
--- a/explorer/testdata/interface/parameterized.carbon
+++ b/explorer/testdata/interface/parameterized.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/tuple_vector_add_scale.carbon b/explorer/testdata/interface/tuple_vector_add_scale.carbon
index c3078c4071d6..40b7960b00f4 100644
--- a/explorer/testdata/interface/tuple_vector_add_scale.carbon
+++ b/explorer/testdata/interface/tuple_vector_add_scale.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/vector_point_add_scale.carbon b/explorer/testdata/interface/vector_point_add_scale.carbon
index 343dfe3d7a19..bb1a8da5483d 100644
--- a/explorer/testdata/interface/vector_point_add_scale.carbon
+++ b/explorer/testdata/interface/vector_point_add_scale.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/interface/with_self.carbon b/explorer/testdata/interface/with_self.carbon
index 0eb27e39f7fd..7b2b595f7414 100644
--- a/explorer/testdata/interface/with_self.carbon
+++ b/explorer/testdata/interface/with_self.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/let/fail_function_args.carbon b/explorer/testdata/let/fail_function_args.carbon
index 81e648bf5402..0c8aa69f89e6 100644
--- a/explorer/testdata/let/fail_function_args.carbon
+++ b/explorer/testdata/let/fail_function_args.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn f((var x: i32, y: i32)) -> i32 {
   x = 0;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_function_args.carbon:[[@LINE+1]]: Cannot assign to rvalue 'y'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_function_args.carbon:[[@LINE+1]]: Cannot assign to rvalue 'y'
   y = 0;
   return x - 1;
 }
diff --git a/explorer/testdata/let/fail_global_assign.carbon b/explorer/testdata/let/fail_global_assign.carbon
index 2b12f25095b4..fb46c145ca32 100644
--- a/explorer/testdata/let/fail_global_assign.carbon
+++ b/explorer/testdata/let/fail_global_assign.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 let x: i32 = 10;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_global_assign.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_global_assign.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
   x = 0;
   return 0;
 }
diff --git a/explorer/testdata/let/fail_global_named_self.carbon b/explorer/testdata/let/fail_global_named_self.carbon
index 5a0b5d5d0cc2..c27ae37714d3 100644
--- a/explorer/testdata/let/fail_global_named_self.carbon
+++ b/explorer/testdata/let/fail_global_named_self.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 // Error: Can't use keyword `Self` as the name of a global.
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_global_named_self.carbon:[[@LINE+1]]: syntax error, unexpected SELF, expecting identifier
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_global_named_self.carbon:[[@LINE+1]]: syntax error, unexpected SELF, expecting identifier
 let Self: i32 = 10;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/let/fail_local_assign.carbon b/explorer/testdata/let/fail_local_assign.carbon
index a9e157d89ed3..6d5bcbd48a4b 100644
--- a/explorer/testdata/let/fail_local_assign.carbon
+++ b/explorer/testdata/let/fail_local_assign.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   let x: auto = 10;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_local_assign.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_local_assign.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
   x = 0;
   return 0;
 }
diff --git a/explorer/testdata/let/fail_local_named_self.carbon b/explorer/testdata/let/fail_local_named_self.carbon
index 0d78917f7658..75e9e33d8060 100644
--- a/explorer/testdata/let/fail_local_named_self.carbon
+++ b/explorer/testdata/let/fail_local_named_self.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   // Error: Can't use keyword `Self` as the name of a local.
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_local_named_self.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_local_named_self.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
   let Self: auto = 10;
   return 0;
 }
diff --git a/explorer/testdata/let/fail_match_choice.carbon b/explorer/testdata/let/fail_match_choice.carbon
index 2625aaed32a1..5c44957fa249 100644
--- a/explorer/testdata/let/fail_match_choice.carbon
+++ b/explorer/testdata/let/fail_match_choice.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -25,7 +25,7 @@ fn Main() -> i32 {
       n = n - 1;
     }
     case Ints.Two(x: auto, y: auto) => {
-      // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_match_choice.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
+      // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_match_choice.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
       x = 0;
     }
   }
diff --git a/explorer/testdata/let/fail_method_args.carbon b/explorer/testdata/let/fail_method_args.carbon
index 7527261dfbef..6fe6738c04c5 100644
--- a/explorer/testdata/let/fail_method_args.carbon
+++ b/explorer/testdata/let/fail_method_args.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ class Point {
   }
 
   fn SetX[me: Point](x: i32) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_method_args.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_method_args.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
     x = 10;
   }
 
diff --git a/explorer/testdata/let/fail_tuple_pattern_let_context.carbon b/explorer/testdata/let/fail_tuple_pattern_let_context.carbon
index acea767427cf..4d0785bad9fe 100644
--- a/explorer/testdata/let/fail_tuple_pattern_let_context.carbon
+++ b/explorer/testdata/let/fail_tuple_pattern_let_context.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,7 +12,7 @@ fn Main() -> i32 {
   let (var a: auto, b: auto, c: auto, d: auto) = (1, 2, 3, 4);
   a = 0;
   // should fail
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context.carbon:[[@LINE+1]]: Cannot assign to rvalue 'c'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context.carbon:[[@LINE+1]]: Cannot assign to rvalue 'c'
   c = 0;
   return 0;
 }
diff --git a/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon b/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon
index 1abe6becf4a7..fe17fa66931e 100644
--- a/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon
+++ b/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon:[[@LINE+1]]: syntax error, unexpected LET
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon:[[@LINE+1]]: syntax error, unexpected LET
   let (var a: auto, b: auto, c: auto, var (d: auto, let e: auto)) =
       (1, 2, 3, (4, 5));
   return 0;
diff --git a/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon b/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon
index ae9e3caf2c36..698d7b0505e0 100644
--- a/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon
+++ b/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon:[[@LINE+1]]: syntax error, unexpected LET
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon:[[@LINE+1]]: syntax error, unexpected LET
   var (var a: auto, b: auto, let c: auto, d: auto) = (1, 2, 3, 4);
   return 0;
 }
diff --git a/explorer/testdata/let/implicit_conversion.carbon b/explorer/testdata/let/implicit_conversion.carbon
index 8c2f4340dc1c..5b3577db1efe 100644
--- a/explorer/testdata/let/implicit_conversion.carbon
+++ b/explorer/testdata/let/implicit_conversion.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/let/implicit_conversion_choice.carbon b/explorer/testdata/let/implicit_conversion_choice.carbon
index ad0823924d3b..75113db4be58 100644
--- a/explorer/testdata/let/implicit_conversion_choice.carbon
+++ b/explorer/testdata/let/implicit_conversion_choice.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/let/nested_tuple_pattern.carbon b/explorer/testdata/let/nested_tuple_pattern.carbon
index 270c0e17d0d2..69da4420b01f 100644
--- a/explorer/testdata/let/nested_tuple_pattern.carbon
+++ b/explorer/testdata/let/nested_tuple_pattern.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/linked_list/linked_list.carbon b/explorer/testdata/linked_list/linked_list.carbon
index d029e6b720a8..e338cc6e2d16 100644
--- a/explorer/testdata/linked_list/linked_list.carbon
+++ b/explorer/testdata/linked_list/linked_list.carbon
@@ -2,18 +2,18 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:1
-// CHECK:2
-// CHECK:3
-// CHECK:4
-// CHECK:1
-// CHECK:2
-// CHECK:3
-// CHECK:4
-// CHECK:result: 0
+// CHECK:STDOUT: 1
+// CHECK:STDOUT: 2
+// CHECK:STDOUT: 3
+// CHECK:STDOUT: 4
+// CHECK:STDOUT: 1
+// CHECK:STDOUT: 2
+// CHECK:STDOUT: 3
+// CHECK:STDOUT: 4
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/any_int.carbon b/explorer/testdata/match/any_int.carbon
index cad9035ad0a3..12101f3bca4d 100644
--- a/explorer/testdata/match/any_int.carbon
+++ b/explorer/testdata/match/any_int.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/convert.carbon b/explorer/testdata/match/convert.carbon
index 8b8a4e3b28b3..6ceb47eed715 100644
--- a/explorer/testdata/match/convert.carbon
+++ b/explorer/testdata/match/convert.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/exhaustive.carbon b/explorer/testdata/match/exhaustive.carbon
index 288ab518fd0a..e8cb98e54873 100644
--- a/explorer/testdata/match/exhaustive.carbon
+++ b/explorer/testdata/match/exhaustive.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/exhaustive_bool.carbon b/explorer/testdata/match/exhaustive_bool.carbon
index bbe2e79bbcdd..1ed547166eb3 100644
--- a/explorer/testdata/match/exhaustive_bool.carbon
+++ b/explorer/testdata/match/exhaustive_bool.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/exhaustive_exponential_8x.carbon b/explorer/testdata/match/exhaustive_exponential_8x.carbon
index ff35cbb8b83b..a0c8fc33dd23 100644
--- a/explorer/testdata/match/exhaustive_exponential_8x.carbon
+++ b/explorer/testdata/match/exhaustive_exponential_8x.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/exhaustive_exponential_series_t.carbon b/explorer/testdata/match/exhaustive_exponential_series_t.carbon
index 86656923b03d..166d8d88efc3 100644
--- a/explorer/testdata/match/exhaustive_exponential_series_t.carbon
+++ b/explorer/testdata/match/exhaustive_exponential_series_t.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/fail_exhaustive_exponential_9x.carbon b/explorer/testdata/match/fail_exhaustive_exponential_9x.carbon
index c6a8cd1f727c..46836bd2b932 100644
--- a/explorer/testdata/match/fail_exhaustive_exponential_9x.carbon
+++ b/explorer/testdata/match/fail_exhaustive_exponential_9x.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -29,5 +29,5 @@ fn Main() -> i32 {
     case (_: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  AB.B(), AB.A()) => { return 8; }
     case (_: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  AB.B()) => { return 9; }
   }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_exhaustive_exponential_9x.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_exhaustive_exponential_9x.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
 }
diff --git a/explorer/testdata/match/fail_exhaustive_exponential_series_t.carbon b/explorer/testdata/match/fail_exhaustive_exponential_series_t.carbon
index a1b0d63f2442..8bf6aac6257d 100644
--- a/explorer/testdata/match/fail_exhaustive_exponential_series_t.carbon
+++ b/explorer/testdata/match/fail_exhaustive_exponential_series_t.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -38,5 +38,5 @@ fn Main() -> i32 {
     case (_: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  AB.A()) => { return 16; }
     case (_: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  _: AB,  AB.B()) => { return 17; }
   }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_exhaustive_exponential_series_t.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_exhaustive_exponential_series_t.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
 }
diff --git a/explorer/testdata/match/fail_exhaustive_int.carbon b/explorer/testdata/match/fail_exhaustive_int.carbon
index 7929e49a1935..232db172d0b3 100644
--- a/explorer/testdata/match/fail_exhaustive_int.carbon
+++ b/explorer/testdata/match/fail_exhaustive_int.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,7 +13,7 @@ fn A(n: i32) -> i32 {
     case 1 => { return 1; }
     case 2 => { return 2; }
   }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_exhaustive_int.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_exhaustive_int.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/match/fail_not_alternative.carbon b/explorer/testdata/match/fail_not_alternative.carbon
index efb0fa8a7e62..bb3e6ddce51e 100644
--- a/explorer/testdata/match/fail_not_alternative.carbon
+++ b/explorer/testdata/match/fail_not_alternative.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var x: i32 = 0;
   match (x) {
-    // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/match/fail_not_alternative.carbon:[[@LINE+1]]: Alternative pattern must have the form of a field access.
+    // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/match/fail_not_alternative.carbon:[[@LINE+1]]: Alternative pattern must have the form of a field access.
     case i32(n: i32) => {
       return 1;
     }
diff --git a/explorer/testdata/match/fail_not_exhaustive.carbon b/explorer/testdata/match/fail_not_exhaustive.carbon
index 1a28373c9f09..0dc9adbc4e27 100644
--- a/explorer/testdata/match/fail_not_exhaustive.carbon
+++ b/explorer/testdata/match/fail_not_exhaustive.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -18,7 +18,7 @@ fn A(a: Opt, b: Opt) -> i32 {
     case (Opt.Some(n: i32), _: auto) => { return n; }
     case (_: auto, Opt.Some(n: i32)) => { return n; }
   }
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_not_exhaustive.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_not_exhaustive.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/match/fail_not_reachable.carbon b/explorer/testdata/match/fail_not_reachable.carbon
index e9a6c100b380..35440d15dcbd 100644
--- a/explorer/testdata/match/fail_not_reachable.carbon
+++ b/explorer/testdata/match/fail_not_reachable.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -17,7 +17,7 @@ fn A(a: Opt, b: Opt) -> i32 {
   match ((a, b)) {
     case (Opt.Some(n: i32), _: auto) => { return n; }
     case (_: auto, Opt.Some(n: i32)) => { return n; }
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_not_reachable.carbon:[[@LINE+1]]: unreachable case: all values matched by this case are matched by earlier cases
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_not_reachable.carbon:[[@LINE+1]]: unreachable case: all values matched by this case are matched by earlier cases
     case (Opt.Some(m: i32), Opt.Some(n: i32)) => { return m + n; }
   }
   return 0;
diff --git a/explorer/testdata/match/fail_not_reachable_default.carbon b/explorer/testdata/match/fail_not_reachable_default.carbon
index c173e8ee3907..fb64bb8591f8 100644
--- a/explorer/testdata/match/fail_not_reachable_default.carbon
+++ b/explorer/testdata/match/fail_not_reachable_default.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -18,7 +18,7 @@ fn A(a: Opt, b: Opt) -> i32 {
     case (Opt.None(), Opt.None()) => { return 0; }
     case (Opt.Some(n: i32), _: auto) => { return n; }
     case (_: auto, Opt.Some(n: i32)) => { return n; }
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_not_reachable_default.carbon:[[@LINE+1]]: unreachable case: all values matched by this case are matched by earlier cases
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_not_reachable_default.carbon:[[@LINE+1]]: unreachable case: all values matched by this case are matched by earlier cases
     default => { return -1; }
   }
   return 0;
diff --git a/explorer/testdata/match/fail_pattern_type_mismatch.carbon b/explorer/testdata/match/fail_pattern_type_mismatch.carbon
index 91b3c972cd62..336a102c6d26 100644
--- a/explorer/testdata/match/fail_pattern_type_mismatch.carbon
+++ b/explorer/testdata/match/fail_pattern_type_mismatch.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -21,9 +21,9 @@ fn Main() -> i32 {
   // expression when checking each pattern.
   match ((a, a)) {
     case (0, n: A) => { return 1; }
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_pattern_type_mismatch.carbon:[[@LINE+3]]: type error in `match` pattern type
-    // CHECK:expected: (i32, class A)
-    // CHECK:actual: (class A, i32)
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_pattern_type_mismatch.carbon:[[@LINE+3]]: type error in `match` pattern type
+    // CHECK:STDERR: expected: (i32, class A)
+    // CHECK:STDERR: actual: (class A, i32)
     case (n: A, 0) => { return 1; }
     case (1, 1) => { return 0; }
   }
diff --git a/explorer/testdata/match/fail_redundant_int.carbon b/explorer/testdata/match/fail_redundant_int.carbon
index 255e4514749b..3cfc320d615d 100644
--- a/explorer/testdata/match/fail_redundant_int.carbon
+++ b/explorer/testdata/match/fail_redundant_int.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn A(n: i32) -> i32 {
   match (n) {
     case 1 => { return 1; }
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_redundant_int.carbon:[[@LINE+1]]: unreachable case: all values matched by this case are matched by earlier cases
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/match/fail_redundant_int.carbon:[[@LINE+1]]: unreachable case: all values matched by this case are matched by earlier cases
     case 1 => { return 2; }
     case (_: i32) => { return 3; }
   }
diff --git a/explorer/testdata/match/int.carbon b/explorer/testdata/match/int.carbon
index c3eff63c9b47..2f541469a55a 100644
--- a/explorer/testdata/match/int.carbon
+++ b/explorer/testdata/match/int.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/int_default.carbon b/explorer/testdata/match/int_default.carbon
index cd5a1336c1e4..95052f312947 100644
--- a/explorer/testdata/match/int_default.carbon
+++ b/explorer/testdata/match/int_default.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/no_match.carbon b/explorer/testdata/match/no_match.carbon
index d2ef6872714a..a8a8ded5207c 100644
--- a/explorer/testdata/match/no_match.carbon
+++ b/explorer/testdata/match/no_match.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/match/placeholder.carbon b/explorer/testdata/match/placeholder.carbon
index fc187e5656cc..cfb13f9c7e97 100644
--- a/explorer/testdata/match/placeholder.carbon
+++ b/explorer/testdata/match/placeholder.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/member_access/convert_lhs_class.carbon b/explorer/testdata/member_access/convert_lhs_class.carbon
index 077f4d4e2803..c428c1e4a653 100644
--- a/explorer/testdata/member_access/convert_lhs_class.carbon
+++ b/explorer/testdata/member_access/convert_lhs_class.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package Foo api;
 
diff --git a/explorer/testdata/member_access/convert_lhs_interface.carbon b/explorer/testdata/member_access/convert_lhs_interface.carbon
index 06dfce8bef45..6503a3fc9827 100644
--- a/explorer/testdata/member_access/convert_lhs_interface.carbon
+++ b/explorer/testdata/member_access/convert_lhs_interface.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package Foo api;
 
diff --git a/explorer/testdata/member_access/evaluate_type_before_dot.carbon b/explorer/testdata/member_access/evaluate_type_before_dot.carbon
index e5acc39c2dbd..4ea7a931bc71 100644
--- a/explorer/testdata/member_access/evaluate_type_before_dot.carbon
+++ b/explorer/testdata/member_access/evaluate_type_before_dot.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Struct OK
-// CHECK:Choice OK
-// CHECK:Class OK
-// CHECK:Interface OK
-// CHECK:Constraint OK
-// CHECK:result: 0
+// CHECK:STDOUT: Struct OK
+// CHECK:STDOUT: Choice OK
+// CHECK:STDOUT: Class OK
+// CHECK:STDOUT: Interface OK
+// CHECK:STDOUT: Constraint OK
+// CHECK:STDOUT: result: 0
 
 package Foo api;
 
diff --git a/explorer/testdata/member_access/fail_qualified_non_member.carbon b/explorer/testdata/member_access/fail_qualified_non_member.carbon
index 4b611ba59a0f..cd03fcb05f93 100644
--- a/explorer/testdata/member_access/fail_qualified_non_member.carbon
+++ b/explorer/testdata/member_access/fail_qualified_non_member.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package Foo api;
@@ -11,7 +11,7 @@ fn F[me: i32]() {}
 fn Main() -> i32 {
   // TODO: It's unclear whether this is valid per the current rules. See
   // https://github.com/carbon-language/carbon-lang/pull/1122
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/member_access/fail_qualified_non_member.carbon:[[@LINE+1]]: expected name of instance member or interface member in compound member access, found fn () -> ()
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/member_access/fail_qualified_non_member.carbon:[[@LINE+1]]: expected name of instance member or interface member in compound member access, found fn () -> ()
   42.(F)();
   return 0;
 }
diff --git a/explorer/testdata/member_access/fail_vacuous_access.carbon b/explorer/testdata/member_access/fail_vacuous_access.carbon
index 9c60231bf968..26e9f51975ee 100644
--- a/explorer/testdata/member_access/fail_vacuous_access.carbon
+++ b/explorer/testdata/member_access/fail_vacuous_access.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package Foo api;
@@ -15,6 +15,6 @@ class X {
 }
 fn Main() -> i32 {
   var a: X = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/member_access/fail_vacuous_access.carbon:[[@LINE+1]]: expected name of instance member or interface member in compound member access, found fn () -> i32
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/member_access/fail_vacuous_access.carbon:[[@LINE+1]]: expected name of instance member or interface member in compound member access, found fn () -> i32
   return a.(X.(A.F))();
 }
diff --git a/explorer/testdata/member_access/fail_vacuous_access_via_type_param.carbon b/explorer/testdata/member_access/fail_vacuous_access_via_type_param.carbon
index c49ce859e52f..359f4f523f7a 100644
--- a/explorer/testdata/member_access/fail_vacuous_access_via_type_param.carbon
+++ b/explorer/testdata/member_access/fail_vacuous_access_via_type_param.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package Foo api;
@@ -14,7 +14,7 @@ class X {
   }
 }
 fn F[T:! A](a: T) -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/member_access/fail_vacuous_access_via_type_param.carbon:[[@LINE+1]]: expected name of instance member or interface member in compound member access, found fn () -> i32
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/member_access/fail_vacuous_access_via_type_param.carbon:[[@LINE+1]]: expected name of instance member or interface member in compound member access, found fn () -> i32
   return a.(T.F)();
 }
 fn Main() -> i32 {
diff --git a/explorer/testdata/member_access/nearly_vacuous_access_with_impl_lookup.carbon b/explorer/testdata/member_access/nearly_vacuous_access_with_impl_lookup.carbon
index c3ba3c8b6219..72f0dd6cb2b5 100644
--- a/explorer/testdata/member_access/nearly_vacuous_access_with_impl_lookup.carbon
+++ b/explorer/testdata/member_access/nearly_vacuous_access_with_impl_lookup.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package Foo api;
 interface A { fn F() -> i32; }
diff --git a/explorer/testdata/member_access/nearly_vacuous_access_with_instance_binding.carbon b/explorer/testdata/member_access/nearly_vacuous_access_with_instance_binding.carbon
index 0c1dbb48f7a9..4d4376a507db 100644
--- a/explorer/testdata/member_access/nearly_vacuous_access_with_instance_binding.carbon
+++ b/explorer/testdata/member_access/nearly_vacuous_access_with_instance_binding.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package Foo api;
 interface A { fn F[me: Self]() -> i32; }
diff --git a/explorer/testdata/member_access/param_qualified_interface_member.carbon b/explorer/testdata/member_access/param_qualified_interface_member.carbon
index 8116279dea64..87042132f132 100644
--- a/explorer/testdata/member_access/param_qualified_interface_member.carbon
+++ b/explorer/testdata/member_access/param_qualified_interface_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package Foo api;
 interface A {
diff --git a/explorer/testdata/member_access/qualified_class_member.carbon b/explorer/testdata/member_access/qualified_class_member.carbon
index 4cb975cd3dde..ebf6413c2ebb 100644
--- a/explorer/testdata/member_access/qualified_class_member.carbon
+++ b/explorer/testdata/member_access/qualified_class_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package Foo api;
 class X {
diff --git a/explorer/testdata/member_access/qualified_constraint_member.carbon b/explorer/testdata/member_access/qualified_constraint_member.carbon
index 3a3bc295c716..e1592346f96f 100644
--- a/explorer/testdata/member_access/qualified_constraint_member.carbon
+++ b/explorer/testdata/member_access/qualified_constraint_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 12
+// CHECK:STDOUT: result: 12
 
 package Foo api;
 interface A {
diff --git a/explorer/testdata/member_access/qualified_interface_member.carbon b/explorer/testdata/member_access/qualified_interface_member.carbon
index 8c7e2cd79b12..7067578d90e7 100644
--- a/explorer/testdata/member_access/qualified_interface_member.carbon
+++ b/explorer/testdata/member_access/qualified_interface_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package Foo api;
 interface A {
diff --git a/explorer/testdata/member_access/qualified_param_member.carbon b/explorer/testdata/member_access/qualified_param_member.carbon
index 9bd0fd2c34a2..dc73a87b8285 100644
--- a/explorer/testdata/member_access/qualified_param_member.carbon
+++ b/explorer/testdata/member_access/qualified_param_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package Foo api;
 interface A {
diff --git a/explorer/testdata/member_access/qualified_struct_member.carbon b/explorer/testdata/member_access/qualified_struct_member.carbon
index e5bc2e4aa615..48ef8b756678 100644
--- a/explorer/testdata/member_access/qualified_struct_member.carbon
+++ b/explorer/testdata/member_access/qualified_struct_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package Foo api;
 fn Main() -> i32 {
diff --git a/explorer/testdata/member_access/type_qualified_interface_member.carbon b/explorer/testdata/member_access/type_qualified_interface_member.carbon
index 78d5509ca69d..1ff905a83416 100644
--- a/explorer/testdata/member_access/type_qualified_interface_member.carbon
+++ b/explorer/testdata/member_access/type_qualified_interface_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 42
+// CHECK:STDOUT: result: 42
 
 package Foo api;
 interface HasDefault {
diff --git a/explorer/testdata/mixin/fail_circular_mixing.carbon b/explorer/testdata/mixin/fail_circular_mixing.carbon
index 2e428995eafd..9a8d016e4a84 100644
--- a/explorer/testdata/mixin/fail_circular_mixing.carbon
+++ b/explorer/testdata/mixin/fail_circular_mixing.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 package ExplorerTest api;
 
@@ -11,7 +11,7 @@ __mixin M1 {
   fn F1[me: Self](x: Self) -> Self{
      return x;
   }
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_circular_mixing.carbon:[[@LINE+1]]: 'M3' has not been declared yet
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_circular_mixing.carbon:[[@LINE+1]]: 'M3' has not been declared yet
   __mix M3;
 }
 
diff --git a/explorer/testdata/mixin/fail_field_member_name_clash.carbon b/explorer/testdata/mixin/fail_field_member_name_clash.carbon
index de9901b6d98f..4c3dbcb9902f 100644
--- a/explorer/testdata/mixin/fail_field_member_name_clash.carbon
+++ b/explorer/testdata/mixin/fail_field_member_name_clash.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -17,7 +17,7 @@ __mixin M {
 class C {
   let F: i32 = 0;
   __mix M;
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:[[@LINE-6]]) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:[[@LINE-2]])
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:[[@LINE-6]]) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:[[@LINE-2]])
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail_method_member_name_clash.carbon b/explorer/testdata/mixin/fail_method_member_name_clash.carbon
index bdd0fc598d7b..e63360347c7b 100644
--- a/explorer/testdata/mixin/fail_method_member_name_clash.carbon
+++ b/explorer/testdata/mixin/fail_method_member_name_clash.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -19,7 +19,7 @@ class C {
      return x;
   }
   __mix M;
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:[[@LINE-8]]) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:[[@LINE-2]])
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:[[@LINE-8]]) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:[[@LINE-2]])
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail_mix_as_type_expr.carbon b/explorer/testdata/mixin/fail_mix_as_type_expr.carbon
index 2871f92f8efa..e6bb362b3427 100644
--- a/explorer/testdata/mixin/fail_mix_as_type_expr.carbon
+++ b/explorer/testdata/mixin/fail_mix_as_type_expr.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 __mixin Operations {}
 
 fn Main() -> i32 {
-   // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_as_type_expr.carbon:[[@LINE+1]]: Expected a type, but got mixin Operations
+   // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_as_type_expr.carbon:[[@LINE+1]]: Expected a type, but got mixin Operations
    var a: Operations;
    return 0;
 }
diff --git a/explorer/testdata/mixin/fail_mix_diamond_clash.carbon b/explorer/testdata/mixin/fail_mix_diamond_clash.carbon
index 5ec512a2b8a9..e3396b39a390 100644
--- a/explorer/testdata/mixin/fail_mix_diamond_clash.carbon
+++ b/explorer/testdata/mixin/fail_mix_diamond_clash.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 package ExplorerTest api;
 
@@ -28,7 +28,7 @@ __mixin M3 {
 class C {
   __mix M2;
   __mix M3;
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_diamond_clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_mix_diamond_clash.carbon:[[@LINE-18]]) is being mixed multiple times into C
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_diamond_clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_mix_diamond_clash.carbon:[[@LINE-18]]) is being mixed multiple times into C
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail_mix_in_global.carbon b/explorer/testdata/mixin/fail_mix_in_global.carbon
index e0912613b361..393ecf32617b 100644
--- a/explorer/testdata/mixin/fail_mix_in_global.carbon
+++ b/explorer/testdata/mixin/fail_mix_in_global.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,7 +14,7 @@ __mixin Operations {
   }
 }
 
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_in_global.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting END_OF_FILE
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_in_global.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting END_OF_FILE
 __mix Operations;
 
 class Point {
diff --git a/explorer/testdata/mixin/fail_mix_in_impl.carbon b/explorer/testdata/mixin/fail_mix_in_impl.carbon
index c1fa313bd457..04d1bd7f5d6c 100644
--- a/explorer/testdata/mixin/fail_mix_in_impl.carbon
+++ b/explorer/testdata/mixin/fail_mix_in_impl.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -19,7 +19,7 @@ interface A {
 }
 
 external impl i32 as A {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_in_impl.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting ALIAS or FN or RIGHT_CURLY_BRACE
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_in_impl.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting ALIAS or FN or RIGHT_CURLY_BRACE
   __mix Operations;
   fn F() {}
 }
diff --git a/explorer/testdata/mixin/fail_mix_members_clash.carbon b/explorer/testdata/mixin/fail_mix_members_clash.carbon
index 95bc8dfa3d4f..ae442e7b8050 100644
--- a/explorer/testdata/mixin/fail_mix_members_clash.carbon
+++ b/explorer/testdata/mixin/fail_mix_members_clash.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 package ExplorerTest api;
 
@@ -21,7 +21,7 @@ __mixin M2 {
 class C {
   __mix M1;
   __mix M2;
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:[[@LINE-6]]) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:[[@LINE-11]])
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:[[@LINE-6]]) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:[[@LINE-11]])
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail_recursive_mixing.carbon b/explorer/testdata/mixin/fail_recursive_mixing.carbon
index be0349d2bf56..d76e8c4f3455 100644
--- a/explorer/testdata/mixin/fail_recursive_mixing.carbon
+++ b/explorer/testdata/mixin/fail_recursive_mixing.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 package ExplorerTest api;
 
@@ -12,7 +12,7 @@ __mixin M1 {
      return x;
   }
   __mix M1;
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_recursive_mixing.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_recursive_mixing.carbon:[[@LINE-2]]) is being mixed multiple times into M1
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_recursive_mixing.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_recursive_mixing.carbon:[[@LINE-2]]) is being mixed multiple times into M1
 }
 
 class C {
diff --git a/explorer/testdata/mixin/fail_self_substitution.carbon b/explorer/testdata/mixin/fail_self_substitution.carbon
index b0834d080f41..ee4e89481535 100644
--- a/explorer/testdata/mixin/fail_self_substitution.carbon
+++ b/explorer/testdata/mixin/fail_self_substitution.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -36,7 +36,7 @@ class Complex {
 fn Main() -> i32 {
   var p: Point = Point.Origin();
   var c: Complex = {.r = 42, .i = 1};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_self_substitution.carbon:[[@LINE+1]]: type error in call: 'class Complex' is not implicitly convertible to 'class Point'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_self_substitution.carbon:[[@LINE+1]]: type error in call: 'class Complex' is not implicitly convertible to 'class Point'
   var p1: Point = p.F(c);
 
   return p1.x - 42;
diff --git a/explorer/testdata/mixin/self_substitution.carbon b/explorer/testdata/mixin/self_substitution.carbon
index e99d89c69c5c..adf91b73bf38 100644
--- a/explorer/testdata/mixin/self_substitution.carbon
+++ b/explorer/testdata/mixin/self_substitution.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/mixin/simple_mix_in_mixin.carbon b/explorer/testdata/mixin/simple_mix_in_mixin.carbon
index dd092736fc7c..a19051553ba6 100644
--- a/explorer/testdata/mixin/simple_mix_in_mixin.carbon
+++ b/explorer/testdata/mixin/simple_mix_in_mixin.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/mixin/simple_reuse.carbon b/explorer/testdata/mixin/simple_reuse.carbon
index 8bfe216f707c..2b4a69c39f70 100644
--- a/explorer/testdata/mixin/simple_reuse.carbon
+++ b/explorer/testdata/mixin/simple_reuse.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/mixin/use_mixin_method_in_class_method.carbon b/explorer/testdata/mixin/use_mixin_method_in_class_method.carbon
index eb70ac72f5fa..a791daf80e17 100644
--- a/explorer/testdata/mixin/use_mixin_method_in_class_method.carbon
+++ b/explorer/testdata/mixin/use_mixin_method_in_class_method.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/name_lookup/class_fn_body_reorder.carbon b/explorer/testdata/name_lookup/class_fn_body_reorder.carbon
index 3b577dfb2c81..683900b47bf6 100644
--- a/explorer/testdata/name_lookup/class_fn_body_reorder.carbon
+++ b/explorer/testdata/name_lookup/class_fn_body_reorder.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/name_lookup/fail_block_duplicate.carbon b/explorer/testdata/name_lookup/fail_block_duplicate.carbon
index 04c9ce4de127..80a2ec8ec5dc 100644
--- a/explorer/testdata/name_lookup/fail_block_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_block_duplicate.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 0;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:[[@LINE-1]]
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:[[@LINE-1]]
   var x: i32 = 0;
   return 0;
 }
diff --git a/explorer/testdata/name_lookup/fail_choice_duplicate.carbon b/explorer/testdata/name_lookup/fail_choice_duplicate.carbon
index 903037dce0d9..51b25cb2db74 100644
--- a/explorer/testdata/name_lookup/fail_choice_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_choice_duplicate.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 choice Ints {
   None,
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_choice_duplicate.carbon:[[@LINE+1]]: Duplicate name `None` in choice type
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_choice_duplicate.carbon:[[@LINE+1]]: Duplicate name `None` in choice type
   None,
 }
 
diff --git a/explorer/testdata/name_lookup/fail_class_duplicate.carbon b/explorer/testdata/name_lookup/fail_class_duplicate.carbon
index ccef41b69da0..0f697b4c26b8 100644
--- a/explorer/testdata/name_lookup/fail_class_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_class_duplicate.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 class Foo {
   var x: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:[[@LINE-1]]
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:[[@LINE-1]]
   var x: i32;
 }
 
diff --git a/explorer/testdata/name_lookup/fail_class_fn_use_before_declaration.carbon b/explorer/testdata/name_lookup/fail_class_fn_use_before_declaration.carbon
index afcbba988c0c..49af9fd89d14 100644
--- a/explorer/testdata/name_lookup/fail_class_fn_use_before_declaration.carbon
+++ b/explorer/testdata/name_lookup/fail_class_fn_use_before_declaration.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -19,7 +19,7 @@ class A {
     return 0;
   }
 
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_fn_use_before_declaration.carbon:[[@LINE+1]]: 'I' has not been declared yet
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_fn_use_before_declaration.carbon:[[@LINE+1]]: 'I' has not been declared yet
   fn H() -> I() {
     return 0;
   }
diff --git a/explorer/testdata/name_lookup/fail_class_use_in_params.carbon b/explorer/testdata/name_lookup/fail_class_use_in_params.carbon
index c31f34f82c43..b7394999c8b8 100644
--- a/explorer/testdata/name_lookup/fail_class_use_in_params.carbon
+++ b/explorer/testdata/name_lookup/fail_class_use_in_params.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_use_in_params.carbon:[[@LINE+1]]: 'A' is not usable until after it has been completely declared
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_use_in_params.carbon:[[@LINE+1]]: 'A' is not usable until after it has been completely declared
 class A(X:! A(i32)) {
 }
 
diff --git a/explorer/testdata/name_lookup/fail_fn_use_in_param.carbon b/explorer/testdata/name_lookup/fail_fn_use_in_param.carbon
index 0fed8e48aa8f..f30eb434dfa0 100644
--- a/explorer/testdata/name_lookup/fail_fn_use_in_param.carbon
+++ b/explorer/testdata/name_lookup/fail_fn_use_in_param.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_fn_use_in_param.carbon:[[@LINE+1]]: 'F' is not usable until after it has been completely declared
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_fn_use_in_param.carbon:[[@LINE+1]]: 'F' is not usable until after it has been completely declared
 fn F(x: F(0)) -> Type {
   return i32;
 }
diff --git a/explorer/testdata/name_lookup/fail_fn_use_in_return_type.carbon b/explorer/testdata/name_lookup/fail_fn_use_in_return_type.carbon
index dbe7aaa688a2..805ea30925d3 100644
--- a/explorer/testdata/name_lookup/fail_fn_use_in_return_type.carbon
+++ b/explorer/testdata/name_lookup/fail_fn_use_in_return_type.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_fn_use_in_return_type.carbon:[[@LINE+1]]: 'F' is not usable until after it has been completely declared
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_fn_use_in_return_type.carbon:[[@LINE+1]]: 'F' is not usable until after it has been completely declared
 fn F() -> F() {
   return Type;
 }
diff --git a/explorer/testdata/name_lookup/fail_global_duplicate.carbon b/explorer/testdata/name_lookup/fail_global_duplicate.carbon
index 968400f71483..7ccb8abe6e11 100644
--- a/explorer/testdata/name_lookup/fail_global_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_global_duplicate.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 var x: i32 = 0;
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:[[@LINE-1]]
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:[[@LINE-1]]
 var x: i32 = 0;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/name_lookup/fail_use_before_declare.carbon b/explorer/testdata/name_lookup/fail_use_before_declare.carbon
index d720ff0e9397..99cf706de9b9 100644
--- a/explorer/testdata/name_lookup/fail_use_before_declare.carbon
+++ b/explorer/testdata/name_lookup/fail_use_before_declare.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_use_before_declare.carbon:[[@LINE+1]]: could not resolve 'x'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_use_before_declare.carbon:[[@LINE+1]]: could not resolve 'x'
   x = 0;
   var x: i32 = 0;
   return 0;
diff --git a/explorer/testdata/name_lookup/fail_var_use_before_declaration.carbon b/explorer/testdata/name_lookup/fail_var_use_before_declaration.carbon
index 445ed8c9a7d9..f13f0a4f8660 100644
--- a/explorer/testdata/name_lookup/fail_var_use_before_declaration.carbon
+++ b/explorer/testdata/name_lookup/fail_var_use_before_declaration.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 // Test that a global variable may not depend on a later global.
 // Error expected.
 
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_var_use_before_declaration.carbon:[[@LINE+1]]: 'y' has not been declared yet
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_var_use_before_declaration.carbon:[[@LINE+1]]: 'y' has not been declared yet
 var x: i32 = y;
 
 var y: i32 = 0;
diff --git a/explorer/testdata/name_lookup/global_shadow.carbon b/explorer/testdata/name_lookup/global_shadow.carbon
index d53f81dffd6f..784438ac1a66 100644
--- a/explorer/testdata/name_lookup/global_shadow.carbon
+++ b/explorer/testdata/name_lookup/global_shadow.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/name_lookup/match_shadow.carbon b/explorer/testdata/name_lookup/match_shadow.carbon
index 88cbd94e4ac0..6b28e6ed24e4 100644
--- a/explorer/testdata/name_lookup/match_shadow.carbon
+++ b/explorer/testdata/name_lookup/match_shadow.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/add.carbon b/explorer/testdata/operators/add.carbon
index a54e57e95011..319ec44989ae 100644
--- a/explorer/testdata/operators/add.carbon
+++ b/explorer/testdata/operators/add.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 6
+// CHECK:STDOUT: result: 6
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/add_builtin.carbon b/explorer/testdata/operators/add_builtin.carbon
index d1fb03952aec..11ea9bd277db 100644
--- a/explorer/testdata/operators/add_builtin.carbon
+++ b/explorer/testdata/operators/add_builtin.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Interface: 5
-// CHECK:Op: 5
-// CHECK:result: 0
+// CHECK:STDOUT: Interface: 5
+// CHECK:STDOUT: Op: 5
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/bit_and.carbon b/explorer/testdata/operators/bit_and.carbon
index 92b7bac58eeb..799586ca38ef 100644
--- a/explorer/testdata/operators/bit_and.carbon
+++ b/explorer/testdata/operators/bit_and.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/bit_complement.carbon b/explorer/testdata/operators/bit_complement.carbon
index e9b2f5ec077b..61ac5493ed67 100644
--- a/explorer/testdata/operators/bit_complement.carbon
+++ b/explorer/testdata/operators/bit_complement.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: -6
+// CHECK:STDOUT: result: -6
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/bit_or.carbon b/explorer/testdata/operators/bit_or.carbon
index d6e4117e69de..d9d27a4f80a7 100644
--- a/explorer/testdata/operators/bit_or.carbon
+++ b/explorer/testdata/operators/bit_or.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 5
+// CHECK:STDOUT: result: 5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/bit_xor.carbon b/explorer/testdata/operators/bit_xor.carbon
index d3f7f61d4421..d58fd484e678 100644
--- a/explorer/testdata/operators/bit_xor.carbon
+++ b/explorer/testdata/operators/bit_xor.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 4
+// CHECK:STDOUT: result: 4
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/bitwise.carbon b/explorer/testdata/operators/bitwise.carbon
index 93de60c8a113..601c78e56079 100644
--- a/explorer/testdata/operators/bitwise.carbon
+++ b/explorer/testdata/operators/bitwise.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/div.carbon b/explorer/testdata/operators/div.carbon
index 0fc7076987e2..a14ded4a698f 100644
--- a/explorer/testdata/operators/div.carbon
+++ b/explorer/testdata/operators/div.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/div_builtin.carbon b/explorer/testdata/operators/div_builtin.carbon
index 4af51e5711dc..3b89e7c05183 100644
--- a/explorer/testdata/operators/div_builtin.carbon
+++ b/explorer/testdata/operators/div_builtin.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Interface: 2
-// CHECK:Op: 2
-// CHECK:result: 0
+// CHECK:STDOUT: Interface: 2
+// CHECK:STDOUT: Op: 2
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/fail_no_add.carbon b/explorer/testdata/operators/fail_no_add.carbon
index 2d225fd319d4..8bb70f07c3ee 100644
--- a/explorer/testdata/operators/fail_no_add.carbon
+++ b/explorer/testdata/operators/fail_no_add.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,8 +12,8 @@ class A {}
 
 fn Main() -> i32 {
   var a: A = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_add.carbon:[[@LINE+2]]: type error in `+`:
-  // CHECK:could not find implementation of interface AddWith(U = i32) for class A
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_add.carbon:[[@LINE+2]]: type error in `+`:
+  // CHECK:STDERR: could not find implementation of interface AddWith(U = i32) for class A
   a + 1;
   return 0;
 }
diff --git a/explorer/testdata/operators/fail_no_mul.carbon b/explorer/testdata/operators/fail_no_mul.carbon
index 53b26126de5d..7fe93db59a98 100644
--- a/explorer/testdata/operators/fail_no_mul.carbon
+++ b/explorer/testdata/operators/fail_no_mul.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,8 +12,8 @@ class A {}
 
 fn Main() -> i32 {
   var a: A = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_mul.carbon:[[@LINE+2]]: type error in `*`:
-  // CHECK:could not find implementation of interface MulWith(U = i32) for class A
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_mul.carbon:[[@LINE+2]]: type error in `*`:
+  // CHECK:STDERR: could not find implementation of interface MulWith(U = i32) for class A
   a * 1;
   return 0;
 }
diff --git a/explorer/testdata/operators/fail_no_negate.carbon b/explorer/testdata/operators/fail_no_negate.carbon
index b5f11b13cfa1..2327abe0b5db 100644
--- a/explorer/testdata/operators/fail_no_negate.carbon
+++ b/explorer/testdata/operators/fail_no_negate.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,8 +12,8 @@ class A {}
 
 fn Main() -> i32 {
   var a: A = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_negate.carbon:[[@LINE+2]]: type error in `-`:
-  // CHECK:could not find implementation of interface Negate for class A
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_negate.carbon:[[@LINE+2]]: type error in `-`:
+  // CHECK:STDERR: could not find implementation of interface Negate for class A
   -a;
   return 0;
 }
diff --git a/explorer/testdata/operators/fail_no_sub.carbon b/explorer/testdata/operators/fail_no_sub.carbon
index 791228f60740..19136b840740 100644
--- a/explorer/testdata/operators/fail_no_sub.carbon
+++ b/explorer/testdata/operators/fail_no_sub.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -12,8 +12,8 @@ class A {}
 
 fn Main() -> i32 {
   var a: A = {};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_sub.carbon:[[@LINE+2]]: type error in `-`:
-  // CHECK:could not find implementation of interface SubWith(U = i32) for class A
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/operators/fail_no_sub.carbon:[[@LINE+2]]: type error in `-`:
+  // CHECK:STDERR: could not find implementation of interface SubWith(U = i32) for class A
   a - 1;
   return 0;
 }
diff --git a/explorer/testdata/operators/left_shift.carbon b/explorer/testdata/operators/left_shift.carbon
index 536ef8f7d45a..a2d603e93590 100644
--- a/explorer/testdata/operators/left_shift.carbon
+++ b/explorer/testdata/operators/left_shift.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 10
+// CHECK:STDOUT: result: 10
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/mod.carbon b/explorer/testdata/operators/mod.carbon
index 5d6cd6a5fb60..d0d2e0276691 100644
--- a/explorer/testdata/operators/mod.carbon
+++ b/explorer/testdata/operators/mod.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/mod_builtin.carbon b/explorer/testdata/operators/mod_builtin.carbon
index 176bfa58b49c..0f79054af306 100644
--- a/explorer/testdata/operators/mod_builtin.carbon
+++ b/explorer/testdata/operators/mod_builtin.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Interface: 2
-// CHECK:Op: 2
-// CHECK:result: 0
+// CHECK:STDOUT: Interface: 2
+// CHECK:STDOUT: Op: 2
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/mod_edges.carbon b/explorer/testdata/operators/mod_edges.carbon
index 2f81cf592f76..248bdd13f461 100644
--- a/explorer/testdata/operators/mod_edges.carbon
+++ b/explorer/testdata/operators/mod_edges.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/mul.carbon b/explorer/testdata/operators/mul.carbon
index a0ebb723cee3..9ac8ed5420eb 100644
--- a/explorer/testdata/operators/mul.carbon
+++ b/explorer/testdata/operators/mul.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 10
+// CHECK:STDOUT: result: 10
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/mul_builtin.carbon b/explorer/testdata/operators/mul_builtin.carbon
index 74de6e22bee6..03a699ff108f 100644
--- a/explorer/testdata/operators/mul_builtin.carbon
+++ b/explorer/testdata/operators/mul_builtin.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Interface: 6
-// CHECK:Op: 6
-// CHECK:result: 0
+// CHECK:STDOUT: Interface: 6
+// CHECK:STDOUT: Op: 6
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/negate.carbon b/explorer/testdata/operators/negate.carbon
index b48ffa6970fc..6b9a6e0b7deb 100644
--- a/explorer/testdata/operators/negate.carbon
+++ b/explorer/testdata/operators/negate.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: -5
+// CHECK:STDOUT: result: -5
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/negate_builtin.carbon b/explorer/testdata/operators/negate_builtin.carbon
index de13666db2fc..15acbfe5656a 100644
--- a/explorer/testdata/operators/negate_builtin.carbon
+++ b/explorer/testdata/operators/negate_builtin.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Interface: -3
-// CHECK:Op: -3
-// CHECK:result: 0
+// CHECK:STDOUT: Interface: -3
+// CHECK:STDOUT: Op: -3
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/right_shift.carbon b/explorer/testdata/operators/right_shift.carbon
index 1a3aae50d4e7..ede1284055fb 100644
--- a/explorer/testdata/operators/right_shift.carbon
+++ b/explorer/testdata/operators/right_shift.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/shift.carbon b/explorer/testdata/operators/shift.carbon
index fbe571c3db32..fd4885c3c494 100644
--- a/explorer/testdata/operators/shift.carbon
+++ b/explorer/testdata/operators/shift.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/shortcircuit_and.carbon b/explorer/testdata/operators/shortcircuit_and.carbon
index 91f64db75e88..75ea29407416 100644
--- a/explorer/testdata/operators/shortcircuit_and.carbon
+++ b/explorer/testdata/operators/shortcircuit_and.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/shortcircuit_and_no_shortcircuit.carbon b/explorer/testdata/operators/shortcircuit_and_no_shortcircuit.carbon
index ad568e46d6f3..0ceea4becaec 100644
--- a/explorer/testdata/operators/shortcircuit_and_no_shortcircuit.carbon
+++ b/explorer/testdata/operators/shortcircuit_and_no_shortcircuit.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/shortcircuit_or.carbon b/explorer/testdata/operators/shortcircuit_or.carbon
index 4572f98195a7..0b950ba0fc26 100644
--- a/explorer/testdata/operators/shortcircuit_or.carbon
+++ b/explorer/testdata/operators/shortcircuit_or.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/shortcircuit_or_no_shortcircuit.carbon b/explorer/testdata/operators/shortcircuit_or_no_shortcircuit.carbon
index 3048b03dc4fe..ddd55d099daf 100644
--- a/explorer/testdata/operators/shortcircuit_or_no_shortcircuit.carbon
+++ b/explorer/testdata/operators/shortcircuit_or_no_shortcircuit.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 2
+// CHECK:STDOUT: result: 2
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/sub.carbon b/explorer/testdata/operators/sub.carbon
index 3f3564950018..f929dabc4f34 100644
--- a/explorer/testdata/operators/sub.carbon
+++ b/explorer/testdata/operators/sub.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 4
+// CHECK:STDOUT: result: 4
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/sub_builtin.carbon b/explorer/testdata/operators/sub_builtin.carbon
index e7ec8990cb0f..65e5c489be65 100644
--- a/explorer/testdata/operators/sub_builtin.carbon
+++ b/explorer/testdata/operators/sub_builtin.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Interface: 5
-// CHECK:Op: 5
-// CHECK:result: 0
+// CHECK:STDOUT: Interface: 5
+// CHECK:STDOUT: Op: 5
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/optional/init_optional_with_non_value.carbon b/explorer/testdata/optional/init_optional_with_non_value.carbon
index ff8f330b1219..88463a8ed255 100644
--- a/explorer/testdata/optional/init_optional_with_non_value.carbon
+++ b/explorer/testdata/optional/init_optional_with_non_value.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/optional/init_optional_with_value.carbon b/explorer/testdata/optional/init_optional_with_value.carbon
index 35725424ae5d..661588889589 100644
--- a/explorer/testdata/optional/init_optional_with_value.carbon
+++ b/explorer/testdata/optional/init_optional_with_value.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Hallo Welt
-// CHECK:result: 0
+// CHECK:STDOUT: Hallo Welt
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/package/fail_missing.carbon b/explorer/testdata/package/fail_missing.carbon
index 5f0e0ff6bd96..15c566c56ae3 100644
--- a/explorer/testdata/package/fail_missing.carbon
+++ b/explorer/testdata/package/fail_missing.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
-// CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/package/fail_missing.carbon:[[@LINE+1]]: syntax error, unexpected FN, expecting PACKAGE
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/package/fail_missing.carbon:[[@LINE+1]]: syntax error, unexpected FN, expecting PACKAGE
 fn Main() -> i32 {
   return 0;
 }
diff --git a/explorer/testdata/package/with_library.carbon b/explorer/testdata/package/with_library.carbon
index 087ee7ea39ff..2d1e282ae86d 100644
--- a/explorer/testdata/package/with_library.carbon
+++ b/explorer/testdata/package/with_library.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest library "Foo" api;
 
diff --git a/explorer/testdata/pointer/aliasing.carbon b/explorer/testdata/pointer/aliasing.carbon
index 62139d8697d7..37c8ee3bc98a 100644
--- a/explorer/testdata/pointer/aliasing.carbon
+++ b/explorer/testdata/pointer/aliasing.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/pointer/basic.carbon b/explorer/testdata/pointer/basic.carbon
index 234d510ced62..95d1d3dd488c 100644
--- a/explorer/testdata/pointer/basic.carbon
+++ b/explorer/testdata/pointer/basic.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/pointer/fail_rvalue_addressof.carbon b/explorer/testdata/pointer/fail_rvalue_addressof.carbon
index d6adf7eb8d37..6146e4bd0e52 100644
--- a/explorer/testdata/pointer/fail_rvalue_addressof.carbon
+++ b/explorer/testdata/pointer/fail_rvalue_addressof.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 5;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_rvalue_addressof.carbon:[[@LINE+1]]: Argument to & should be an lvalue.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_rvalue_addressof.carbon:[[@LINE+1]]: Argument to & should be an lvalue.
   var p: i32* = &5;
   return 0;
 }
diff --git a/explorer/testdata/pointer/fail_use_after_free.carbon b/explorer/testdata/pointer/fail_use_after_free.carbon
index 1ab45be58ed5..228500c62949 100644
--- a/explorer/testdata/pointer/fail_use_after_free.carbon
+++ b/explorer/testdata/pointer/fail_use_after_free.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,6 +11,6 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var p: i32* = heap.New(5);
   heap.Delete(p);
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/pointer/fail_use_after_free.carbon:[[@LINE+1]]: undefined behavior: access to dead value 5
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/pointer/fail_use_after_free.carbon:[[@LINE+1]]: undefined behavior: access to dead value 5
   return *p;
 }
diff --git a/explorer/testdata/pointer/heap_alloc_lifetime.carbon b/explorer/testdata/pointer/heap_alloc_lifetime.carbon
index c724d96f4e77..d87f89270a68 100644
--- a/explorer/testdata/pointer/heap_alloc_lifetime.carbon
+++ b/explorer/testdata/pointer/heap_alloc_lifetime.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/pointer/intrinsic_new.carbon b/explorer/testdata/pointer/intrinsic_new.carbon
index 1887a7d6fe56..b795d3f11589 100644
--- a/explorer/testdata/pointer/intrinsic_new.carbon
+++ b/explorer/testdata/pointer/intrinsic_new.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/pointer/new_and_delete.carbon b/explorer/testdata/pointer/new_and_delete.carbon
index 14af052d813c..77b8fdfad806 100644
--- a/explorer/testdata/pointer/new_and_delete.carbon
+++ b/explorer/testdata/pointer/new_and_delete.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/pointer/struct_pointer.carbon b/explorer/testdata/pointer/struct_pointer.carbon
index 38dd6a04b0ff..2380879fdc05 100644
--- a/explorer/testdata/pointer/struct_pointer.carbon
+++ b/explorer/testdata/pointer/struct_pointer.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/print/associated_constant.carbon b/explorer/testdata/print/associated_constant.carbon
index e125227a6314..49f8e8b9ef22 100644
--- a/explorer/testdata/print/associated_constant.carbon
+++ b/explorer/testdata/print/associated_constant.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:i32
-// CHECK:String
-// CHECK:result: 0
+// CHECK:STDOUT: i32
+// CHECK:STDOUT: String
+// CHECK:STDOUT: result: 0
 
 package Testcase api;
 
diff --git a/explorer/testdata/print/fail_no_args.carbon b/explorer/testdata/print/fail_no_args.carbon
index a6619e10fb53..bbdbcaca935f 100644
--- a/explorer/testdata/print/fail_no_args.carbon
+++ b/explorer/testdata/print/fail_no_args.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_no_args.carbon:[[@LINE+1]]: Print takes 1 or 2 arguments, received 0
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_no_args.carbon:[[@LINE+1]]: Print takes 1 or 2 arguments, received 0
   Print();
   return 0;
 }
diff --git a/explorer/testdata/print/fail_not_str.carbon b/explorer/testdata/print/fail_not_str.carbon
index 933667109f6e..e102be9ffb3d 100644
--- a/explorer/testdata/print/fail_not_str.carbon
+++ b/explorer/testdata/print/fail_not_str.carbon
@@ -2,16 +2,16 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_not_str.carbon:[[@LINE+3]]: type error in Print argument 0
-  // CHECK:expected: String
-  // CHECK:actual: {.x: i32}
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_not_str.carbon:[[@LINE+3]]: type error in Print argument 0
+  // CHECK:STDERR: expected: String
+  // CHECK:STDERR: actual: {.x: i32}
   Print({.x = 1});
   return 0;
 }
diff --git a/explorer/testdata/print/fail_too_many_args.carbon b/explorer/testdata/print/fail_too_many_args.carbon
index 3b44282c33ce..b3574eb54400 100644
--- a/explorer/testdata/print/fail_too_many_args.carbon
+++ b/explorer/testdata/print/fail_too_many_args.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_too_many_args.carbon:[[@LINE+1]]: Print takes 1 or 2 arguments, received 5
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_too_many_args.carbon:[[@LINE+1]]: Print takes 1 or 2 arguments, received 5
   Print("too", "many", "args", "to", "print");
   return 0;
 }
diff --git a/explorer/testdata/print/fail_type.carbon b/explorer/testdata/print/fail_type.carbon
index d81d27b6b412..3352fa6bc8ec 100644
--- a/explorer/testdata/print/fail_type.carbon
+++ b/explorer/testdata/print/fail_type.carbon
@@ -2,16 +2,16 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_type.carbon:[[@LINE+3]]: type error in Print argument 1
-  // CHECK:expected: i32
-  // CHECK:actual: {.x: i32}
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/print/fail_type.carbon:[[@LINE+3]]: type error in Print argument 1
+  // CHECK:STDERR: expected: i32
+  // CHECK:STDERR: actual: {.x: i32}
   Print("{0}", {.x = 1});
   return 0;
 }
diff --git a/explorer/testdata/print/format_only.carbon b/explorer/testdata/print/format_only.carbon
index f8f4b52d6209..c95bdc23a09a 100644
--- a/explorer/testdata/print/format_only.carbon
+++ b/explorer/testdata/print/format_only.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Hello world!
-// CHECK:result: 0
+// CHECK:STDOUT: Hello world!
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/print/i32.carbon b/explorer/testdata/print/i32.carbon
index 12cbcf9306a6..bee6ea62b2b4 100644
--- a/explorer/testdata/print/i32.carbon
+++ b/explorer/testdata/print/i32.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Printing 1
-// CHECK:result: 0
+// CHECK:STDOUT: Printing 1
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/random/random.carbon b/explorer/testdata/random/random.carbon
index 00b612ec6fcc..70fa986b9da9 100644
--- a/explorer/testdata/random/random.carbon
+++ b/explorer/testdata/random/random.carbon
@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:Nice!
-// CHECK:result: 0
+// CHECK:STDOUT: Nice!
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 fn test(para: i32)->bool{
diff --git a/explorer/testdata/return/convert_return_value.carbon b/explorer/testdata/return/convert_return_value.carbon
index 639e1661a7d4..f72f7d535041 100644
--- a/explorer/testdata/return/convert_return_value.carbon
+++ b/explorer/testdata/return/convert_return_value.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 42
+// CHECK:STDOUT: result: 42
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/return/explicit_empty.carbon b/explorer/testdata/return/explicit_empty.carbon
index 87ad498bc9b5..11ad24ee12e0 100644
--- a/explorer/testdata/return/explicit_empty.carbon
+++ b/explorer/testdata/return/explicit_empty.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/return/fail_explicit_with_no_return.carbon b/explorer/testdata/return/fail_explicit_with_no_return.carbon
index c768289f03d7..e69ea126a507 100644
--- a/explorer/testdata/return/fail_explicit_with_no_return.carbon
+++ b/explorer/testdata/return/fail_explicit_with_no_return.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn F() -> () {
-// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/return/fail_explicit_with_plain_return.carbon b/explorer/testdata/return/fail_explicit_with_plain_return.carbon
index 569130c437ce..28efb8b816fd 100644
--- a/explorer/testdata/return/fail_explicit_with_plain_return.carbon
+++ b/explorer/testdata/return/fail_explicit_with_plain_return.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn F() -> () {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_plain_return.carbon:[[@LINE+1]]: return; should provide a return value, to match the function's signature.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_plain_return.carbon:[[@LINE+1]]: return; should provide a return value, to match the function's signature.
   return;
 }
 
diff --git a/explorer/testdata/return/fail_implicit_with_explicit_return.carbon b/explorer/testdata/return/fail_implicit_with_explicit_return.carbon
index 43078f13232b..81e214865bc4 100644
--- a/explorer/testdata/return/fail_implicit_with_explicit_return.carbon
+++ b/explorer/testdata/return/fail_implicit_with_explicit_return.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn F() {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_implicit_with_explicit_return.carbon:[[@LINE+1]]: return (); should not provide a return value, to match the function's signature.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_implicit_with_explicit_return.carbon:[[@LINE+1]]: return (); should not provide a return value, to match the function's signature.
   return ();
 }
 
diff --git a/explorer/testdata/return/implicit_with_no_return.carbon b/explorer/testdata/return/implicit_with_no_return.carbon
index f9469bc270bb..70a6130b5ed7 100644
--- a/explorer/testdata/return/implicit_with_no_return.carbon
+++ b/explorer/testdata/return/implicit_with_no_return.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/return/implicit_with_plain_return.carbon b/explorer/testdata/return/implicit_with_plain_return.carbon
index 397c0f39814d..13f7d5148ef9 100644
--- a/explorer/testdata/return/implicit_with_plain_return.carbon
+++ b/explorer/testdata/return/implicit_with_plain_return.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/returned_var/fail_duplicate_return_var.carbon b/explorer/testdata/returned_var/fail_duplicate_return_var.carbon
index a8fe757c0051..6856535e144b 100644
--- a/explorer/testdata/returned_var/fail_duplicate_return_var.carbon
+++ b/explorer/testdata/returned_var/fail_duplicate_return_var.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn AddInt(a: i32, b: i32) -> i32 {
   returned var ret: i32 = a + b;
   if (a == b) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_duplicate_return_var.carbon:[[@LINE+1]]: Duplicate definition of returned var also found at {{.*}}/explorer/testdata/returned_var/fail_duplicate_return_var.carbon:[[@LINE-2]]
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_duplicate_return_var.carbon:[[@LINE+1]]: Duplicate definition of returned var also found at {{.*}}/explorer/testdata/returned_var/fail_duplicate_return_var.carbon:[[@LINE-2]]
     returned var ret2: i32 = a + b;
   }
   return var;
diff --git a/explorer/testdata/returned_var/fail_missing_declaration.carbon b/explorer/testdata/returned_var/fail_missing_declaration.carbon
index 1b64d3b81c73..8e45d1a64671 100644
--- a/explorer/testdata/returned_var/fail_missing_declaration.carbon
+++ b/explorer/testdata/returned_var/fail_missing_declaration.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn AddInt(a: i32, b: i32) -> i32 {
   var ret: i32 = a + b;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_missing_declaration.carbon:[[@LINE+1]]: `return var` is not allowed without a returned var defined in scope.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_missing_declaration.carbon:[[@LINE+1]]: `return var` is not allowed without a returned var defined in scope.
   return var;
 }
 
diff --git a/explorer/testdata/returned_var/fail_missing_return.carbon b/explorer/testdata/returned_var/fail_missing_return.carbon
index f0555e7a3a8d..5684ebbc9adf 100644
--- a/explorer/testdata/returned_var/fail_missing_return.carbon
+++ b/explorer/testdata/returned_var/fail_missing_return.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn AddInt(a: i32, b: i32) -> i32 {
   returned var ret: i32 = a + b;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_missing_return.carbon:[[@LINE+1]]: `return <expression>` is not allowed with a returned var defined in scope: {{.*}}/explorer/testdata/returned_var/fail_missing_return.carbon:[[@LINE-1]]
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_missing_return.carbon:[[@LINE+1]]: `return <expression>` is not allowed with a returned var defined in scope: {{.*}}/explorer/testdata/returned_var/fail_missing_return.carbon:[[@LINE-1]]
   return ret;
 }
 
diff --git a/explorer/testdata/returned_var/fail_returned_var_mismatch_signature.carbon b/explorer/testdata/returned_var/fail_returned_var_mismatch_signature.carbon
index ebe72f801ac0..082083450c0f 100644
--- a/explorer/testdata/returned_var/fail_returned_var_mismatch_signature.carbon
+++ b/explorer/testdata/returned_var/fail_returned_var_mismatch_signature.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Foo() {
   returned var x: () = ();
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_returned_var_mismatch_signature.carbon:[[@LINE+1]]: return var; should not provide a return value, to match the function's signature.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_returned_var_mismatch_signature.carbon:[[@LINE+1]]: return var; should not provide a return value, to match the function's signature.
   return var;
 }
 
diff --git a/explorer/testdata/returned_var/fail_returned_var_multi_auto.carbon b/explorer/testdata/returned_var/fail_returned_var_multi_auto.carbon
index 7cc4ec015d46..25e2c7b8d02b 100644
--- a/explorer/testdata/returned_var/fail_returned_var_multi_auto.carbon
+++ b/explorer/testdata/returned_var/fail_returned_var_multi_auto.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,7 +15,7 @@ fn Foo(a: i32, b: i32) -> auto {
     return var;
   }
   ret = a - b;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_returned_var_multi_auto.carbon:[[@LINE+1]]: Only one return is allowed in a function with an `auto` return type.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_returned_var_multi_auto.carbon:[[@LINE+1]]: Only one return is allowed in a function with an `auto` return type.
   return var;
 }
 
diff --git a/explorer/testdata/returned_var/fail_returned_var_type_mismatch.carbon b/explorer/testdata/returned_var/fail_returned_var_type_mismatch.carbon
index 05f45632666d..1c0b78e5e2d4 100644
--- a/explorer/testdata/returned_var/fail_returned_var_type_mismatch.carbon
+++ b/explorer/testdata/returned_var/fail_returned_var_type_mismatch.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn AddInt(a: i32, b: i32) -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_returned_var_type_mismatch.carbon:[[@LINE+1]]: type of returned var `bool` does not match return type `i32`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/returned_var/fail_returned_var_type_mismatch.carbon:[[@LINE+1]]: type of returned var `bool` does not match return type `i32`
   returned var ret: bool = true;
   return var;
 }
diff --git a/explorer/testdata/returned_var/multiple_returned_var_control_flow.carbon b/explorer/testdata/returned_var/multiple_returned_var_control_flow.carbon
index d0ee4c0becce..ad2e52762ef0 100644
--- a/explorer/testdata/returned_var/multiple_returned_var_control_flow.carbon
+++ b/explorer/testdata/returned_var/multiple_returned_var_control_flow.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/returned_var/normal_return_control_flow.carbon b/explorer/testdata/returned_var/normal_return_control_flow.carbon
index dcb85df3be6c..2e1139f66299 100644
--- a/explorer/testdata/returned_var/normal_return_control_flow.carbon
+++ b/explorer/testdata/returned_var/normal_return_control_flow.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/returned_var/returned_var_auto.carbon b/explorer/testdata/returned_var/returned_var_auto.carbon
index 26e30ca6ac99..cf32fef59798 100644
--- a/explorer/testdata/returned_var/returned_var_auto.carbon
+++ b/explorer/testdata/returned_var/returned_var_auto.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/returned_var/returned_var_basic.carbon b/explorer/testdata/returned_var/returned_var_basic.carbon
index f5277918f4b5..8c5966a0e246 100644
--- a/explorer/testdata/returned_var/returned_var_basic.carbon
+++ b/explorer/testdata/returned_var/returned_var_basic.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/returned_var/returned_var_name_lookup.carbon b/explorer/testdata/returned_var/returned_var_name_lookup.carbon
index b512fd71c5a3..8f2f95a3a8a6 100644
--- a/explorer/testdata/returned_var/returned_var_name_lookup.carbon
+++ b/explorer/testdata/returned_var/returned_var_name_lookup.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/returned_var/returned_var_omitted_expression.carbon b/explorer/testdata/returned_var/returned_var_omitted_expression.carbon
index 2087e12e2378..4d926d560c76 100644
--- a/explorer/testdata/returned_var/returned_var_omitted_expression.carbon
+++ b/explorer/testdata/returned_var/returned_var_omitted_expression.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/basic.carbon b/explorer/testdata/string/basic.carbon
index 2c598952df93..404c68f8e0e8 100644
--- a/explorer/testdata/string/basic.carbon
+++ b/explorer/testdata/string/basic.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/block.carbon b/explorer/testdata/string/block.carbon
index 9755f11afffe..fa8697a8913d 100644
--- a/explorer/testdata/string/block.carbon
+++ b/explorer/testdata/string/block.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/block_escaped_triple_quotes.carbon b/explorer/testdata/string/block_escaped_triple_quotes.carbon
index 112cf460caae..19206973079d 100644
--- a/explorer/testdata/string/block_escaped_triple_quotes.carbon
+++ b/explorer/testdata/string/block_escaped_triple_quotes.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/block_file_type_indicator.carbon b/explorer/testdata/string/block_file_type_indicator.carbon
index f30cfe03241f..9a4d7e5c3db9 100644
--- a/explorer/testdata/string/block_file_type_indicator.carbon
+++ b/explorer/testdata/string/block_file_type_indicator.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon b/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon
index 2e3d4aea913d..f13c687d07e1 100644
--- a/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon
+++ b/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing """
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing """
   var s: String = """
     error: closing """ is not on its own line.
   """;
diff --git a/explorer/testdata/string/fail_hex_lower.carbon b/explorer/testdata/string/fail_hex_lower.carbon
index 1bf1501eecd2..cdf47f468a9f 100644
--- a/explorer/testdata/string/fail_hex_lower.carbon
+++ b/explorer/testdata/string/fail_hex_lower.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_hex_lower.carbon:[[@LINE+1]]: Invalid escaping in string: "str\xaa"
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_hex_lower.carbon:[[@LINE+1]]: Invalid escaping in string: "str\xaa"
   Print("str\xaa");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_hex_truncated.carbon b/explorer/testdata/string/fail_hex_truncated.carbon
index 196d09defec1..9d8a3a3d198f 100644
--- a/explorer/testdata/string/fail_hex_truncated.carbon
+++ b/explorer/testdata/string/fail_hex_truncated.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_hex_truncated.carbon:[[@LINE+1]]: Invalid escaping in string: "str\x"
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_hex_truncated.carbon:[[@LINE+1]]: Invalid escaping in string: "str\x"
   Print("str\x");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_invalid_escape.carbon b/explorer/testdata/string/fail_invalid_escape.carbon
index 6326c1665e7c..bbb5e5d448cb 100644
--- a/explorer/testdata/string/fail_invalid_escape.carbon
+++ b/explorer/testdata/string/fail_invalid_escape.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_invalid_escape.carbon:[[@LINE+1]]: Invalid escaping in string: "str\e"
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_invalid_escape.carbon:[[@LINE+1]]: Invalid escaping in string: "str\e"
   Print("str\e");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_newline.carbon b/explorer/testdata/string/fail_newline.carbon
index f5446d579944..ec01d61cbd9f 100644
--- a/explorer/testdata/string/fail_newline.carbon
+++ b/explorer/testdata/string/fail_newline.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_newline.carbon:[[@LINE+2]]: missing closing quote in single-line string: "new
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_newline.carbon:[[@LINE+2]]: missing closing quote in single-line string: "new
   // CHECK-EMPTY:
   Print("new
 line");
diff --git a/explorer/testdata/string/fail_octal.carbon b/explorer/testdata/string/fail_octal.carbon
index 5500266a4377..70b0357bd02c 100644
--- a/explorer/testdata/string/fail_octal.carbon
+++ b/explorer/testdata/string/fail_octal.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_octal.carbon:[[@LINE+1]]: Invalid escaping in string: "str\01"
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_octal.carbon:[[@LINE+1]]: Invalid escaping in string: "str\01"
   Print("str\01");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon b/explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon
index f9c0fa7e36cd..fa9c09e52bf0 100644
--- a/explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon
+++ b/explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon:[[@LINE+1]]: Unexpected end of file
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon:[[@LINE+1]]: Unexpected end of file
   var s: String = ##"""
     error: there are more #s on the left than the right.
   """#;
diff --git a/explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon b/explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon
index 3300c40b5827..9ea2092359eb 100644
--- a/explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon
+++ b/explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
   var s: String = #"""
     error: there are more #s on the right than the left.
   """##;
diff --git a/explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon b/explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon
index da754c84affc..7aff5322a830 100644
--- a/explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon
+++ b/explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing """
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing """
   var s: String = #"""
     error: closing """# is not on its own line.
   """#;
diff --git a/explorer/testdata/string/fail_raw_block_single_line.carbon b/explorer/testdata/string/fail_raw_block_single_line.carbon
index 4a0a427cd84f..8b5728f0391e 100644
--- a/explorer/testdata/string/fail_raw_block_single_line.carbon
+++ b/explorer/testdata/string/fail_raw_block_single_line.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn CompareStr(s: String) -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_single_line.carbon:[[@LINE+1]]: Invalid block string: Too few lines
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_block_single_line.carbon:[[@LINE+1]]: Invalid block string: Too few lines
   if (s == #"""raw string literal starting with """#) {
     return 0;
   }
diff --git a/explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon b/explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon
index 3eb44ae759fd..129877c9cf23 100644
--- a/explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon
+++ b/explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn CompareStr(s: String) -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon:[[@LINE+2]]: missing closing quote in single-line string: ##"str"#) {
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon:[[@LINE+2]]: missing closing quote in single-line string: ##"str"#) {
   // CHECK-EMPTY:
   if (s == ##"str"#) {
     return 0;
diff --git a/explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon b/explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon
index deca2950e19a..a2e58deddd3b 100644
--- a/explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon
+++ b/explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn CompareStr(s: String) -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
   if (s == "str"#) {
     return 0;
   }
diff --git a/explorer/testdata/string/fail_tab.carbon b/explorer/testdata/string/fail_tab.carbon
index bf71e9c5f629..b25404479231 100644
--- a/explorer/testdata/string/fail_tab.carbon
+++ b/explorer/testdata/string/fail_tab.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_tab.carbon:[[@LINE+1]]: Invalid escaping in string: "new	line"
+  // CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/string/fail_tab.carbon:[[@LINE+1]]: Invalid escaping in string: "new	line"
   Print("new	line");
   return 0;
 }
diff --git a/explorer/testdata/string/hex.carbon b/explorer/testdata/string/hex.carbon
index d94474b364b3..f3d424d81f06 100644
--- a/explorer/testdata/string/hex.carbon
+++ b/explorer/testdata/string/hex.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/newline.carbon b/explorer/testdata/string/newline.carbon
index 27f0f6fec070..d5442b5f4d71 100644
--- a/explorer/testdata/string/newline.carbon
+++ b/explorer/testdata/string/newline.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/raw_basic.carbon b/explorer/testdata/string/raw_basic.carbon
index 0c33d1bcce2a..73b9f5fc0c51 100644
--- a/explorer/testdata/string/raw_basic.carbon
+++ b/explorer/testdata/string/raw_basic.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/raw_basic_multi_hashtag.carbon b/explorer/testdata/string/raw_basic_multi_hashtag.carbon
index 7a508c74c633..7f787c8ecfa0 100644
--- a/explorer/testdata/string/raw_basic_multi_hashtag.carbon
+++ b/explorer/testdata/string/raw_basic_multi_hashtag.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/raw_block.carbon b/explorer/testdata/string/raw_block.carbon
index 4b12eb2460f0..b112f8a62f1a 100644
--- a/explorer/testdata/string/raw_block.carbon
+++ b/explorer/testdata/string/raw_block.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/raw_block_escaped_back_slash.carbon b/explorer/testdata/string/raw_block_escaped_back_slash.carbon
index 76eaf4ff268e..6fcf60870729 100644
--- a/explorer/testdata/string/raw_block_escaped_back_slash.carbon
+++ b/explorer/testdata/string/raw_block_escaped_back_slash.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/raw_block_escaped_triple_quotes.carbon b/explorer/testdata/string/raw_block_escaped_triple_quotes.carbon
index 9da98e9efccf..6c3dc888e875 100644
--- a/explorer/testdata/string/raw_block_escaped_triple_quotes.carbon
+++ b/explorer/testdata/string/raw_block_escaped_triple_quotes.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/raw_nesting.carbon b/explorer/testdata/string/raw_nesting.carbon
index eefec8b29007..cd62aa55de08 100644
--- a/explorer/testdata/string/raw_nesting.carbon
+++ b/explorer/testdata/string/raw_nesting.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/string/unicode.carbon b/explorer/testdata/string/unicode.carbon
index 7c8c0113aac5..a5834fe64e93 100644
--- a/explorer/testdata/string/unicode.carbon
+++ b/explorer/testdata/string/unicode.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/assign.carbon b/explorer/testdata/struct/assign.carbon
index 00894ce9bd5b..8eeef86a04a3 100644
--- a/explorer/testdata/struct/assign.carbon
+++ b/explorer/testdata/struct/assign.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/assign_member.carbon b/explorer/testdata/struct/assign_member.carbon
index 77c6dd6d4fc1..51ade3941a88 100644
--- a/explorer/testdata/struct/assign_member.carbon
+++ b/explorer/testdata/struct/assign_member.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/empty.carbon b/explorer/testdata/struct/empty.carbon
index 9254e06850d1..75b9aca5aa77 100644
--- a/explorer/testdata/struct/empty.carbon
+++ b/explorer/testdata/struct/empty.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/ending_comma.carbon b/explorer/testdata/struct/ending_comma.carbon
index 0a5e1544f5ca..69076a1898a7 100644
--- a/explorer/testdata/struct/ending_comma.carbon
+++ b/explorer/testdata/struct/ending_comma.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/equality.carbon b/explorer/testdata/struct/equality.carbon
index 5cbb5111b03d..ddecaaf824d2 100644
--- a/explorer/testdata/struct/equality.carbon
+++ b/explorer/testdata/struct/equality.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:t1 == t2: 1
-// CHECK:t1 != t2: 0
-// CHECK:result: 0
+// CHECK:STDOUT: t1 == t2: 1
+// CHECK:STDOUT: t1 != t2: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/equality_false.carbon b/explorer/testdata/struct/equality_false.carbon
index d6bc27076802..ae52b7d42fe9 100644
--- a/explorer/testdata/struct/equality_false.carbon
+++ b/explorer/testdata/struct/equality_false.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/fail_equality_type.carbon b/explorer/testdata/struct/fail_equality_type.carbon
index aeec008cff7c..6b0a6be7c8ad 100644
--- a/explorer/testdata/struct/fail_equality_type.carbon
+++ b/explorer/testdata/struct/fail_equality_type.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -21,7 +21,7 @@ external impl {.x: i32, .y: i32} as EqWith(Self) {
 fn Main() -> i32 {
   var t1: {.x: i32, .y: i32} = {.x = 5, .y = 2};
   var t2: {.x: i32,} = {.x = 5,};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_equality_type.carbon:[[@LINE+1]]: {.x: i32, .y: i32} is not equality comparable with {.x: i32} (could not find implementation of interface EqWith(U = {.x: i32}) for {.x: i32, .y: i32})
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_equality_type.carbon:[[@LINE+1]]: {.x: i32, .y: i32} is not equality comparable with {.x: i32} (could not find implementation of interface EqWith(U = {.x: i32}) for {.x: i32, .y: i32})
   if (t1 == t2) {
     return 1;
   } else {
diff --git a/explorer/testdata/struct/fail_field_access_mismatch.carbon b/explorer/testdata/struct/fail_field_access_mismatch.carbon
index 1c6fecf57d8a..472d14af7823 100644
--- a/explorer/testdata/struct/fail_field_access_mismatch.carbon
+++ b/explorer/testdata/struct/fail_field_access_mismatch.carbon
@@ -2,13 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_field_access_mismatch.carbon:[[@LINE+1]]: struct {.x: i32, .y: i32} does not have a field named z
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_field_access_mismatch.carbon:[[@LINE+1]]: struct {.x: i32, .y: i32} does not have a field named z
   return {.x = 1, .y = 2}.z - 1;
 }
diff --git a/explorer/testdata/struct/name_order.carbon b/explorer/testdata/struct/name_order.carbon
index 420903be6422..c85f69a0c34c 100644
--- a/explorer/testdata/struct/name_order.carbon
+++ b/explorer/testdata/struct/name_order.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/temp.carbon b/explorer/testdata/struct/temp.carbon
index 773d8162a237..ad7ad5e22687 100644
--- a/explorer/testdata/struct/temp.carbon
+++ b/explorer/testdata/struct/temp.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/struct/var.carbon b/explorer/testdata/struct/var.carbon
index 35fd41a84edb..68a893b8e6a2 100644
--- a/explorer/testdata/struct/var.carbon
+++ b/explorer/testdata/struct/var.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/ending_comma.carbon b/explorer/testdata/tuple/ending_comma.carbon
index d560804befc2..3cecdfbd23fd 100644
--- a/explorer/testdata/tuple/ending_comma.carbon
+++ b/explorer/testdata/tuple/ending_comma.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/equality.carbon b/explorer/testdata/tuple/equality.carbon
index a9b06b23708d..f445ed0dd323 100644
--- a/explorer/testdata/tuple/equality.carbon
+++ b/explorer/testdata/tuple/equality.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/equality_false.carbon b/explorer/testdata/tuple/equality_false.carbon
index ac7f577d1b19..f4d506afd579 100644
--- a/explorer/testdata/tuple/equality_false.carbon
+++ b/explorer/testdata/tuple/equality_false.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/fail_equality_type.carbon b/explorer/testdata/tuple/fail_equality_type.carbon
index f92c75d5bc73..a4567f99db84 100644
--- a/explorer/testdata/tuple/fail_equality_type.carbon
+++ b/explorer/testdata/tuple/fail_equality_type.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var t1: (i32, i32) = (5, 2);
   var t2: (i32,) = (5,);
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_equality_type.carbon:[[@LINE+1]]: (i32, i32) is not equality comparable with (i32) (could not find implementation of interface EqWith(U = (i32)) for (i32, i32))
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_equality_type.carbon:[[@LINE+1]]: (i32, i32) is not equality comparable with (i32) (could not find implementation of interface EqWith(U = (i32)) for (i32, i32))
   if (t1 == t2) {
     return 1;
   } else {
diff --git a/explorer/testdata/tuple/fail_index.carbon b/explorer/testdata/tuple/fail_index.carbon
index d9267d25c667..44c00162720e 100644
--- a/explorer/testdata/tuple/fail_index.carbon
+++ b/explorer/testdata/tuple/fail_index.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: auto = (0, 1);
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_index.carbon:[[@LINE+1]]: index 2 is out of range for type (i32, i32)
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_index.carbon:[[@LINE+1]]: index 2 is out of range for type (i32, i32)
   return x[2];
 }
diff --git a/explorer/testdata/tuple/fail_index_var.carbon b/explorer/testdata/tuple/fail_index_var.carbon
index fcf97a325ff7..f0520a63f848 100644
--- a/explorer/testdata/tuple/fail_index_var.carbon
+++ b/explorer/testdata/tuple/fail_index_var.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,6 +11,6 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var x: auto = (0, 1);
   var index: i32 = 0;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_index_var.carbon:[[@LINE+1]]: could not find `index: i32`
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_index_var.carbon:[[@LINE+1]]: could not find `index: i32`
   return x[index];
 }
diff --git a/explorer/testdata/tuple/fail_to_array.carbon b/explorer/testdata/tuple/fail_to_array.carbon
index 436f6d6151c6..eee5b123604b 100644
--- a/explorer/testdata/tuple/fail_to_array.carbon
+++ b/explorer/testdata/tuple/fail_to_array.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var t: auto = (1, 2);
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_to_array.carbon:[[@LINE+1]]: type error in name binding: '(i32, i32)' is not implicitly convertible to '[i32; 3]'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_to_array.carbon:[[@LINE+1]]: type error in name binding: '(i32, i32)' is not implicitly convertible to '[i32; 3]'
   var a: [i32; 3] = t;
 }
diff --git a/explorer/testdata/tuple/index.carbon b/explorer/testdata/tuple/index.carbon
index ec0571ce6c2c..af01033ac4a7 100644
--- a/explorer/testdata/tuple/index.carbon
+++ b/explorer/testdata/tuple/index.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/match.carbon b/explorer/testdata/tuple/match.carbon
index e9d9ad446661..3b3cac31b740 100644
--- a/explorer/testdata/tuple/match.carbon
+++ b/explorer/testdata/tuple/match.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/match_nested.carbon b/explorer/testdata/tuple/match_nested.carbon
index 04892b4c5aed..5e1a8237a5b4 100644
--- a/explorer/testdata/tuple/match_nested.carbon
+++ b/explorer/testdata/tuple/match_nested.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/no_ending_comma.carbon b/explorer/testdata/tuple/no_ending_comma.carbon
index c8c66cfc1f25..ab27c3e72518 100644
--- a/explorer/testdata/tuple/no_ending_comma.carbon
+++ b/explorer/testdata/tuple/no_ending_comma.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/tuple/to_array.carbon b/explorer/testdata/tuple/to_array.carbon
index d7c95fa7ac76..74ed068d1e8c 100644
--- a/explorer/testdata/tuple/to_array.carbon
+++ b/explorer/testdata/tuple/to_array.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 3
+// CHECK:STDOUT: result: 3
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/control_flow_defer_to_dynamic.carbon b/explorer/testdata/unformed/control_flow_defer_to_dynamic.carbon
index a02d5b40933f..86e60e58b709 100644
--- a/explorer/testdata/unformed/control_flow_defer_to_dynamic.carbon
+++ b/explorer/testdata/unformed/control_flow_defer_to_dynamic.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/dynamic/fail_global.carbon b/explorer/testdata/unformed/dynamic/fail_global.carbon
index 1918c7ab3165..fef3a23ca6db 100644
--- a/explorer/testdata/unformed/dynamic/fail_global.carbon
+++ b/explorer/testdata/unformed/dynamic/fail_global.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,6 +11,6 @@ package ExplorerTest api;
 var x: i32;
 
 fn Main() -> i32 {
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_global.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_global.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
   return x;
 }
diff --git a/explorer/testdata/unformed/dynamic/fail_param.carbon b/explorer/testdata/unformed/dynamic/fail_param.carbon
index 8f37ffd5e05c..e7d8f03aa125 100644
--- a/explorer/testdata/unformed/dynamic/fail_param.carbon
+++ b/explorer/testdata/unformed/dynamic/fail_param.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -18,6 +18,6 @@ fn Main() -> i32 {
     x = 0;
   }
   // Static analysis thinks `x` may be formed, defer the check to run-time.
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_param.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_param.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
   return AddInt(x, 2);
 }
diff --git a/explorer/testdata/unformed/dynamic/fail_pattern_declare.carbon b/explorer/testdata/unformed/dynamic/fail_pattern_declare.carbon
index f9220d702ab8..213ca64d3aa8 100644
--- a/explorer/testdata/unformed/dynamic/fail_pattern_declare.carbon
+++ b/explorer/testdata/unformed/dynamic/fail_pattern_declare.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,6 +15,6 @@ fn Main() -> i32 {
     y = 0;
   }
   // Static analysis thinks `y` may be formed, defer the check to run-time.
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_pattern_declare.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<y>>
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_pattern_declare.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<y>>
   return y;
 }
diff --git a/explorer/testdata/unformed/dynamic/fail_return.carbon b/explorer/testdata/unformed/dynamic/fail_return.carbon
index c91452716973..76328ddcc376 100644
--- a/explorer/testdata/unformed/dynamic/fail_return.carbon
+++ b/explorer/testdata/unformed/dynamic/fail_return.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,6 +14,6 @@ fn Main() -> i32 {
     x = 0;
   }
   // Static analysis thinks `x` may be formed, defer the check to run-time.
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_return.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_return.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
   return x;
 }
diff --git a/explorer/testdata/unformed/dynamic/fail_returned_var.carbon b/explorer/testdata/unformed/dynamic/fail_returned_var.carbon
index a1962aa3b542..c85757d1e9d9 100644
--- a/explorer/testdata/unformed/dynamic/fail_returned_var.carbon
+++ b/explorer/testdata/unformed/dynamic/fail_returned_var.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,6 +14,6 @@ fn Main() -> i32 {
     x = 0;
   }
   // Static analysis thinks `x` may be formed, defer the check to run-time.
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_returned_var.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_returned_var.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
   return var;
 }
diff --git a/explorer/testdata/unformed/dynamic/fail_rhs_assign.carbon b/explorer/testdata/unformed/dynamic/fail_rhs_assign.carbon
index 8c53f694b4b8..c3f2cd6043b1 100644
--- a/explorer/testdata/unformed/dynamic/fail_rhs_assign.carbon
+++ b/explorer/testdata/unformed/dynamic/fail_rhs_assign.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,7 +15,7 @@ fn Main() -> i32 {
     x = 0;
   }
   // Static analysis thinks `x` may be formed, defer the check to run-time.
-  // CHECK:RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_rhs_assign.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/unformed/dynamic/fail_rhs_assign.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
   y = x;
   return y;
 }
diff --git a/explorer/testdata/unformed/global_assign_before_use.carbon b/explorer/testdata/unformed/global_assign_before_use.carbon
index 0626bd56368b..687209e489af 100644
--- a/explorer/testdata/unformed/global_assign_before_use.carbon
+++ b/explorer/testdata/unformed/global_assign_before_use.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/global_escape.carbon b/explorer/testdata/unformed/global_escape.carbon
index 1175ec1778f8..62af1da6bb71 100644
--- a/explorer/testdata/unformed/global_escape.carbon
+++ b/explorer/testdata/unformed/global_escape.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 42
+// CHECK:STDOUT: result: 42
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/global_unformed_without_use.carbon b/explorer/testdata/unformed/global_unformed_without_use.carbon
index cdd0cea1f1fb..a96ceb6ae2f4 100644
--- a/explorer/testdata/unformed/global_unformed_without_use.carbon
+++ b/explorer/testdata/unformed/global_unformed_without_use.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/local_assign_before_use.carbon b/explorer/testdata/unformed/local_assign_before_use.carbon
index 39100eb60d17..4fda3d7a7e7c 100644
--- a/explorer/testdata/unformed/local_assign_before_use.carbon
+++ b/explorer/testdata/unformed/local_assign_before_use.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/local_escape.carbon b/explorer/testdata/unformed/local_escape.carbon
index 29ea8ba0580b..c757589cc2f0 100644
--- a/explorer/testdata/unformed/local_escape.carbon
+++ b/explorer/testdata/unformed/local_escape.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 42
+// CHECK:STDOUT: result: 42
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/local_unformed_without_use.carbon b/explorer/testdata/unformed/local_unformed_without_use.carbon
index 0c96fdafcc88..7ef6b9f9dd98 100644
--- a/explorer/testdata/unformed/local_unformed_without_use.carbon
+++ b/explorer/testdata/unformed/local_unformed_without_use.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/pattern_declare.carbon b/explorer/testdata/unformed/pattern_declare.carbon
index 496ec6d8e520..335a9e186b98 100644
--- a/explorer/testdata/unformed/pattern_declare.carbon
+++ b/explorer/testdata/unformed/pattern_declare.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/returned_var_assign_before_use.carbon b/explorer/testdata/unformed/returned_var_assign_before_use.carbon
index 83f4677b2beb..099615dd81ed 100644
--- a/explorer/testdata/unformed/returned_var_assign_before_use.carbon
+++ b/explorer/testdata/unformed/returned_var_assign_before_use.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 1
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/unformed/static/fail_field_value.carbon b/explorer/testdata/unformed/static/fail_field_value.carbon
index 7d8df13dae40..f13d1cfc0fcc 100644
--- a/explorer/testdata/unformed/static/fail_field_value.carbon
+++ b/explorer/testdata/unformed/static/fail_field_value.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_field_value.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_field_value.carbon:[[@LINE+1]]: use of uninitialized variable x
   var p: auto = {.x = x,};
   return 0;
 }
diff --git a/explorer/testdata/unformed/static/fail_if_cond.carbon b/explorer/testdata/unformed/static/fail_if_cond.carbon
index 12d1193b0a30..ee19243533ab 100644
--- a/explorer/testdata/unformed/static/fail_if_cond.carbon
+++ b/explorer/testdata/unformed/static/fail_if_cond.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_if_cond.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_if_cond.carbon:[[@LINE+1]]: use of uninitialized variable x
   if (x == 0) {
     return 0;
   }
diff --git a/explorer/testdata/unformed/static/fail_if_else.carbon b/explorer/testdata/unformed/static/fail_if_else.carbon
index 04a4cd4e1f95..d8c6a94432e4 100644
--- a/explorer/testdata/unformed/static/fail_if_else.carbon
+++ b/explorer/testdata/unformed/static/fail_if_else.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -15,7 +15,7 @@ fn Main() -> i32 {
   if (Foo() == 0) {
     return 0;
   } else {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_if_else.carbon:[[@LINE+1]]: use of uninitialized variable x
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_if_else.carbon:[[@LINE+1]]: use of uninitialized variable x
     return x;
   }
 }
diff --git a/explorer/testdata/unformed/static/fail_if_then.carbon b/explorer/testdata/unformed/static/fail_if_then.carbon
index af9258da65fe..90e797db8672 100644
--- a/explorer/testdata/unformed/static/fail_if_then.carbon
+++ b/explorer/testdata/unformed/static/fail_if_then.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,7 +13,7 @@ fn Foo() -> i32;
 fn Main() -> i32 {
   var x: i32;
   if (Foo() == 0) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_if_then.carbon:[[@LINE+1]]: use of uninitialized variable x
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_if_then.carbon:[[@LINE+1]]: use of uninitialized variable x
     return x;
   }
   return 1;
diff --git a/explorer/testdata/unformed/static/fail_local_pattern_declare.carbon b/explorer/testdata/unformed/static/fail_local_pattern_declare.carbon
index 81d9477ff4b4..010a855f1fcc 100644
--- a/explorer/testdata/unformed/static/fail_local_pattern_declare.carbon
+++ b/explorer/testdata/unformed/static/fail_local_pattern_declare.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,6 +11,6 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var (x: i32, y: i32);
   x = 1;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_local_pattern_declare.carbon:[[@LINE+1]]: use of uninitialized variable y
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_local_pattern_declare.carbon:[[@LINE+1]]: use of uninitialized variable y
   return y;
 }
diff --git a/explorer/testdata/unformed/static/fail_local_rhs_assign.carbon b/explorer/testdata/unformed/static/fail_local_rhs_assign.carbon
index c7ab8e006cab..d4fd5849767c 100644
--- a/explorer/testdata/unformed/static/fail_local_rhs_assign.carbon
+++ b/explorer/testdata/unformed/static/fail_local_rhs_assign.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -11,7 +11,7 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var x: i32;
   var y: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_local_rhs_assign.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_local_rhs_assign.carbon:[[@LINE+1]]: use of uninitialized variable x
   y = x;
   return y;
 }
diff --git a/explorer/testdata/unformed/static/fail_match_clause.carbon b/explorer/testdata/unformed/static/fail_match_clause.carbon
index 213b19a62e4c..9da58ea7fa2f 100644
--- a/explorer/testdata/unformed/static/fail_match_clause.carbon
+++ b/explorer/testdata/unformed/static/fail_match_clause.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -17,7 +17,7 @@ fn Main() -> i32 {
       return 2;
     }
     case 1 => {
-      // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_match_clause.carbon:[[@LINE+1]]: use of uninitialized variable x
+      // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_match_clause.carbon:[[@LINE+1]]: use of uninitialized variable x
       return x;
     }
     default => {
diff --git a/explorer/testdata/unformed/static/fail_match_expression.carbon b/explorer/testdata/unformed/static/fail_match_expression.carbon
index 6fd85fb39d3f..0cce364a94dc 100644
--- a/explorer/testdata/unformed/static/fail_match_expression.carbon
+++ b/explorer/testdata/unformed/static/fail_match_expression.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x : i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_match_expression.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_match_expression.carbon:[[@LINE+1]]: use of uninitialized variable x
   match (x) {
     case 0 => {
       return 1;
diff --git a/explorer/testdata/unformed/static/fail_param.carbon b/explorer/testdata/unformed/static/fail_param.carbon
index 4eabadec7ab7..117c3db83e5c 100644
--- a/explorer/testdata/unformed/static/fail_param.carbon
+++ b/explorer/testdata/unformed/static/fail_param.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -14,6 +14,6 @@ fn AddInt(a: i32, b: i32) -> auto {
 
 fn Main() -> i32 {
   var x: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_param.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_param.carbon:[[@LINE+1]]: use of uninitialized variable x
   return AddInt(x, 2);
 }
diff --git a/explorer/testdata/unformed/static/fail_return.carbon b/explorer/testdata/unformed/static/fail_return.carbon
index b872e2257dfb..3506ab095fb0 100644
--- a/explorer/testdata/unformed/static/fail_return.carbon
+++ b/explorer/testdata/unformed/static/fail_return.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_return.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_return.carbon:[[@LINE+1]]: use of uninitialized variable x
   return x;
 }
diff --git a/explorer/testdata/unformed/static/fail_returned_var.carbon b/explorer/testdata/unformed/static/fail_returned_var.carbon
index dae83edcca1f..247010881d51 100644
--- a/explorer/testdata/unformed/static/fail_returned_var.carbon
+++ b/explorer/testdata/unformed/static/fail_returned_var.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   returned var x: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_returned_var.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_returned_var.carbon:[[@LINE+1]]: use of uninitialized variable x
   return var;
 }
diff --git a/explorer/testdata/unformed/static/fail_rhs_def.carbon b/explorer/testdata/unformed/static/fail_rhs_def.carbon
index 463009fac440..086b5e521aa4 100644
--- a/explorer/testdata/unformed/static/fail_rhs_def.carbon
+++ b/explorer/testdata/unformed/static/fail_rhs_def.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32;
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_rhs_def.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_rhs_def.carbon:[[@LINE+1]]: use of uninitialized variable x
   var y: i32 = x;
   return x;
 }
diff --git a/explorer/testdata/unformed/static/fail_struct_member_access.carbon b/explorer/testdata/unformed/static/fail_struct_member_access.carbon
index 6587c9ad65af..20460a47638f 100644
--- a/explorer/testdata/unformed/static/fail_struct_member_access.carbon
+++ b/explorer/testdata/unformed/static/fail_struct_member_access.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var p: {.x: i32, .y: i32};
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_struct_member_access.carbon:[[@LINE+1]]: use of uninitialized variable p
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_struct_member_access.carbon:[[@LINE+1]]: use of uninitialized variable p
   p.x = p.y;
   return 0;
 }
diff --git a/explorer/testdata/unformed/static/fail_while_body.carbon b/explorer/testdata/unformed/static/fail_while_body.carbon
index d66d70986676..d093c0a67d25 100644
--- a/explorer/testdata/unformed/static/fail_while_body.carbon
+++ b/explorer/testdata/unformed/static/fail_while_body.carbon
@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -13,7 +13,7 @@ fn Foo() -> i32;
 fn Main() -> i32 {
   var (x: i32, y: i32);
   while (Foo() == 0) {
-    // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_while_body.carbon:[[@LINE+1]]: use of uninitialized variable x
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_while_body.carbon:[[@LINE+1]]: use of uninitialized variable x
     y = x;
   }
   return 1;
diff --git a/explorer/testdata/unformed/static/fail_while_cond.carbon b/explorer/testdata/unformed/static/fail_while_cond.carbon
index 5711ef2651bc..5cf5279d5c43 100644
--- a/explorer/testdata/unformed/static/fail_while_cond.carbon
+++ b/explorer/testdata/unformed/static/fail_while_cond.carbon
@@ -2,15 +2,15 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{not} %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var (x: i32, y: i32);
-  // CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_while_cond.carbon:[[@LINE+1]]: use of uninitialized variable x
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/unformed/static/fail_while_cond.carbon:[[@LINE+1]]: use of uninitialized variable x
   while (x == 0) {
     y = 1;
   }
diff --git a/explorer/testdata/while/basic.carbon b/explorer/testdata/while/basic.carbon
index 12c89e4dbaba..4d9ec3f3027e 100644
--- a/explorer/testdata/while/basic.carbon
+++ b/explorer/testdata/while/basic.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/while/break.carbon b/explorer/testdata/while/break.carbon
index 402fc8ebd522..b56ef83bfe14 100644
--- a/explorer/testdata/while/break.carbon
+++ b/explorer/testdata/while/break.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/while/continue.carbon b/explorer/testdata/while/continue.carbon
index 1b57e7060db0..64896f562e84 100644
--- a/explorer/testdata/while/continue.carbon
+++ b/explorer/testdata/while/continue.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/while/convert_condition.carbon b/explorer/testdata/while/convert_condition.carbon
index 9ee13ae871b2..08ea21aee452 100644
--- a/explorer/testdata/while/convert_condition.carbon
+++ b/explorer/testdata/while/convert_condition.carbon
@@ -2,10 +2,10 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{explorer} %s 2>&1 | %{FileCheck-strict} %s
-// RUN: %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
+// RUN: %{explorer} %s | %{FileCheck-strict} %s
+// RUN: %{explorer-trace} %s | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK:result: 0
+// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
diff --git a/toolchain/driver/testdata/BUILD b/toolchain/driver/testdata/BUILD
index e1261e0ad6a1..bf123a919e30 100644
--- a/toolchain/driver/testdata/BUILD
+++ b/toolchain/driver/testdata/BUILD
@@ -6,6 +6,7 @@ load("//bazel/testing:lit.bzl", "glob_lit_tests")
 
 glob_lit_tests(
     data = [
+        "//bazel/testing:merge_output",
         "//toolchain/driver:carbon",
         "@llvm-project//llvm:FileCheck",
         "@llvm-project//llvm:not",
diff --git a/toolchain/driver/testdata/errors_sorted_test.carbon b/toolchain/driver/testdata/errors_sorted_test.carbon
index cf76cc2a4181..aad3411a2166 100644
--- a/toolchain/driver/testdata/errors_sorted_test.carbon
+++ b/toolchain/driver/testdata/errors_sorted_test.carbon
@@ -2,12 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{carbon} dump tokens %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{not} %{carbon} dump tokens %s | %{FileCheck-strict} %s
+// CHECK-COUNT-17:STDOUT: token: {{.*}}
 
-// CHECK:{{.*}}/errors_sorted_test.carbon:[[@LINE+1]]:24: Closing symbol does not match most recent opening symbol.
+// CHECK:STDERR: {{.*}}/errors_sorted_test.carbon:[[@LINE+1]]:24: Closing symbol does not match most recent opening symbol.
 fn run(String program) {
   return True;
 
-// CHECK:{{.*}}/errors_sorted_test.carbon:[[@LINE+1]]:10: Invalid digit 'a' in decimal numeric literal.
+// CHECK:STDERR: {{.*}}/errors_sorted_test.carbon:[[@LINE+1]]:10: Invalid digit 'a' in decimal numeric literal.
 var x = 3a;
-// CHECK-COUNT-17:token: {{.*}}
diff --git a/toolchain/driver/testdata/errors_streamed_test.carbon b/toolchain/driver/testdata/errors_streamed_test.carbon
index 040ad8cf24f4..eea1be098886 100644
--- a/toolchain/driver/testdata/errors_streamed_test.carbon
+++ b/toolchain/driver/testdata/errors_streamed_test.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{carbon} --print-errors=streamed dump tokens %s 2>&1 | \
+// RUN: %{not} %{carbon} --print-errors=streamed dump tokens %s | \
 // RUN:   %{FileCheck-strict} %s
+// CHECK-COUNT-17:STDOUT: token: {{.*}}
 
 fn run(String program) {
   return True;
 
 var x = 3a;
 
-// CHECK:{{.*}}/errors_streamed_test.carbon:[[@LINE-2]]:10: Invalid digit 'a' in decimal numeric literal.
-// CHECK:{{.*}}/errors_streamed_test.carbon:[[@LINE-6]]:24: Closing symbol does not match most recent opening symbol.
-// CHECK-COUNT-17:token: {{.*}}
+// CHECK:STDERR:{{.*}}/errors_streamed_test.carbon:[[@LINE-2]]:10: Invalid digit 'a' in decimal numeric literal.
+// CHECK:STDERR:{{.*}}/errors_streamed_test.carbon:[[@LINE-6]]:24: Closing symbol does not match most recent opening symbol.
diff --git a/toolchain/lexer/lit_autoupdate.py b/toolchain/lexer/lit_autoupdate.py
index 02a398c097e9..25715e30c416 100755
--- a/toolchain/lexer/lit_autoupdate.py
+++ b/toolchain/lexer/lit_autoupdate.py
@@ -14,7 +14,7 @@ from pathlib import Path
 
 
 def main() -> None:
-    # Calls the main script with explorer settings. This uses execv in order to
+    # Calls the main script with lexer settings. This uses execv in order to
     # avoid Python import behaviors.
     this_py = Path(__file__).resolve()
     actual_py = this_py.parent.parent.parent.joinpath(
@@ -22,12 +22,9 @@ def main() -> None:
     )
     args = [
         sys.argv[0],
-        # Flags to configure for explorer testing.
+        # Flags to configure for lexer testing.
         "--build_target",
         "//toolchain/driver:carbon",
-        "--cmd_replace",
-        "%{carbon}",
-        "./bazel-bin/toolchain/driver/carbon",
         # Ignore the resulting column of EndOfFile because it's typically the
         # end of the CHECK comment.
         "--extra_check_replacement",
diff --git a/toolchain/lexer/testdata/BUILD b/toolchain/lexer/testdata/BUILD
index e1261e0ad6a1..bf123a919e30 100644
--- a/toolchain/lexer/testdata/BUILD
+++ b/toolchain/lexer/testdata/BUILD
@@ -6,6 +6,7 @@ load("//bazel/testing:lit.bzl", "glob_lit_tests")
 
 glob_lit_tests(
     data = [
+        "//bazel/testing:merge_output",
         "//toolchain/driver:carbon",
         "@llvm-project//llvm:FileCheck",
         "@llvm-project//llvm:not",
diff --git a/toolchain/lexer/testdata/carbon_test.carbon b/toolchain/lexer/testdata/carbon_test.carbon
index 545a73e445d5..8d20ea6506ab 100644
--- a/toolchain/lexer/testdata/carbon_test.carbon
+++ b/toolchain/lexer/testdata/carbon_test.carbon
@@ -2,21 +2,21 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{carbon} dump tokens %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{carbon} dump tokens %s | %{FileCheck-strict} %s
 // AUTOUPDATE: %{carbon} dump tokens %s
 
-// CHECK:token: { index:  0, kind:              'Fn', line: {{ *}}[[@LINE+7]], column:   1, indent: 1, spelling: 'fn', has_trailing_space: true }
-// CHECK:token: { index:  1, kind:      'Identifier', line: {{ *}}[[@LINE+6]], column:   4, indent: 1, spelling: 'run', identifier: 0 }
-// CHECK:token: { index:  2, kind:       'OpenParen', line: {{ *}}[[@LINE+5]], column:   7, indent: 1, spelling: '(', closing_token: 5 }
-// CHECK:token: { index:  3, kind:      'Identifier', line: {{ *}}[[@LINE+4]], column:   8, indent: 1, spelling: 'String', identifier: 1, has_trailing_space: true }
-// CHECK:token: { index:  4, kind:      'Identifier', line: {{ *}}[[@LINE+3]], column:  15, indent: 1, spelling: 'program', identifier: 2 }
-// CHECK:token: { index:  5, kind:      'CloseParen', line: {{ *}}[[@LINE+2]], column:  22, indent: 1, spelling: ')', opening_token: 2, has_trailing_space: true }
-// CHECK:token: { index:  6, kind:  'OpenCurlyBrace', line: {{ *}}[[@LINE+1]], column:  24, indent: 1, spelling: '{', closing_token: 10, has_trailing_space: true }
+// CHECK:STDOUT: token: { index:  0, kind:              'Fn', line: {{ *}}[[@LINE+7]], column:   1, indent: 1, spelling: 'fn', has_trailing_space: true }
+// CHECK:STDOUT: token: { index:  1, kind:      'Identifier', line: {{ *}}[[@LINE+6]], column:   4, indent: 1, spelling: 'run', identifier: 0 }
+// CHECK:STDOUT: token: { index:  2, kind:       'OpenParen', line: {{ *}}[[@LINE+5]], column:   7, indent: 1, spelling: '(', closing_token: 5 }
+// CHECK:STDOUT: token: { index:  3, kind:      'Identifier', line: {{ *}}[[@LINE+4]], column:   8, indent: 1, spelling: 'String', identifier: 1, has_trailing_space: true }
+// CHECK:STDOUT: token: { index:  4, kind:      'Identifier', line: {{ *}}[[@LINE+3]], column:  15, indent: 1, spelling: 'program', identifier: 2 }
+// CHECK:STDOUT: token: { index:  5, kind:      'CloseParen', line: {{ *}}[[@LINE+2]], column:  22, indent: 1, spelling: ')', opening_token: 2, has_trailing_space: true }
+// CHECK:STDOUT: token: { index:  6, kind:  'OpenCurlyBrace', line: {{ *}}[[@LINE+1]], column:  24, indent: 1, spelling: '{', closing_token: 10, has_trailing_space: true }
 fn run(String program) {
-  // CHECK:token: { index:  7, kind:          'Return', line: {{ *}}[[@LINE+3]], column:   3, indent: 3, spelling: 'return', has_trailing_space: true }
-  // CHECK:token: { index:  8, kind:      'Identifier', line: {{ *}}[[@LINE+2]], column:  10, indent: 3, spelling: 'True', identifier: 3 }
-  // CHECK:token: { index:  9, kind:            'Semi', line: {{ *}}[[@LINE+1]], column:  14, indent: 3, spelling: ';', has_trailing_space: true }
+  // CHECK:STDOUT: token: { index:  7, kind:          'Return', line: {{ *}}[[@LINE+3]], column:   3, indent: 3, spelling: 'return', has_trailing_space: true }
+  // CHECK:STDOUT: token: { index:  8, kind:      'Identifier', line: {{ *}}[[@LINE+2]], column:  10, indent: 3, spelling: 'True', identifier: 3 }
+  // CHECK:STDOUT: token: { index:  9, kind:            'Semi', line: {{ *}}[[@LINE+1]], column:  14, indent: 3, spelling: ';', has_trailing_space: true }
   return True;
-// CHECK:token: { index: 10, kind: 'CloseCurlyBrace', line: {{ *}}[[@LINE+1]], column:   1, indent: 1, spelling: '}', opening_token: 6, has_trailing_space: true }
+// CHECK:STDOUT: token: { index: 10, kind: 'CloseCurlyBrace', line: {{ *}}[[@LINE+1]], column:   1, indent: 1, spelling: '}', opening_token: 6, has_trailing_space: true }
 }
-// CHECK:token: { index: 11, kind:       'EndOfFile', line: {{ *}}[[@LINE+0]], column: {{[0-9]+}}, indent: 1, spelling: '' }
+// CHECK:STDOUT: token: { index: 11, kind:       'EndOfFile', line: {{ *}}[[@LINE+0]], column: {{[0-9]+}}, indent: 1, spelling: '' }
diff --git a/toolchain/semantics/lit_autoupdate.py b/toolchain/semantics/lit_autoupdate.py
index 728484581a22..98f09e4b988c 100755
--- a/toolchain/semantics/lit_autoupdate.py
+++ b/toolchain/semantics/lit_autoupdate.py
@@ -25,9 +25,6 @@ def main() -> None:
         # Flags to configure for explorer testing.
         "--build_target",
         "//toolchain/driver:carbon",
-        "--cmd_replace",
-        "%{carbon}",
-        "./bazel-bin/toolchain/driver/carbon",
         # TODO: This should eventually have lines in output, but it doesn't
         # right now.
         "--line_number_pattern",
diff --git a/toolchain/semantics/testdata/BUILD b/toolchain/semantics/testdata/BUILD
index e1261e0ad6a1..bf123a919e30 100644
--- a/toolchain/semantics/testdata/BUILD
+++ b/toolchain/semantics/testdata/BUILD
@@ -6,6 +6,7 @@ load("//bazel/testing:lit.bzl", "glob_lit_tests")
 
 glob_lit_tests(
     data = [
+        "//bazel/testing:merge_output",
         "//toolchain/driver:carbon",
         "@llvm-project//llvm:FileCheck",
         "@llvm-project//llvm:not",
diff --git a/toolchain/semantics/testdata/empty.carbon b/toolchain/semantics/testdata/empty.carbon
index 490565112f08..004cd8679fb8 100644
--- a/toolchain/semantics/testdata/empty.carbon
+++ b/toolchain/semantics/testdata/empty.carbon
@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{carbon} dump semantics-ir %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{carbon} dump semantics-ir %s | %{FileCheck-strict} %s
 // AUTOUPDATE: %{carbon} dump semantics-ir %s
-// CHECK:{
-// CHECK:}
+// CHECK:STDOUT: {
+// CHECK:STDOUT: }
diff --git a/toolchain/semantics/testdata/function/basic.carbon b/toolchain/semantics/testdata/function/basic.carbon
index 992ef3a09010..93f60b925b55 100644
--- a/toolchain/semantics/testdata/function/basic.carbon
+++ b/toolchain/semantics/testdata/function/basic.carbon
@@ -2,14 +2,14 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{carbon} dump semantics-ir %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{carbon} dump semantics-ir %s | %{FileCheck-strict} %s
 // AUTOUPDATE: %{carbon} dump semantics-ir %s
-// CHECK:{
-// CHECK:  Function(
-// CHECK:      %0,
-// CHECK:      {
-// CHECK:      }),
-// CHECK:  SetName(`Foo`, %0),
-// CHECK:}
+// CHECK:STDOUT: {
+// CHECK:STDOUT:   Function(
+// CHECK:STDOUT:       %0,
+// CHECK:STDOUT:       {
+// CHECK:STDOUT:       }),
+// CHECK:STDOUT:   SetName(`Foo`, %0),
+// CHECK:STDOUT: }
 
 fn Foo() {}
diff --git a/toolchain/semantics/testdata/function/order.carbon b/toolchain/semantics/testdata/function/order.carbon
index 5c457dcb512a..c8bbd32d227c 100644
--- a/toolchain/semantics/testdata/function/order.carbon
+++ b/toolchain/semantics/testdata/function/order.carbon
@@ -2,25 +2,25 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{carbon} dump semantics-ir %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{carbon} dump semantics-ir %s | %{FileCheck-strict} %s
 // AUTOUPDATE: %{carbon} dump semantics-ir %s
-// CHECK:{
-// CHECK:  Function(
-// CHECK:      %2,
-// CHECK:      {
-// CHECK:      }),
-// CHECK:  SetName(`Foo`, %2),
-// CHECK:  Function(
-// CHECK:      %1,
-// CHECK:      {
-// CHECK:      }),
-// CHECK:  SetName(`Bar`, %1),
-// CHECK:  Function(
-// CHECK:      %0,
-// CHECK:      {
-// CHECK:      }),
-// CHECK:  SetName(`Baz`, %0),
-// CHECK:}
+// CHECK:STDOUT: {
+// CHECK:STDOUT:   Function(
+// CHECK:STDOUT:       %2,
+// CHECK:STDOUT:       {
+// CHECK:STDOUT:       }),
+// CHECK:STDOUT:   SetName(`Foo`, %2),
+// CHECK:STDOUT:   Function(
+// CHECK:STDOUT:       %1,
+// CHECK:STDOUT:       {
+// CHECK:STDOUT:       }),
+// CHECK:STDOUT:   SetName(`Bar`, %1),
+// CHECK:STDOUT:   Function(
+// CHECK:STDOUT:       %0,
+// CHECK:STDOUT:       {
+// CHECK:STDOUT:       }),
+// CHECK:STDOUT:   SetName(`Baz`, %0),
+// CHECK:STDOUT: }
 
 fn Foo() {}
 fn Bar() {}
diff --git a/toolchain/semantics/testdata/return/binary_op.carbon b/toolchain/semantics/testdata/return/binary_op.carbon
index 207174c83b72..246ac438fce9 100644
--- a/toolchain/semantics/testdata/return/binary_op.carbon
+++ b/toolchain/semantics/testdata/return/binary_op.carbon
@@ -2,19 +2,19 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{carbon} dump semantics-ir %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{carbon} dump semantics-ir %s | %{FileCheck-strict} %s
 // AUTOUPDATE: %{carbon} dump semantics-ir %s
-// CHECK:{
-// CHECK:  Function(
-// CHECK:      %0,
-// CHECK:      {
-// CHECK:        IntegerLiteral(%3, 12),
-// CHECK:        IntegerLiteral(%2, 34),
-// CHECK:        BinaryOperator(%1, +, %3, %2),
-// CHECK:        Return(%1),
-// CHECK:      }),
-// CHECK:  SetName(`Main`, %0),
-// CHECK:}
+// CHECK:STDOUT: {
+// CHECK:STDOUT:   Function(
+// CHECK:STDOUT:       %0,
+// CHECK:STDOUT:       {
+// CHECK:STDOUT:         IntegerLiteral(%3, 12),
+// CHECK:STDOUT:         IntegerLiteral(%2, 34),
+// CHECK:STDOUT:         BinaryOperator(%1, +, %3, %2),
+// CHECK:STDOUT:         Return(%1),
+// CHECK:STDOUT:       }),
+// CHECK:STDOUT:   SetName(`Main`, %0),
+// CHECK:STDOUT: }
 
 fn Main() {
   return 12 + 34;
diff --git a/toolchain/semantics/testdata/return/literal.carbon b/toolchain/semantics/testdata/return/literal.carbon
index b52fd587cd48..89caf13fe9d8 100644
--- a/toolchain/semantics/testdata/return/literal.carbon
+++ b/toolchain/semantics/testdata/return/literal.carbon
@@ -2,17 +2,17 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{carbon} dump semantics-ir %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{carbon} dump semantics-ir %s | %{FileCheck-strict} %s
 // AUTOUPDATE: %{carbon} dump semantics-ir %s
-// CHECK:{
-// CHECK:  Function(
-// CHECK:      %0,
-// CHECK:      {
-// CHECK:        IntegerLiteral(%1, 0),
-// CHECK:        Return(%1),
-// CHECK:      }),
-// CHECK:  SetName(`Main`, %0),
-// CHECK:}
+// CHECK:STDOUT: {
+// CHECK:STDOUT:   Function(
+// CHECK:STDOUT:       %0,
+// CHECK:STDOUT:       {
+// CHECK:STDOUT:         IntegerLiteral(%1, 0),
+// CHECK:STDOUT:         Return(%1),
+// CHECK:STDOUT:       }),
+// CHECK:STDOUT:   SetName(`Main`, %0),
+// CHECK:STDOUT: }
 
 fn Main() {
   return 0;
diff --git a/toolchain/semantics/testdata/return/trivial.carbon b/toolchain/semantics/testdata/return/trivial.carbon
index b260779a98ec..22bcbeff1609 100644
--- a/toolchain/semantics/testdata/return/trivial.carbon
+++ b/toolchain/semantics/testdata/return/trivial.carbon
@@ -2,16 +2,16 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{carbon} dump semantics-ir %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{carbon} dump semantics-ir %s | %{FileCheck-strict} %s
 // AUTOUPDATE: %{carbon} dump semantics-ir %s
-// CHECK:{
-// CHECK:  Function(
-// CHECK:      %0,
-// CHECK:      {
-// CHECK:        Return(None),
-// CHECK:      }),
-// CHECK:  SetName(`Main`, %0),
-// CHECK:}
+// CHECK:STDOUT: {
+// CHECK:STDOUT:   Function(
+// CHECK:STDOUT:       %0,
+// CHECK:STDOUT:       {
+// CHECK:STDOUT:         Return(None),
+// CHECK:STDOUT:       }),
+// CHECK:STDOUT:   SetName(`Main`, %0),
+// CHECK:STDOUT: }
 
 fn Main() {
   return;