📄 ai-sdk/cookbook/api-servers/fastify

import { openai } from '@ai-sdk/openai';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-4o'),    prompt: 'Invent a new holiday and describe its traditions.',  });
  // Mark the response as a v1 data stream:  reply.header('X-Vercel-AI-Data-Stream', 'v1');  reply.header('Content-Type', 'text/plain; charset=utf-8');
  return reply.send(result.toDataStream({ data }));});
fastify.listen({ port: 8080 });

Sending Custom Data

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

index.ts

import { openai } from '@ai-sdk/openai';import { createDataStream, streamText } 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 dataStream = createDataStream({    execute: async dataStreamWriter => {      dataStreamWriter.writeData('initialized call');
      const result = streamText({        model: openai('gpt-4o'),        prompt: 'Invent a new holiday and describe its traditions.',      });
      result.mergeIntoDataStream(dataStreamWriter);    },    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);    },  });
  // Mark the response as a v1 data stream:  reply.header('X-Vercel-AI-Data-Stream', 'v1');  reply.header('Content-Type', 'text/plain; charset=utf-8');
  return reply.send(dataStream);});
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 { openai } from '@ai-sdk/openai';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-4o'),    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

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