langchain-ai/langgraph · error · RuntimeError

Protocol error [{code}]: {message}

Error message

Protocol error [{code}]: {message}

What it means

Error "Protocol error [{code}]: {message}" thrown in langchain-ai/langgraph.

Source

Thrown at libs/sdk-py/langgraph_sdk/_sync/stream.py:1482

        finally:
            self._unregister_subscription(sub.id)

    def _send_command(self, method: str, params: dict[str, Any]) -> dict[str, Any]:
        """Send a protocol command and return the `result` payload."""
        if self._transport is None:
            raise RuntimeError("SyncThreadStream not entered — use `with`.")
        with self._command_id_lock:
            command_id = self._next_command_id
            self._next_command_id += 1
        response = self._transport.send_command(
            {"id": command_id, "method": method, "params": params}
        )
        if response is None:
            return {}
        if response.get("type") == "error":
            code = response.get("error", "unknown")
            message = response.get("message", "")
            raise RuntimeError(f"Protocol error [{code}]: {message}")
        meta = response.get("meta")
        if isinstance(meta, dict):
            applied_through_seq = meta.get("applied_through_seq")
            if self._controller is not None:
                self._controller.observe_applied_through_seq(applied_through_seq)
        return response.get("result", {})

    def _ensure_lifecycle_watcher_running(self) -> None:
        if self._lifecycle_watcher_thread is not None:
            return
        self._lifecycle_watcher_thread = threading.Thread(
            target=self._run_lifecycle_watcher,
            name="langgraph-sdk-sync-lifecycle",
            daemon=True,
        )
        self._lifecycle_watcher_thread.start()

    def _observe_lifecycle_event(self, event: Event) -> None:

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/sdk-py/langgraph_sdk/_sync/stream.py:1482 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of langchain-ai/langgraph@38031739e5 (2026-08-26). Data as JSON: /api/errors/abd841187db9adb6. Report an issue: GitHub.