bug(core): Fix for "no changes" edits. (#6836)

packages/core/src/tools/edit.test.ts

@@ -632,6 +632,33 @@ describe('EditTool', () => {
       expect(result.llmContent).toMatch(/No changes to apply/);
       expect(result.returnDisplay).toMatch(/No changes to apply/);
     });
+
+    it('should return EDIT_NO_CHANGE error if replacement results in identical content', async () => {
+      // This can happen if ensureCorrectEdit finds a fuzzy match, but the literal
+      // string replacement with `replaceAll` results in no change.
+      const initialContent = 'line 1\nline  2\nline 3'; // Note the double space
+      fs.writeFileSync(filePath, initialContent, 'utf8');
+      const params: EditToolParams = {
+        file_path: filePath,
+        // old_string has a single space, so it won't be found by replaceAll
+        old_string: 'line 1\nline 2\nline 3',
+        new_string: 'line 1\nnew line 2\nline 3',
+      };
+
+      // Mock ensureCorrectEdit to simulate it finding a match (e.g., via fuzzy matching)
+      // but it doesn't correct the old_string to the literal content.
+      mockEnsureCorrectEdit.mockResolvedValueOnce({ params, occurrences: 1 });
+
+      const invocation = tool.build(params);
+      const result = await invocation.execute(new AbortController().signal);
+
+      expect(result.error?.type).toBe(ToolErrorType.EDIT_NO_CHANGE);
+      expect(result.returnDisplay).toMatch(
+        /No changes to apply. The new content is identical to the current content./,
+      );
+      // Ensure the file was not actually changed
+      expect(fs.readFileSync(filePath, 'utf8')).toBe(initialContent);
+    });
   });
 
   describe('Error Scenarios', () => {

packages/core/src/tools/edit.ts

@@ -204,12 +204,23 @@ class EditToolInvocation implements ToolInvocation<EditToolParams, ToolResult> {
       };
     }
 
-    const newContent = applyReplacement(
-      currentContent,
-      finalOldString,
-      finalNewString,
-      isNewFile,
-    );
+    const newContent = !error
+      ? applyReplacement(
+          currentContent,
+          finalOldString,
+          finalNewString,
+          isNewFile,
+        )
+      : (currentContent ?? '');
+
+    if (!error && fileExists && currentContent === newContent) {
+      error = {
+        display:
+          'No changes to apply. The new content is identical to the current content.',
+        raw: `No changes to apply. The new content is identical to the current content in file: ${params.file_path}`,
+        type: ToolErrorType.EDIT_NO_CHANGE,
+      };
+    }
 
     return {
       currentContent,