diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 9c5c81ae9c34..445f5b70308e 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -95,6 +95,16 @@ repos:
         entry: scripts/check_sha_filenames.py
         language: python
         files: ^.*/fuzzer_corpus/.*$
+      - id: check-toolchain-diagnostics
+        name: Check toolchain diagnostics
+        entry: toolchain/diagnostics/check_diagnostics.py
+        language: python
+        files: |
+          (?x)^(
+              toolchain/.*\.cpp|
+              toolchain/.*\.h|
+              toolchain/diagnostics/diagnostic_registry\.def
+          )$
 
   # Run linters last, as formatters and other checks may fix issues.
   - repo: local
diff --git a/toolchain/diagnostics/check_diagnostics.py b/toolchain/diagnostics/check_diagnostics.py
new file mode 100755
index 000000000000..b202c61e5241
--- /dev/null
+++ b/toolchain/diagnostics/check_diagnostics.py
@@ -0,0 +1,117 @@
+#!/usr/bin/env python3
+
+"""Checks diagnostic use.
+
+Validates that each diagnostic declared with CARBON_DIAGNOSTIC_KIND is
+referenced by one (and only one) CARBON_DIAGNOSTIC.
+"""
+
+__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 collections
+from concurrent import futures
+import itertools
+from pathlib import Path
+import os
+import re
+import sys
+from typing import Dict, List, NamedTuple, Set
+
+
+# Example or test diagnostics, ignored because they're expected to not pass.
+IGNORED = set(["MyDiagnostic", "TestDiagnostic"])
+
+
+class Location(NamedTuple):
+    """A location for a diagnostic."""
+
+    def __str__(self) -> str:
+        return f"{str(self.path)}:{self.line}"
+
+    path: Path
+    line: int
+
+
+def load_diagnostic_kind() -> Set[str]:
+    """Returns the set of declared diagnostic kinds.
+
+    This isn't validated for uniqueness because the compiler does that.
+    """
+    path = Path("toolchain/diagnostics/diagnostic_registry.def")
+    content = path.read_text()
+    decls = set(re.findall(r"CARBON_DIAGNOSTIC_KIND\((.+)\)", content))
+    return decls.difference(IGNORED)
+
+
+def load_diagnostic_uses_in(
+    path: Path,
+) -> Dict[str, List[Location]]:
+    """Returns the path's CARBON_DIAGNOSTIC uses."""
+    content = path.read_text()
+
+    # Keep a line cursor so that we don't keep re-scanning the file.
+    line = 1
+    line_offset = 0
+
+    found: Dict[str, List[Location]] = collections.defaultdict(lambda: [])
+    for m in re.finditer(r"CARBON_DIAGNOSTIC\(\s*(\w+),", content):
+        diag = m.group(1)
+        if diag in IGNORED:
+            continue
+        line += content.count("\n", line_offset, m.start())
+        line_offset = m.start()
+        found[diag].append(Location(path, line))
+    return found
+
+
+def load_diagnostic_uses() -> Dict[str, List[Location]]:
+    """Returns all CARBON_DIAGNOSTIC uses."""
+    globs = itertools.chain(
+        *[Path("toolchain").glob(f"**/*.{ext}") for ext in ("h", "cpp")]
+    )
+    with futures.ThreadPoolExecutor() as exec:
+        results = exec.map(load_diagnostic_uses_in, globs)
+
+    found: Dict[str, List[Location]] = collections.defaultdict(lambda: [])
+    for result in results:
+        for diag, locations in result.items():
+            found[diag].extend(locations)
+    return found
+
+
+def check_uniqueness(uses: Dict[str, List[Location]]) -> bool:
+    """If any diagnostic is non-unique, prints an error and returns true."""
+    has_errors = False
+    for diag in sorted(uses.keys()):
+        if len(uses[diag]) > 1:
+            print(f"Non-unique diagnostic {diag}:", file=sys.stderr)
+            for loc in uses[diag]:
+                print(f"  - {loc}", file=sys.stderr)
+            has_errors = True
+    return has_errors
+
+
+def check_unused(decls: Set[str], uses: Dict[str, List[Location]]) -> bool:
+    """If any diagnostic is unused, prints an error and returns true."""
+    unused = decls.difference(uses.keys())
+    for diag in sorted(unused):
+        print(f"Unused diagnostic: {diag}")
+    return False
+
+
+def main() -> None:
+    # Run from the repo root.
+    os.chdir(Path(__file__).parent.parent.parent)
+    decls = load_diagnostic_kind()
+    uses = load_diagnostic_uses()
+
+    if any([check_uniqueness(uses), check_unused(decls, uses)]):
+        exit(1)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/toolchain/diagnostics/diagnostic_registry.def b/toolchain/diagnostics/diagnostic_registry.def
index 75fca2a5947d..f312395b1b51 100644
--- a/toolchain/diagnostics/diagnostic_registry.def
+++ b/toolchain/diagnostics/diagnostic_registry.def
@@ -61,11 +61,9 @@ CARBON_DIAGNOSTIC_KIND(ExpectedStructLiteralField)
 CARBON_DIAGNOSTIC_KIND(ExpectedVariableDeclaration)
 CARBON_DIAGNOSTIC_KIND(ExpectedVariableName)
 CARBON_DIAGNOSTIC_KIND(OperatorRequiresParentheses)
-CARBON_DIAGNOSTIC_KIND(StackLimitExceeded)
 CARBON_DIAGNOSTIC_KIND(UnaryOperatorHasWhitespace)
 CARBON_DIAGNOSTIC_KIND(UnaryOperatorRequiresWhitespace)
 CARBON_DIAGNOSTIC_KIND(UnexpectedTokenAfterListElement)
-CARBON_DIAGNOSTIC_KIND(UnexpectedTokenInCodeBlock)
 CARBON_DIAGNOSTIC_KIND(UnrecognizedDeclaration)
 
 // Package-related diagnostics.
@@ -77,6 +75,7 @@ CARBON_DIAGNOSTIC_KIND(ExpectedSemiToEndPackageDirective)
 
 // For-specific diagnostics.
 CARBON_DIAGNOSTIC_KIND(ExpectedIn)
+CARBON_DIAGNOSTIC_KIND(ExpectedInNotColon)
 
 // Interface-specific diagnostics.
 CARBON_DIAGNOSTIC_KIND(ExpectedInterfaceName)
diff --git a/toolchain/parser/parser.cpp b/toolchain/parser/parser.cpp
index 2f4fc62f57e5..65ae76458c92 100644
--- a/toolchain/parser/parser.cpp
+++ b/toolchain/parser/parser.cpp
@@ -28,6 +28,9 @@ CARBON_DIAGNOSTIC(ExpectedParenAfter, Error, "Expected `(` after `{0}`.",
 CARBON_DIAGNOSTIC(ExpectedSemiAfterExpression, Error,
                   "Expected `;` after expression.");
 
+CARBON_DIAGNOSTIC(UnrecognizedDeclaration, Error,
+                  "Unrecognized declaration introducer.");
+
 // A relative location for characters in errors.
 enum class RelativeLocation : int8_t {
   Around,
@@ -738,8 +741,6 @@ auto Parser::HandleDeclarationLoopState() -> void {
       break;
     }
     default: {
-      CARBON_DIAGNOSTIC(UnrecognizedDeclaration, Error,
-                        "Unrecognized declaration introducer.");
       emitter_->Emit(*position_, UnrecognizedDeclaration);
       auto cursor = *position_;
       auto semi = SkipPastLikelyEnd(cursor);
@@ -1236,8 +1237,6 @@ auto Parser::HandleInterfaceDefinitionLoopState() -> void {
       break;
     }
     default: {
-      CARBON_DIAGNOSTIC(UnrecognizedDeclaration, Error,
-                        "Unrecognized declaration introducer.");
       emitter_->Emit(*position_, UnrecognizedDeclaration);
       if (auto semi = SkipPastLikelyEnd(*position_)) {
         AddLeafNode(ParseNodeKind::EmptyDeclaration(), *semi,
@@ -1851,8 +1850,9 @@ auto Parser::HandleVarFinishAsForState() -> void {
   if (PositionIs(TokenKind::In())) {
     end_token = Consume();
   } else if (PositionIs(TokenKind::Colon())) {
-    CARBON_DIAGNOSTIC(ExpectedIn, Error, "`:` should be replaced by `in`.");
-    emitter_->Emit(*position_, ExpectedIn);
+    CARBON_DIAGNOSTIC(ExpectedInNotColon, Error,
+                      "`:` should be replaced by `in`.");
+    emitter_->Emit(*position_, ExpectedInNotColon);
     state.has_error = true;
     end_token = Consume();
   } else {