onFinish not called when stream is aborted

Issue

When using toUIMessageStreamResponse with an onFinish callback, the callback may not execute when the stream is aborted. This happens because the abort handler immediately terminates the response, preventing the onFinish callback from being triggered.

// Server-side code where onFinish isn't called on abort
export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages: convertToModelMessages(messages),
    abortSignal: req.signal,
  });

  return result.toUIMessageStreamResponse({
    onFinish: async ({ isAborted }) => {
      // This isn't called when the stream is aborted!
      if (isAborted) {
        console.log('Stream was aborted');
        // Handle abort-specific cleanup
      } else {
        console.log('Stream completed normally');
        // Handle normal completion
      }
    },
  });
}

Background

When a stream is aborted, the response is immediately terminated. Without proper handling, the onFinish callback has no chance to execute, preventing important cleanup operations like saving partial results or logging abort events.

Solution

Add consumeStream to the toUIMessageStreamResponse configuration. This ensures that abort events are properly captured and forwarded to the onFinish callback, allowing it to execute even when the stream is aborted.

// other imports...
import { consumeStream } from 'ai';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages: convertToModelMessages(messages),
    abortSignal: req.signal,
  });

  return result.toUIMessageStreamResponse({
    onFinish: async ({ isAborted }) => {
      // Now this WILL be called even when aborted!
      if (isAborted) {
        console.log('Stream was aborted');
        // Handle abort-specific cleanup
      } else {
        console.log('Stream completed normally');
        // Handle normal completion
      }
    },
    consumeSseStream: consumeStream, // This enables onFinish to be called on abort
  });
}