rootkiller6788 · Aug 30, 2026 · Aug 30, 2026
diff --git a/src/kimi_cli/tools/file/replace.py b/src/kimi_cli/tools/file/replace.py
@@ -134,6 +134,18 @@ async def __call__(self, params: Params) -> ToolReturnValue:
             original_content = content
             edits = [params.edit] if isinstance(params.edit, Edit) else params.edit
 
+            # An empty `old` makes str.replace() insert `new` at every position
+            # instead of matching nothing, silently mangling the file. Reject it.
+            for edit in edits:
+                if not edit.old:
+                    return ToolError(
+                        message=(
+                            "The `old` string cannot be empty. "
+                            "Give the exact text to replace, or use WriteFile to insert text."
+                        ),
+                        brief="Empty old string",
+                    )
+
             # Apply all edits
             for edit in edits:
                 content = self._apply_edit(content, edit)

diff --git a/tests/tools/test_str_replace_file.py b/tests/tools/test_str_replace_file.py
@@ -246,3 +246,56 @@ async def test_replace_empty_strings(
     assert not result.is_error
     assert "successfully edited" in result.message
     assert await file_path.read_text() == "Hello !"
+
+
+async def test_replace_rejects_empty_old_string(
+    str_replace_file_tool: StrReplaceFile, temp_work_dir: KaosPath
+):
+    """An empty `old` would silently insert `new` via str.replace() rather than
+    match nothing, so it must be rejected instead of corrupting the file."""
+    file_path = temp_work_dir / "test.txt"
+    original_content = "Hello world!"
+    await file_path.write_text(original_content)
+
+    result = await str_replace_file_tool(
+        Params(path=str(file_path), edit=Edit(old="", new="X"))
+    )
+
+    assert result.is_error
+    assert "old" in result.message and "empty" in result.message
+    assert await file_path.read_text() == original_content  # Content unchanged
+
+
+async def test_replace_rejects_empty_old_string_replace_all(
+    str_replace_file_tool: StrReplaceFile, temp_work_dir: KaosPath
+):
+    """Same guard applies when replace_all is set."""
+    file_path = temp_work_dir / "test.txt"
+    original_content = "Hello world!"
+    await file_path.write_text(original_content)
+
+    result = await str_replace_file_tool(
+        Params(path=str(file_path), edit=Edit(old="", new="X", replace_all=True))
+    )
+
+    assert result.is_error
+    assert await file_path.read_text() == original_content  # Content unchanged
+
+
+async def test_replace_rejects_empty_old_string_in_list(
+    str_replace_file_tool: StrReplaceFile, temp_work_dir: KaosPath
+):
+    """An empty `old` anywhere in a list of edits aborts the whole call."""
+    file_path = temp_work_dir / "test.txt"
+    original_content = "Hello world!"
+    await file_path.write_text(original_content)
+
+    result = await str_replace_file_tool(
+        Params(
+            path=str(file_path),
+            edit=[Edit(old="Hello", new="Hi"), Edit(old="", new="X")],
+        )
+    )
+
+    assert result.is_error
+    assert await file_path.read_text() == original_content  # Content unchanged