langchain-ai/langgraph · error · RuntimeError

Protocol command did not return a valid response.

Error message

Protocol command did not return a valid response.

What it means

Error "Protocol command did not return a valid response." thrown in langchain-ai/langgraph.

Source

Thrown at libs/sdk-py/langgraph_sdk/stream/transport/sync_http.py:59

        self._default_headers: dict[str, str] = dict(headers or {})
        self._closed = False
        self._open_responses: list[httpx.Response] = []

    def send_command(self, command: dict[str, Any]) -> dict[str, Any] | None:
        if self._closed:
            raise RuntimeError("Protocol transport is closed.")
        merged_headers = {**self._default_headers, "content-type": "application/json"}
        response = self._client.post(
            self._commands_url,
            content=orjson.dumps(command),
            headers=merged_headers,
        )
        response.raise_for_status()
        if response.status_code in (202, 204):
            return None
        payload = orjson.loads(response.content)
        if not isinstance(payload, dict) or "id" not in payload:
            raise RuntimeError("Protocol command did not return a valid response.")
        return payload

    def open_event_stream(self, params: dict[str, Any]) -> SyncEventStreamHandle:
        if self._closed:
            raise RuntimeError("Protocol transport is closed.")
        sse_headers = {
            **self._default_headers,
            "content-type": "application/json",
            "accept": "text/event-stream",
            "cache-control": "no-store",
        }
        request = self._client.build_request(
            "POST",
            self._stream_url,
            content=orjson.dumps(build_event_stream_body(params)),
            headers=sse_headers,
        )
        stream_cm = self._client.send(request, stream=True)

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/sdk-py/langgraph_sdk/stream/transport/sync_http.py:59 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/e43b3feb75fee228. Report an issue: GitHub.