How to Provide chat message cost without blocking Streaming?

AI SDK
1

I’m using the Vercel AI SDK (ai package) to stream responses from a model, and I want to include extra provider metadata — specifically the cost — in the final finish metadata sent to the client.

Here’s the simplified version of my code:

const result = streamText({
    prompt,
    model: gateway(ai_model)}
);

const providerMetadataPromise = result.providerMetadata;

return result.toUIMessageStreamResponse({
  originalMessages: messages,
  sendReasoning: true,
  messageMetadata: ({ part }): Record<string, string> | undefined => {
    if (part.type === 'start') {
      return { model: ai_model };
    }

    if (part.type === 'finish') {
      let answerCost: string;
      providerMetadataPromise.then(data => {
        answerCost = (data!.gateway as any).cost;
      });
      return { model: ai_model, cost: answerCost! };
    }
  },
});

  • If I use await result.providerMetadata before streaming starts, I get the cost correctly — but it blocks the start of the stream.

  • If I remove the await and use .then(), streaming starts instantly — but the cost in the finish metadata is always undefined on the client.

  • I also tried making messageMetadata async, but it seems the SDK doesn’t await it, so the cost never arrives.

I want:

  • The streaming to start immediately (no delay).

  • The final finish metadata to contain both { model, cost } values.

  • The client to actually receive the cost along with the last message.

Is there a correct, non-blocking way to include async provider metadata (like cost) in the finish metadata for a streaming response in the Vercel AI SDK?