fix(patch): ignore inert context-only hunks (#63678)

tests/tools/test_patch_parser.py

@@ -401,6 +401,74 @@ class TestValidationPhase:
         assert result.success is True
         assert set(written.keys()) == {"a.py", "b.py"}
 
+    def test_context_only_hunk_does_not_reject_later_real_hunk(self):
+        patch = """\
+*** Begin Patch
+*** Update File: a.py
+@@ anchor @@
+ anchor
+@@ value @@
+-value = 1
++value = 2
+*** End Patch"""
+        ops, err = parse_v4a_patch(patch)
+        assert err is None
+
+        class FakeFileOps:
+            written = None
+            def read_file_raw(self, path):
+                return SimpleNamespace(content="anchor\nvalue = 1\n", error=None)
+            def write_file(self, path, content):
+                self.written = content
+                return SimpleNamespace(error=None)
+
+        file_ops = FakeFileOps()
+        result = apply_v4a_operations(ops, file_ops)
+        assert result.success is True
+        assert file_ops.written == "anchor\nvalue = 2\n"
+
+    def test_patch_with_only_context_hunks_reports_no_changes(self):
+        patch = """\
+*** Begin Patch
+*** Update File: a.py
+@@ anchor @@
+ anchor
+*** End Patch"""
+        ops, err = parse_v4a_patch(patch)
+        assert err is None
+
+        class FakeFileOps:
+            def read_file_raw(self, path):
+                return SimpleNamespace(content="anchor\n", error=None)
+            def write_file(self, path, content):
+                raise AssertionError("no-op patch must not write")
+
+        result = apply_v4a_operations(ops, FakeFileOps())
+        assert result.success is False
+        assert "no changes" in result.error.lower()
+
+    def test_validation_error_identifies_hunk_number(self):
+        patch = """\
+*** Begin Patch
+*** Update File: a.py
+@@ first @@
+-first = 1
++first = 2
+@@ missing @@
+-does_not_exist = 1
++does_not_exist = 2
+*** End Patch"""
+        ops, err = parse_v4a_patch(patch)
+        assert err is None
+
+        class FakeFileOps:
+            def read_file_raw(self, path):
+                return SimpleNamespace(content="first = 1\n", error=None)
+
+        result = apply_v4a_operations(ops, FakeFileOps())
+        assert result.success is False
+        assert "hunk 2" in result.error.lower()
+
 
 class TestApplyDelete:
     """Tests for _apply_delete producing a real unified diff."""

tools/patch_parser.py

@@ -253,8 +253,11 @@ def _validate_operations(
     from tools.fuzzy_match import fuzzy_find_and_replace
 
     errors: List[str] = []
+    real_change_count = 0
 
     for op in operations:
+        if op.operation != OperationType.UPDATE:
+            real_change_count += 1
         if op.operation == OperationType.UPDATE:
             read_result = file_ops.read_file_raw(op.file_path)
             if read_result.error:
@@ -262,8 +265,15 @@ def _validate_operations(
                 continue
 
             simulated = read_result.content
-            for hunk in op.hunks:
+            for hunk_index, hunk in enumerate(op.hunks, start=1):
                 search_lines = [l.content for l in hunk.lines if l.prefix in {' ', '-'}]
+                removed_lines = [l.content for l in hunk.lines if l.prefix == '-']
+                added_lines = [l.content for l in hunk.lines if l.prefix == '+']
+                if not removed_lines and not added_lines:
+                    # Models occasionally emit inert anchor hunks between real
+                    # changes. Ignore them without poisoning the atomic patch.
+                    continue
+                real_change_count += 1
                 if not search_lines:
                     # Addition-only hunk: validate context hint uniqueness
                     if hunk.context_hint:
@@ -291,7 +301,7 @@ def _validate_operations(
                 if count == 0:
                     label = f"'{hunk.context_hint}'" if hunk.context_hint else "(no hint)"
                     msg = (
-                        f"{op.file_path}: hunk {label} not found"
+                        f"{op.file_path}: hunk {hunk_index} {label} not found"
                         + (f" — {match_error}" if match_error else "")
                     )
                     try:
@@ -325,6 +335,9 @@ def _validate_operations(
 
         # ADD: parent directory creation handled by write_file; no pre-check needed.
 
+    if not errors and real_change_count == 0:
+        errors.append("Patch contains no changes (only context lines were provided)")
+
     return errors
 
 
@@ -545,6 +558,8 @@ def _apply_update(op: PatchOperation, file_ops: Any) -> Tuple[bool, str, Optiona
             elif line.prefix == '+':
                 replace_lines.append(line.content)
 
+        if search_lines and search_lines == replace_lines:
+            continue
         if search_lines:
             search_pattern = '\n'.join(search_lines)
             replacement = '\n'.join(replace_lines)