From b4d00ab4fb338631de039dc445936f7af2c5b79e Mon Sep 17 00:00:00 2001
From: Wen Sun <iwendellsun@gmail.com>
Date: Tue, 22 Jul 2025 06:24:02 +0900
Subject: [PATCH] feat(edit): Prevent no-op edits (#3520)

---
 packages/core/src/tools/edit.test.ts | 13 +++++++++++++
 packages/core/src/tools/edit.ts      |  5 +++++
 2 files changed, 18 insertions(+)

diff --git a/packages/core/src/tools/edit.test.ts b/packages/core/src/tools/edit.test.ts
index 84ad1daf..4ff33ff4 100644
--- a/packages/core/src/tools/edit.test.ts
+++ b/packages/core/src/tools/edit.test.ts
@@ -608,6 +608,19 @@ describe('EditTool', () => {
         /User modified the `new_string` content/,
       );
     });
+
+    it('should return error if old_string and new_string are identical', async () => {
+      const initialContent = 'This is some identical text.';
+      fs.writeFileSync(filePath, initialContent, 'utf8');
+      const params: EditToolParams = {
+        file_path: filePath,
+        old_string: 'identical',
+        new_string: 'identical',
+      };
+      const result = await tool.execute(params, new AbortController().signal);
+      expect(result.llmContent).toMatch(/No changes to apply/);
+      expect(result.returnDisplay).toMatch(/No changes to apply/);
+    });
   });
 
   describe('getDescription', () => {
diff --git a/packages/core/src/tools/edit.ts b/packages/core/src/tools/edit.ts
index ccba3d72..87a2ea44 100644
--- a/packages/core/src/tools/edit.ts
+++ b/packages/core/src/tools/edit.ts
@@ -246,6 +246,11 @@ Expectation for required parameters:
           display: `Failed to edit, expected ${expectedReplacements} ${occurenceTerm} but found ${occurrences}.`,
           raw: `Failed to edit, Expected ${expectedReplacements} ${occurenceTerm} but found ${occurrences} for old_string in file: ${params.file_path}`,
         };
+      } else if (finalOldString === finalNewString) {
+        error = {
+          display: `No changes to apply. The old_string and new_string are identical.`,
+          raw: `No changes to apply. The old_string and new_string are identical in file: ${params.file_path}`,
+        };
       }
     } else {
       // Should not happen if fileExists and no exception was thrown, but defensively: