diff --git a/tsc/internal/diagnostics/diagnosticMessages.generated.json b/tsc/internal/diagnostics/diagnosticMessages.generated.json
index bf7f327e371c0..ae749e01b85b5 100644
--- a/tsc/internal/diagnostics/diagnosticMessages.generated.json
+++ b/tsc/internal/diagnostics/diagnosticMessages.generated.json
@@ -1900,7 +1900,7 @@
     "Duplicate_named_capturing_groups_are_only_available_when_targeting_0_or_later_18063": "Duplicate named capturing groups are only available when targeting '{0}' or later.",
     "Private_identifiers_cannot_be_used_in_destructuring_patterns_18064": "Private identifiers cannot be used in destructuring patterns.",
     "Content_mapper_file_extension_0_must_begin_with_a_18065": "Content mapper file extension '{0}' must begin with a '.'.",
-    "Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper_18066": "Content mapper file extension '{0}' is a built-in extension and cannot be registered by a content mapper.",
+    "Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension_18066": "Content mapper file extension '{0}' cannot be registered because it ends in a built-in extension.",
     "Content_mapper_file_extension_0_is_registered_by_more_than_one_content_mapper_18067": "Content mapper file extension '{0}' is registered by more than one content mapper.",
     "Content_mappers_require_the_runExternalCode_command_line_flag_to_be_enabled_18068": "Content mappers require the '--runExternalCode' command line flag to be enabled.",
     "The_content_mapper_0_failed_to_transform_this_file_18069": "The content mapper '{0}' failed to transform this file.",
diff --git a/tsc/internal/diagnostics/diagnosticMessages.json b/tsc/internal/diagnostics/diagnosticMessages.json
index 895ede2e4fa92..d4bf59ab839de 100644
--- a/tsc/internal/diagnostics/diagnosticMessages.json
+++ b/tsc/internal/diagnostics/diagnosticMessages.json
@@ -8720,7 +8720,7 @@
         "category": "Error",
         "code": 18065
     },
-    "Content mapper file extension '{0}' is a built-in extension and cannot be registered by a content mapper.": {
+    "Content mapper file extension '{0}' cannot be registered because it ends in a built-in extension.": {
         "category": "Error",
         "code": 18066
     },
diff --git a/tsc/internal/diagnostics/diagnostics_generated.go b/tsc/internal/diagnostics/diagnostics_generated.go
index fbd75c71bbe08..0e08cf23701b7 100644
--- a/tsc/internal/diagnostics/diagnostics_generated.go
+++ b/tsc/internal/diagnostics/diagnostics_generated.go
@@ -3804,7 +3804,7 @@ var Private_identifiers_cannot_be_used_in_destructuring_patterns = &Message{code
 
 var Content_mapper_file_extension_0_must_begin_with_a = &Message{code: 18065, category: CategoryError, key: "Content_mapper_file_extension_0_must_begin_with_a_18065", text: "Content mapper file extension '{0}' must begin with a '.'."}
 
-var Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper = &Message{code: 18066, category: CategoryError, key: "Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper_18066", text: "Content mapper file extension '{0}' is a built-in extension and cannot be registered by a content mapper."}
+var Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension = &Message{code: 18066, category: CategoryError, key: "Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension_18066", text: "Content mapper file extension '{0}' cannot be registered because it ends in a built-in extension."}
 
 var Content_mapper_file_extension_0_is_registered_by_more_than_one_content_mapper = &Message{code: 18067, category: CategoryError, key: "Content_mapper_file_extension_0_is_registered_by_more_than_one_content_mapper_18067", text: "Content mapper file extension '{0}' is registered by more than one content mapper."}
 
@@ -6364,7 +6364,7 @@ var allMessages = [...]**Message{
 	&Duplicate_named_capturing_groups_are_only_available_when_targeting_0_or_later,
 	&Private_identifiers_cannot_be_used_in_destructuring_patterns,
 	&Content_mapper_file_extension_0_must_begin_with_a,
-	&Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper,
+	&Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension,
 	&Content_mapper_file_extension_0_is_registered_by_more_than_one_content_mapper,
 	&Content_mappers_require_the_runExternalCode_command_line_flag_to_be_enabled,
 	&The_content_mapper_0_failed_to_transform_this_file,
diff --git a/tsc/internal/tsoptions/tsconfigparsing.go b/tsc/internal/tsoptions/tsconfigparsing.go
index f2ea2fdc07f73..0490a5542b4a7 100644
--- a/tsc/internal/tsoptions/tsconfigparsing.go
+++ b/tsc/internal/tsoptions/tsconfigparsing.go
@@ -1357,7 +1357,7 @@ func parseJsonConfigFileContentWorker(
 			case strings.ContainsAny(ext, "/\\"):
 				errors = append(errors, setContentMapperDiagnosticLocation(ast.NewCompilerDiagnostic(diagnostics.Content_mapper_extension_0_must_not_contain_path_separators, ext), contentMapperSourceFile, extNode))
 			case contentmapper.HasReservedSourceExtension(ext):
-				errors = append(errors, setContentMapperDiagnosticLocation(ast.NewCompilerDiagnostic(diagnostics.Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper, ext), contentMapperSourceFile, extNode))
+				errors = append(errors, setContentMapperDiagnosticLocation(ast.NewCompilerDiagnostic(diagnostics.Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension, ext), contentMapperSourceFile, extNode))
 			default:
 				if _, seen := seenContentMapperExtensions[canonicalExt]; seen {
 					errors = append(errors, setContentMapperDiagnosticLocation(ast.NewCompilerDiagnostic(diagnostics.Content_mapper_file_extension_0_is_registered_by_more_than_one_content_mapper, ext), contentMapperSourceFile, extNode))
diff --git a/tsc/internal/tsoptions/tsconfigparsing_test.go b/tsc/internal/tsoptions/tsconfigparsing_test.go
index c5f50dc795f43..1eb344588c20a 100644
--- a/tsc/internal/tsoptions/tsconfigparsing_test.go
+++ b/tsc/internal/tsoptions/tsconfigparsing_test.go
@@ -1458,7 +1458,7 @@ func TestContentMappersValidation(t *testing.T) {
 		{
 			name:           "built-in extension",
 			contentMappers: `[{ "package": "x", "extensions": [".ts"] }]`,
-			expectedCode:   diagnostics.Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper.Code(),
+			expectedCode:   diagnostics.Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension.Code(),
 		},
 		{
 			name:           "missing extensions",
@@ -1562,7 +1562,7 @@ func TestContentMapperExtensionValidationUsesHostCaseSensitivity(t *testing.T) {
 			name:            "built-in extension on case-insensitive host",
 			caseSensitivity: tspath.CaseInsensitive,
 			contentMappers:  `[{ "package": "mapper", "extensions": [".TS"] }]`,
-			expectedCode:    diagnostics.Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper.Code(),
+			expectedCode:    diagnostics.Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension.Code(),
 		},
 		{
 			name:            "duplicate extension on case-insensitive host",
@@ -1574,7 +1574,7 @@ func TestContentMapperExtensionValidationUsesHostCaseSensitivity(t *testing.T) {
 			name:            "built-in extension on case-sensitive host",
 			caseSensitivity: tspath.CaseSensitive,
 			contentMappers:  `[{ "package": "mapper", "extensions": [".TS"] }]`,
-			expectedCode:    diagnostics.Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper.Code(),
+			expectedCode:    diagnostics.Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension.Code(),
 		},
 		{
 			name:            "mapper extension casing is distinct on case-sensitive host",
@@ -1643,7 +1643,7 @@ func TestContentMapperCompoundExtensionValidation(t *testing.T) {
 					assert.DeepEqual(t, parsed.ContentMapperExtensions(), []string{extension})
 				} else {
 					assert.Equal(t, len(parsed.Errors), 1)
-					assert.Equal(t, parsed.Errors[0].Code(), diagnostics.Content_mapper_file_extension_0_is_a_built_in_extension_and_cannot_be_registered_by_a_content_mapper.Code())
+					assert.Equal(t, parsed.Errors[0].Code(), diagnostics.Content_mapper_file_extension_0_cannot_be_registered_because_it_ends_in_a_built_in_extension.Code())
 					assert.Equal(t, len(parsed.ContentMapperExtensions()), 0)
 				}
 			})