diff --git a/backend/packages/harness/deerflow/sandbox/tools.py b/backend/packages/harness/deerflow/sandbox/tools.py
index af08f5402..6f86629f7 100644
--- a/backend/packages/harness/deerflow/sandbox/tools.py
+++ b/backend/packages/harness/deerflow/sandbox/tools.py
@@ -2268,7 +2268,9 @@ def str_replace_tool(
         with get_file_operation_lock(sandbox, path):
             content = sandbox.read_file(path)
             if not content:
-                return "OK"
+                if not old_str:
+                    return "OK"
+                return f"Error: String to replace not found in file: {requested_path}"
             if old_str not in content:
                 return f"Error: String to replace not found in file: {requested_path}"
             if replace_all:
diff --git a/backend/tests/test_str_replace_empty_file.py b/backend/tests/test_str_replace_empty_file.py
new file mode 100644
index 000000000..a68068d0e
--- /dev/null
+++ b/backend/tests/test_str_replace_empty_file.py
@@ -0,0 +1,54 @@
+"""str_replace tool behaviour on empty files.
+
+An empty file used to short-circuit to ``"OK"`` regardless of ``old_str``,
+so a real substring replacement silently "succeeded" without changing anything
+and without telling the model the target was missing. The fix only returns
+``"OK"`` on an empty file when ``old_str`` is itself empty (a no-op edit);
+a non-empty ``old_str`` now reports the string was not found.
+"""
+
+from pathlib import Path
+from types import SimpleNamespace
+
+from deerflow.sandbox.local.local_sandbox import LocalSandbox
+from deerflow.sandbox.tools import str_replace_tool
+
+
+def _local_runtime(tmp_path: Path) -> SimpleNamespace:
+    for sub in ("workspace", "uploads", "outputs"):
+        (tmp_path / sub).mkdir(parents=True, exist_ok=True)
+    thread_data = {
+        "workspace_path": str(tmp_path / "workspace"),
+        "uploads_path": str(tmp_path / "uploads"),
+        "outputs_path": str(tmp_path / "outputs"),
+    }
+    return SimpleNamespace(
+        state={"sandbox": {"sandbox_id": "local:t1"}, "thread_data": thread_data},
+        context={"thread_id": "t1"},
+    )
+
+
+def _str_replace(tmp_path, monkeypatch, *, old_str: str, new_str: str = "x") -> str:
+    runtime = _local_runtime(tmp_path)
+    (tmp_path / "outputs" / "empty.txt").write_text("", encoding="utf-8")
+    monkeypatch.setattr("deerflow.sandbox.tools.ensure_sandbox_initialized", lambda runtime: LocalSandbox("t1"))
+    monkeypatch.setattr("deerflow.sandbox.tools.ensure_thread_directories_exist", lambda runtime: None)
+    return str_replace_tool.func(
+        runtime=runtime,
+        description="replace in empty file",
+        path="/mnt/user-data/outputs/empty.txt",
+        old_str=old_str,
+        new_str=new_str,
+    )
+
+
+def test_empty_file_with_non_empty_old_str_reports_not_found(tmp_path, monkeypatch) -> None:
+    result = _str_replace(tmp_path, monkeypatch, old_str="something")
+    assert result.startswith("Error: String to replace not found in file")
+    assert "empty.txt" in result
+
+
+def test_empty_file_with_empty_old_str_returns_ok(tmp_path, monkeypatch) -> None:
+    # An empty old_str is a no-op edit and remains a benign "OK" on an empty file.
+    result = _str_replace(tmp_path, monkeypatch, old_str="")
+    assert result == "OK"