fix(tools): clarify identical old and new string error

tests/tools/test_file_tools_live.py

@@ -179,6 +179,19 @@ class TestPatchReplace:
         assert Path(path).read_text() == "hello earth\n"
 
 
+    def test_identical_replacement_explains_no_change(self, ops, tmp_path):
+        path = str(tmp_path / "unchanged.txt")
+        Path(path).write_text("hello world\n")
+
+        result = ops.patch_replace(path, "world", "world")
+
+        assert result.success is False
+        assert result.error is not None
+        assert "No edit was applied" in result.error
+        assert "existing text to replace in old_string" in result.error
+        assert "replacement text in new_string" in result.error
+        assert Path(path).read_text() == "hello world\n"
+
     def test_multiline_patch(self, ops, tmp_path):
         path = str(tmp_path / "multi.txt")
         Path(path).write_text("line1\nline2\nline3\n")

tests/tools/test_fuzzy_match.py

@@ -25,6 +25,15 @@ class TestExactMatch:
         assert count == 0
         assert err is not None
 
+    def test_identical_strings(self):
+        new, count, _, err = fuzzy_find_and_replace("abc", "abc", "abc")
+        assert count == 0
+        assert new == "abc"
+        assert err == (
+            "No edit was applied because old_string and new_string are identical. "
+            "Provide the existing text to replace in old_string and the changed "
+            "replacement text in new_string."
+        )
 
     def test_multiline_exact(self):
         content = "line1\nline2\nline3"
@@ -431,6 +440,28 @@ class TestFormatNoMatchHint:
         )
         assert result == ""
 
+    def test_silent_on_identical_strings(self):
+        """old_string == new_string — hint irrelevant."""
+        result = self.fmt(
+            "No edit was applied because old_string and new_string are identical. "
+            "Provide the existing text to replace in old_string and the changed "
+            "replacement text in new_string.",
+            0, "foo", "foo bar\n",
+        )
+        assert result == ""
+
+    def test_silent_when_match_count_nonzero(self):
+        """If match succeeded, we shouldn't be in the error path — defense in depth."""
+        result = self.fmt(
+            "Could not find a match for old_string in the file",
+            1, "foo", "foo bar\n",
+        )
+        assert result == ""
+
+    def test_silent_on_none_error(self):
+        """No error at all — no hint."""
+        result = self.fmt(None, 0, "foo", "bar\n")
+        assert result == ""
 
     def test_silent_when_no_similar_content(self):
         """Even for a valid no-match error, skip hint when nothing similar exists."""

tools/fuzzy_match.py

@@ -143,7 +143,11 @@ def fuzzy_find_and_replace(content: str, old_string: str, new_string: str,
         return content, 0, None, "old_string is only whitespace — provide non-blank text to match"
 
     if old_string == new_string:
-        return content, 0, None, "old_string and new_string are identical"
+        return content, 0, None, (
+            "No edit was applied because old_string and new_string are identical. "
+            "Provide the existing text to replace in old_string and the changed "
+            "replacement text in new_string."
+        )
 
     # Try each matching strategy in order
     strategies: List[Tuple[str, Callable]] = [