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Express

You can use the AI SDK in an Express server to generate and stream text and objects to the client.

Examples

The examples start a simple HTTP server that listens on port 8080. You can e.g. test it using curl:

curl -X POST http://localhost:8080

Note

The examples use the Vercel AI Gateway. Ensure that your AI Gateway API key is set in the AI_GATEWAY_API_KEY environment variable.

Full example: github.com/vercel/ai/examples/express

UI Message Stream

You can use the pipeUIMessageStreamToResponse helper to pipe the stream data to the server response.

import {
  pipeUIMessageStreamToResponse,
  streamText,
  toUIMessageStream,
} from 'ai';
import express, { Request, Response } from 'express';

const app = express();

app.post('/', async (req: Request, res: Response) => {
  const result = streamText({
    model: 'openai/gpt-6-astra',
    prompt: 'Invent a new holiday and describe its traditions.',
  });

  pipeUIMessageStreamToResponse({
    response: res,
    stream: toUIMessageStream({ stream: result.stream }),
  });
});

app.listen(8080, () => {
  console.log(`Example app listening on port ${8080}`);
});

Sending Custom Data

pipeUIMessageStreamToResponse can be used to send custom data to the client.

import {
  createUIMessageStream,
  pipeUIMessageStreamToResponse,
  streamText,
  toUIMessageStream,
} from 'ai';
import express, { Request, Response } from 'express';

const app = express();

app.post('/custom-data-parts', async (req: Request, res: Response) => {
  pipeUIMessageStreamToResponse({
    response: res,
    stream: createUIMessageStream({
      execute: async ({ writer }) => {
        writer.write({ type: 'start' });

        writer.write({
          type: 'data-custom',
          data: {
            custom: 'Hello, world!',
          },
        });

        const result = streamText({
          model: 'openai/gpt-6-astra',
          prompt: 'Invent a new holiday and describe its traditions.',
        });

        writer.merge(
          toUIMessageStream({ stream: result.stream, sendStart: false }),
        );
      },
    }),
  });
});

app.listen(8080, () => {
  console.log(`Example app listening on port ${8080}`);
});

Text Stream

You can send a text stream to the client using pipeTextStreamToResponse.

import { pipeTextStreamToResponse, streamText, toTextStream } from 'ai';
import express, { Request, Response } from 'express';

const app = express();

app.post('/', async (req: Request, res: Response) => {
  const result = streamText({
    model: 'openai/gpt-6-astra',
    prompt: 'Invent a new holiday and describe its traditions.',
  });

  pipeTextStreamToResponse({
    response: res,
    stream: toTextStream({ stream: result.stream }),
  });
});

app.listen(8080, () => {
  console.log(`Example app listening on port ${8080}`);
});

Troubleshooting

  • Streaming not working when proxied
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