What causes "No response body" in mastra-ai/mastra?

Ensure the server's thread subscribe endpoint streams data (test with curl -N).

No response body

Error message

No response body

What it means

In agent.ts subscribeToThread(), after subscribing to a thread's stream the client checks streamResponse.body and throws 'No response body' if it is missing, since the returned subscription object needs a ReadableStream to process agent thread data chunks. This is an early guard before wiring up processDataStream and abort handlers.

Source

Thrown at client-sdks/client-js/src/resources/agent.ts:642

      unsubscribe: () => void;
    }
  > {
    const { resourceId, threadId } = params;
    const requestSubscription = () =>
      this.request(`/agents/${this.agentId}/threads/subscribe`, {
        method: 'POST',
        body: { resourceId, threadId },
        stream: true,
      }) as Promise<Response>;

    const streamResponse = (await requestSubscription()) as Response & {
      processDataStream: (options: ProcessAgentThreadStreamOptions) => Promise<void>;
      abort: () => Promise<boolean>;
      unsubscribe: () => void;
    };

    if (!streamResponse.body) {
      throw new Error('No response body');
    }

    const agent = this;
    streamResponse.abort = async () => (await agent.abortThread({ resourceId, threadId })).aborted;

    let unsubscribed = false;
    let processAbortController: AbortController | undefined;
    let processStarted = false;

    streamResponse.unsubscribe = () => {
      if (unsubscribed) return;
      unsubscribed = true;
      processAbortController?.abort();
      if (!processStarted) {
        void streamResponse.body?.cancel().catch(() => {});
      }
    };

View on GitHub (pinned to 75dd419e61)

Solutions

Ensure the server's thread subscribe endpoint streams data (test with curl -N).
Fix test mocks to supply a ReadableStream body.
Upgrade to an environment with native streaming fetch (modern browsers, Node 18+).
Check proxies/CDNs (e.g. buffering middleware) aren't stripping the stream.
Try/catch around subscribeToThread and fall back to polling thread messages.

Example fix

// before
const sub = await agent.subscribeToThread({ resourceId, threadId });
// after
try {
  const sub = await agent.subscribeToThread({ resourceId, threadId });
} catch (e) {
  if ((e as Error).message === 'No response body') {
    console.error('Subscribe returned no body — falling back to polling');
  }
  throw e;
}

Defensive patterns

Strategy: try-catch

Validate before calling

const res = await fetch(subscribeUrl);
if (res.ok && !res.body) throw new Error('Thread subscribe returns no body in this environment');

Type guard

function hasBody(res: Response): res is Response & { body: ReadableStream<Uint8Array> } { return res.body != null; }

Try / catch

try {
  const sub = await agent.subscribeToThread({ resourceId, threadId });
} catch (err) {
  if ((err as Error).message === 'No response body') {
    console.error('Subscribe got no stream body — check mocks/proxies, consider polling thread messages');
  }
  throw err;
}

Prevention

Mock subscribe endpoints with ReadableStream bodies
Verify subscribe endpoint streams in the target environment
Avoid CDN/proxy buffering on SSE routes
Provide a polling fallback for thread messages

When it happens

Trigger: The thread subscription request returns ok but body is null: mocks without a body, buffering infrastructure on the subscribe route, or a fetch implementation without streaming bodies.

Common situations: Frontend tests mocking the thread subscribe endpoint with only { ok: true }; gateways/CDNs buffering the SSE stream to empty; older polyfilled fetch in legacy browsers or constrained runtimes.

Related errors

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/54e62c389daee9ca. Report an issue: GitHub.