diff --git a/core/prelude/types/string.carbon b/core/prelude/types/string.carbon
index d204621597af..2e2aa3be7af1 100644
--- a/core/prelude/types/string.carbon
+++ b/core/prelude/types/string.carbon
@@ -14,7 +14,7 @@ import library "prelude/types/int";
 import library "prelude/operators/as";
 
 class String {
-  fn Size(self) -> u64 { return self.size; }
+  fn Size(self) -> i64 { return self.size; }
 
   impl as Copy {
     fn Op(self) -> Self { return {.ptr = self.ptr, .size = self.size}; }
@@ -22,7 +22,7 @@ class String {
   // TODO: This should be an array iterator.
   private var ptr: Char*;
   // TODO: This should be a word-sized integer.
-  private var size: u64;
+  private var size: i64;
 }
 
 impl String as UnformedInit {}
diff --git a/toolchain/check/testdata/interop/cpp/macros/macros.carbon b/toolchain/check/testdata/interop/cpp/macros/macros.carbon
index fc06d33e9ca9..2e2110532589 100644
--- a/toolchain/check/testdata/interop/cpp/macros/macros.carbon
+++ b/toolchain/check/testdata/interop/cpp/macros/macros.carbon
@@ -1375,36 +1375,36 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %str.3d4: type = class_type @String [concrete]
 // CHECK:STDOUT:   %int_64: Core.IntLiteral = int_value 64 [concrete]
-// CHECK:STDOUT:   %u64: type = class_type @UInt, @UInt(%int_64) [concrete]
+// CHECK:STDOUT:   %i64: type = class_type @Int, @Int(%int_64) [concrete]
 // CHECK:STDOUT:   %char: type = class_type @Char [concrete]
 // CHECK:STDOUT:   %ptr.b46: type = ptr_type %char [concrete]
 // CHECK:STDOUT:   %pattern_type.adf: type = pattern_type %str.3d4 [concrete]
 // CHECK:STDOUT:   %a.patt: %pattern_type.adf = value_binding_pattern a [concrete]
 // CHECK:STDOUT:   %str.d76: %ptr.b46 = string_literal "abc" [concrete]
-// CHECK:STDOUT:   %int_3: %u64 = int_value 3 [concrete]
-// CHECK:STDOUT:   %String.val.6e1: %str.3d4 = struct_value (%str.d76, %int_3) [concrete]
+// CHECK:STDOUT:   %int_3: %i64 = int_value 3 [concrete]
+// CHECK:STDOUT:   %String.val.2dc: %str.3d4 = struct_value (%str.d76, %int_3) [concrete]
 // CHECK:STDOUT:   %b.patt: %pattern_type.adf = value_binding_pattern b [concrete]
 // CHECK:STDOUT:   %str.3d5: %ptr.b46 = string_literal "" [concrete]
-// CHECK:STDOUT:   %int_0: %u64 = int_value 0 [concrete]
-// CHECK:STDOUT:   %String.val.9fd: %str.3d4 = struct_value (%str.3d5, %int_0) [concrete]
+// CHECK:STDOUT:   %int_0: %i64 = int_value 0 [concrete]
+// CHECK:STDOUT:   %String.val.b11: %str.3d4 = struct_value (%str.3d5, %int_0) [concrete]
 // CHECK:STDOUT:   %c.patt: %pattern_type.adf = value_binding_pattern c [concrete]
 // CHECK:STDOUT:   %str.4fa: %ptr.b46 = string_literal " \t " [concrete]
-// CHECK:STDOUT:   %String.val.86a: %str.3d4 = struct_value (%str.4fa, %int_3) [concrete]
+// CHECK:STDOUT:   %String.val.5de: %str.3d4 = struct_value (%str.4fa, %int_3) [concrete]
 // CHECK:STDOUT:   %d.patt: %pattern_type.adf = value_binding_pattern d [concrete]
 // CHECK:STDOUT:   %str.475: %ptr.b46 = string_literal "ab" [concrete]
-// CHECK:STDOUT:   %int_2: %u64 = int_value 2 [concrete]
-// CHECK:STDOUT:   %String.val.efd: %str.3d4 = struct_value (%str.475, %int_2) [concrete]
+// CHECK:STDOUT:   %int_2: %i64 = int_value 2 [concrete]
+// CHECK:STDOUT:   %String.val.c5e: %str.3d4 = struct_value (%str.475, %int_2) [concrete]
 // CHECK:STDOUT:   %e.patt: %pattern_type.adf = value_binding_pattern e [concrete]
 // CHECK:STDOUT:   %str.434: %ptr.b46 = string_literal " foo: \"bar\" { 123 } " [concrete]
-// CHECK:STDOUT:   %int_20: %u64 = int_value 20 [concrete]
-// CHECK:STDOUT:   %String.val.70a: %str.3d4 = struct_value (%str.434, %int_20) [concrete]
+// CHECK:STDOUT:   %int_20: %i64 = int_value 20 [concrete]
+// CHECK:STDOUT:   %String.val.593: %str.3d4 = struct_value (%str.434, %int_20) [concrete]
 // CHECK:STDOUT:   %f.patt: %pattern_type.adf = value_binding_pattern f [concrete]
 // CHECK:STDOUT:   %str.fa3: %ptr.b46 = string_literal "\xD0\xB0\xD0\xB1\xD0\xB2" [concrete]
-// CHECK:STDOUT:   %int_6: %u64 = int_value 6 [concrete]
-// CHECK:STDOUT:   %String.val.711: %str.3d4 = struct_value (%str.fa3, %int_6) [concrete]
+// CHECK:STDOUT:   %int_6: %i64 = int_value 6 [concrete]
+// CHECK:STDOUT:   %String.val.7f0: %str.3d4 = struct_value (%str.fa3, %int_6) [concrete]
 // CHECK:STDOUT:   %g.patt: %pattern_type.adf = value_binding_pattern g [concrete]
 // CHECK:STDOUT:   %str.618: %ptr.b46 = string_literal "xy" [concrete]
-// CHECK:STDOUT:   %String.val.fce: %str.3d4 = struct_value (%str.618, %int_2) [concrete]
+// CHECK:STDOUT:   %String.val.41b: %str.3d4 = struct_value (%str.618, %int_2) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -1424,7 +1424,7 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Cpp.ref.loc8: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %SimpleString.ref: %str.3d4 = name_ref SimpleString, %String.val.1 [concrete = constants.%String.val.6e1]
+// CHECK:STDOUT:   %SimpleString.ref: %str.3d4 = name_ref SimpleString, %String.val.1 [concrete = constants.%String.val.2dc]
 // CHECK:STDOUT:   %str.loc8: type = type_literal constants.%str.3d4 [concrete = constants.%str.3d4]
 // CHECK:STDOUT:   %a: %str.3d4 = wrapper_binding a, %SimpleString.ref
 // CHECK:STDOUT:   name_binding_decl {
@@ -1432,7 +1432,7 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Cpp.ref.loc9: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %EmptyString.ref: %str.3d4 = name_ref EmptyString, %String.val.2 [concrete = constants.%String.val.9fd]
+// CHECK:STDOUT:   %EmptyString.ref: %str.3d4 = name_ref EmptyString, %String.val.2 [concrete = constants.%String.val.b11]
 // CHECK:STDOUT:   %str.loc9: type = type_literal constants.%str.3d4 [concrete = constants.%str.3d4]
 // CHECK:STDOUT:   %b: %str.3d4 = wrapper_binding b, %EmptyString.ref
 // CHECK:STDOUT:   name_binding_decl {
@@ -1440,7 +1440,7 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Cpp.ref.loc10: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %EscapeCharacter.ref: %str.3d4 = name_ref EscapeCharacter, %String.val.3 [concrete = constants.%String.val.86a]
+// CHECK:STDOUT:   %EscapeCharacter.ref: %str.3d4 = name_ref EscapeCharacter, %String.val.3 [concrete = constants.%String.val.5de]
 // CHECK:STDOUT:   %str.loc10: type = type_literal constants.%str.3d4 [concrete = constants.%str.3d4]
 // CHECK:STDOUT:   %c: %str.3d4 = wrapper_binding c, %EscapeCharacter.ref
 // CHECK:STDOUT:   name_binding_decl {
@@ -1448,7 +1448,7 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Cpp.ref.loc11: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %Concatenated.ref: %str.3d4 = name_ref Concatenated, %String.val.4 [concrete = constants.%String.val.efd]
+// CHECK:STDOUT:   %Concatenated.ref: %str.3d4 = name_ref Concatenated, %String.val.4 [concrete = constants.%String.val.c5e]
 // CHECK:STDOUT:   %str.loc11: type = type_literal constants.%str.3d4 [concrete = constants.%str.3d4]
 // CHECK:STDOUT:   %d: %str.3d4 = wrapper_binding d, %Concatenated.ref
 // CHECK:STDOUT:   name_binding_decl {
@@ -1456,7 +1456,7 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Cpp.ref.loc12: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %RawString.ref: %str.3d4 = name_ref RawString, %String.val.5 [concrete = constants.%String.val.70a]
+// CHECK:STDOUT:   %RawString.ref: %str.3d4 = name_ref RawString, %String.val.5 [concrete = constants.%String.val.593]
 // CHECK:STDOUT:   %str.loc12: type = type_literal constants.%str.3d4 [concrete = constants.%str.3d4]
 // CHECK:STDOUT:   %e: %str.3d4 = wrapper_binding e, %RawString.ref
 // CHECK:STDOUT:   name_binding_decl {
@@ -1464,7 +1464,7 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Cpp.ref.loc13: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %Utf8String.ref: %str.3d4 = name_ref Utf8String, %String.val.6 [concrete = constants.%String.val.711]
+// CHECK:STDOUT:   %Utf8String.ref: %str.3d4 = name_ref Utf8String, %String.val.6 [concrete = constants.%String.val.7f0]
 // CHECK:STDOUT:   %str.loc13: type = type_literal constants.%str.3d4 [concrete = constants.%str.3d4]
 // CHECK:STDOUT:   %f: %str.3d4 = wrapper_binding f, %Utf8String.ref
 // CHECK:STDOUT:   name_binding_decl {
@@ -1472,7 +1472,7 @@ let _: C(1) = {} as C(Cpp.COUNT);
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Cpp.ref.loc14: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %Indirect.ref: %str.3d4 = name_ref Indirect, %String.val.7 [concrete = constants.%String.val.fce]
+// CHECK:STDOUT:   %Indirect.ref: %str.3d4 = name_ref Indirect, %String.val.7 [concrete = constants.%String.val.41b]
 // CHECK:STDOUT:   %str.loc14: type = type_literal constants.%str.3d4 [concrete = constants.%str.3d4]
 // CHECK:STDOUT:   %g: %str.3d4 = wrapper_binding g, %Indirect.ref
 // CHECK:STDOUT:   name_binding_decl {
diff --git a/toolchain/check/testdata/operators/overloaded/string_indexing.carbon b/toolchain/check/testdata/operators/overloaded/string_indexing.carbon
index 01b3c9a4a543..9364e63dce5f 100644
--- a/toolchain/check/testdata/operators/overloaded/string_indexing.carbon
+++ b/toolchain/check/testdata/operators/overloaded/string_indexing.carbon
@@ -83,11 +83,11 @@ fn TestStringIndexing() {
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
 // CHECK:STDOUT:   %str.3d4: type = class_type @String [concrete]
 // CHECK:STDOUT:   %int_64: Core.IntLiteral = int_value 64 [concrete]
-// CHECK:STDOUT:   %u64: type = class_type @UInt, @UInt(%int_64) [concrete]
+// CHECK:STDOUT:   %i64: type = class_type @Int, @Int(%int_64) [concrete]
 // CHECK:STDOUT:   %char: type = class_type @Char [concrete]
 // CHECK:STDOUT:   %ptr.b46: type = ptr_type %char [concrete]
 // CHECK:STDOUT:   %str.4f8: %ptr.b46 = string_literal "Test" [concrete]
-// CHECK:STDOUT:   %int_4: %u64 = int_value 4 [concrete]
+// CHECK:STDOUT:   %int_4: %i64 = int_value 4 [concrete]
 // CHECK:STDOUT:   %String.val: %str.3d4 = struct_value (%str.4f8, %int_4) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -95,7 +95,7 @@ fn TestStringIndexing() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT:   %str: %ptr.b46 = string_literal "Test" [concrete = constants.%str.4f8]
-// CHECK:STDOUT:   %int_4: %u64 = int_value 4 [concrete = constants.%int_4]
+// CHECK:STDOUT:   %int_4: %i64 = int_value 4 [concrete = constants.%int_4]
 // CHECK:STDOUT:   %String.val: %str.3d4 = struct_value (%str, %int_4) [concrete = constants.%String.val]
 // CHECK:STDOUT:   %C.ref: type = name_ref C, %C.decl [concrete = constants.%C]
 // CHECK:STDOUT:   <elided>
@@ -109,14 +109,13 @@ fn TestStringIndexing() {
 // CHECK:STDOUT:   %c.patt: %pattern_type.2ff = value_binding_pattern c [concrete]
 // CHECK:STDOUT:   %str.3d4: type = class_type @String [concrete]
 // CHECK:STDOUT:   %int_64: Core.IntLiteral = int_value 64 [concrete]
-// CHECK:STDOUT:   %u64: type = class_type @UInt, @UInt(%int_64) [concrete]
+// CHECK:STDOUT:   %i64: type = class_type @Int, @Int(%int_64) [concrete]
 // CHECK:STDOUT:   %ptr.b46: type = ptr_type %char [concrete]
 // CHECK:STDOUT:   %str.4f8: %ptr.b46 = string_literal "Test" [concrete]
-// CHECK:STDOUT:   %int_4: %u64 = int_value 4 [concrete]
+// CHECK:STDOUT:   %int_4: %i64 = int_value 4 [concrete]
 // CHECK:STDOUT:   %String.val: %str.3d4 = struct_value (%str.4f8, %int_4) [concrete]
 // CHECK:STDOUT:   %int_0: Core.IntLiteral = int_value 0 [concrete]
 // CHECK:STDOUT:   %IndexWith.type.385: type = facet_type <@IndexWith, @IndexWith(Core.IntLiteral)> [concrete]
-// CHECK:STDOUT:   %i64: type = class_type @Int, @Int(%int_64) [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.221: type = facet_type <@ImplicitAs, @ImplicitAs(%i64)> [concrete]
 // CHECK:STDOUT:   %T.2f3: %ImplicitAs.type.221 = symbolic_binding T, 0 [symbolic]
 // CHECK:STDOUT:   %str.as.IndexWith.impl.At.type.62e: type = fn_type @str.as.IndexWith.impl.At, @str.as.IndexWith.impl(%T.2f3) [symbolic]
@@ -149,7 +148,7 @@ fn TestStringIndexing() {
 // CHECK:STDOUT: fn @TestStringIndexing() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %str.loc5: %ptr.b46 = string_literal "Test" [concrete = constants.%str.4f8]
-// CHECK:STDOUT:   %int_4.loc5: %u64 = int_value 4 [concrete = constants.%int_4]
+// CHECK:STDOUT:   %int_4.loc5: %i64 = int_value 4 [concrete = constants.%int_4]
 // CHECK:STDOUT:   %String.val.loc5: %str.3d4 = struct_value (%str.loc5, %int_4.loc5) [concrete = constants.%String.val]
 // CHECK:STDOUT:   %int_0: Core.IntLiteral = int_value 0 [concrete = constants.%int_0]
 // CHECK:STDOUT:   %impl.elem1.loc5: %.197 = impl_witness_access constants.%IndexWith.impl_witness.47e, element1 [concrete = constants.%str.as.IndexWith.impl.At.19c]
@@ -169,7 +168,7 @@ fn TestStringIndexing() {
 // CHECK:STDOUT:     %c.patt: %pattern_type.2ff = value_binding_pattern c [concrete = constants.%c.patt]
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %str.loc6: %ptr.b46 = string_literal "Test" [concrete = constants.%str.4f8]
-// CHECK:STDOUT:   %int_4.loc6: %u64 = int_value 4 [concrete = constants.%int_4]
+// CHECK:STDOUT:   %int_4.loc6: %i64 = int_value 4 [concrete = constants.%int_4]
 // CHECK:STDOUT:   %String.val.loc6: %str.3d4 = struct_value (%str.loc6, %int_4.loc6) [concrete = constants.%String.val]
 // CHECK:STDOUT:   %int_3: Core.IntLiteral = int_value 3 [concrete = constants.%int_3]
 // CHECK:STDOUT:   %impl.elem1.loc6: %.197 = impl_witness_access constants.%IndexWith.impl_witness.47e, element1 [concrete = constants.%str.as.IndexWith.impl.At.19c]
diff --git a/toolchain/lower/testdata/primitives/string.carbon b/toolchain/lower/testdata/primitives/string.carbon
index edffdcba7110..86d8d4b7a73f 100644
--- a/toolchain/lower/testdata/primitives/string.carbon
+++ b/toolchain/lower/testdata/primitives/string.carbon
@@ -41,29 +41,29 @@ fn Copy(s: str) -> str {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @0 = private unnamed_addr constant [6 x i8] c"Hello\00", align 1
 // CHECK:STDOUT: @1 = private unnamed_addr constant [6 x i8] c"World\00", align 1
-// CHECK:STDOUT: @String.val.75d.String.val = internal constant { ptr, i64 } { ptr @0, i64 5 }
-// CHECK:STDOUT: @String.val.2f1.String.val = internal constant { ptr, i64 } { ptr @1, i64 5 }
+// CHECK:STDOUT: @String.val.a16.String.val = internal constant { ptr, i64 } { ptr @0, i64 5 }
+// CHECK:STDOUT: @String.val.296.String.val = internal constant { ptr, i64 } { ptr @1, i64 5 }
 // CHECK:STDOUT:
 // CHECK:STDOUT: declare void @_CF.Main(ptr)
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define void @_CG.Main() #0 !dbg !4 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @_CF.Main(ptr @String.val.75d.String.val), !dbg !7
+// CHECK:STDOUT:   call void @_CF.Main(ptr @String.val.a16.String.val), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define void @_CH.Main() #0 !dbg !9 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @_CF.Main(ptr @String.val.75d.String.val), !dbg !10
+// CHECK:STDOUT:   call void @_CF.Main(ptr @String.val.a16.String.val), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define void @_CI.Main() #0 !dbg !12 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @_CF.Main(ptr @String.val.2f1.String.val), !dbg !13
+// CHECK:STDOUT:   call void @_CF.Main(ptr @String.val.296.String.val), !dbg !13
 // CHECK:STDOUT:   ret void, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT: