Fastify | AI SDK

You can use the AI SDK in a Fastify 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

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/fastify

UI Message Stream

You can use the toUIMessageStream helper to convert the result stream to a UI message stream and then send it in the response.

index.ts

import { streamText, toUIMessageStream } from 'ai';
import Fastify from 'fastify';

const fastify = Fastify({ logger: true });

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

  reply.header('Content-Type', 'text/plain; charset=utf-8');

  return reply.send(toUIMessageStream({ stream: result.stream }));
});

fastify.listen({ port: 8080 });

Sending Custom Data

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

index.ts

import { createUIMessageStream, streamText, toUIMessageStream } from 'ai';
import Fastify from 'fastify';

const fastify = Fastify({ logger: true });

fastify.post('/stream-data', async function (request, reply) {
  // immediately start streaming the response
  const stream = createUIMessageStream({
    execute: async ({ writer }) => {
      writer.write({ type: 'start' });

      writer.write({
        type: 'data-custom',
        data: {
          custom: 'initialized call',
        },
      });

      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 }),
      );
    },
    onError: error => {
      // Error messages are masked by default for security reasons.
      // If you want to expose the error message to the client, you can do so here:
      return error instanceof Error ? error.message : String(error);
    },
  });

  reply.header('Content-Type', 'text/plain; charset=utf-8');

  return reply.send(stream);
});

fastify.listen({ port: 8080 });

Text Stream

You can use the textStream property to get a text stream from the result and then pipe it to the response.

index.ts

import { streamText } from 'ai';
import Fastify from 'fastify';

const fastify = Fastify({ logger: true });

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

  reply.header('Content-Type', 'text/plain; charset=utf-8');

  return reply.send(result.textStream);
});

fastify.listen({ port: 8080 });

Troubleshooting

Streaming not working when proxied