vercel/ai · error · Error

The response body is empty.

Error message

The response body is empty.

What it means

The Svelte structured-object client checks response.body after a successful status; if the response has no readable stream body it throws 'The response body is empty.'. The library expects a streamed JSON response to incrementally parse into a partial object.

Source

Thrown at packages/svelte/src/structured-object.svelte.ts:178

      const response = await actualFetch(this.#options.api, {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
          ...this.#options.headers,
        },
        credentials: this.#options.credentials,
        signal: abortController.signal,
        body: JSON.stringify(input),
      });

      if (!response.ok) {
        throw new Error(
          (await response.text()) ?? 'Failed to fetch the response.',
        );
      }

      if (response.body == null) {
        throw new Error('The response body is empty.');
      }

      let accumulatedText = '';
      let latestObject: DeepPartial<RESULT> | undefined = undefined;

      await response.body.pipeThrough(new TextDecoderStream()).pipeTo(
        new WritableStream<string>({
          write: async chunk => {
            if (abortController?.signal.aborted) {
              throw new DOMException('Stream aborted', 'AbortError');
            }
            accumulatedText += chunk;

            const { value } = await parsePartialJson(accumulatedText);
            const currentObject = value as DeepPartial<RESULT>;

            if (!isDeepEqualData(latestObject, currentObject)) {
              latestObject = currentObject;

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Make the API endpoint return a streamed body (200 with content).
2. Check for 204/redirect responses from your route and fix the handler.
3. Run in a runtime that supports streaming Response.body (modern browsers/Node 18+).
4. Add a server-side check that the structured-generation handler actually streams the result.

Example fix

// before (endpoint)
return new Response(null, { status: 204 });
// after
return new Response(JSON.stringify(resultStream), { status: 200 });

// server-side guard
export function POST() {
  const body = buildStructuredStream();
  if (!body) return new Response('no body', { status: 500 });
  return new Response(body, { status: 200 });
}

null

try {
  startGeneration();
} catch (e) {
  if (e?.message === 'The response body is empty.') {
    console.error('Endpoint returned no stream body — check the API handler.');
  } else throw e;
}

When it happens

Trigger: The fetch resolves with !response.ok false but response.body === null — e.g. HTTP 204 responses, opaque/no-store contexts, or environments without streaming response bodies (some runtimes/polyfills).

Common situations: Endpoint returns 204 No Content or an empty-body redirect; running in an environment where Response.body is not implemented; a misconfigured proxy that strips the body.

Related errors

• Failed to fetch the response.
• Failed to fetch the response.
• The response body is empty.
• AbortError
• 'element streams in no-schema mode' functionality not suppor

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/70fea231bb638ee8. Report an issue: GitHub.