diff --git a/.changeset/lowercase-edit-error-message.md b/.changeset/lowercase-edit-error-message.md
new file mode 100644
index 0000000000..b1a9b32570
--- /dev/null
+++ b/.changeset/lowercase-edit-error-message.md
@@ -0,0 +1,6 @@
+---
+"@moonshot-ai/agent-core": patch
+"@moonshot-ai/kimi-code": patch
+---
+
+Lowercase the stale file content message in edit tool errors.
\ No newline at end of file
diff --git a/packages/agent-core/src/tools/builtin/file/edit.ts b/packages/agent-core/src/tools/builtin/file/edit.ts
index 5c4b32a468..a5c77a2a42 100644
--- a/packages/agent-core/src/tools/builtin/file/edit.ts
+++ b/packages/agent-core/src/tools/builtin/file/edit.ts
@@ -117,7 +117,7 @@ export class EditTool implements BuiltinTool<EditInput> {
         }
 
         if (count === 0) {
-          return { isError: true, output: `old_string not found in ${args.path}, The file contents may be out of date. Please use the Read Tool to reload the content.
+          return { isError: true, output: `old_string not found in ${args.path}, the file contents may be out of date. Please use the Read Tool to reload the content.
 ` };
         }
         if (count > 1) {
@@ -140,7 +140,7 @@ export class EditTool implements BuiltinTool<EditInput> {
       const parts = content.split(args.old_string);
       const replacementCount = parts.length - 1;
       if (replacementCount === 0) {
-        return { isError: true, output: `old_string not found in ${args.path}, The file contents may be out of date. Please use the Read Tool to reload the content.
+        return { isError: true, output: `old_string not found in ${args.path}, the file contents may be out of date. Please use the Read Tool to reload the content.
 ` };
       }