Fastify with AI SDK

Learn how to integrate the AI SDK into Fastify applications for high-performance AI endpoints.

Why Fastify?

Fastify is an excellent choice for AI applications:

Fast: Low overhead framework optimized for speed
Schema-based: Built-in JSON schema validation
Plugin System: Extensible architecture
TypeScript: First-class TypeScript support
Logging: Built-in structured logging

Prerequisites

Node.js 18+
Basic knowledge of Fastify
Vercel AI Gateway API key

Quick Start

Create a new project:

mkdir my-ai-server
cd my-ai-server
pnpm init

Install dependencies:

pnpm add fastify ai
pnpm add -D tsx typescript

Configure TypeScript:

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "Node16",
    "moduleResolution": "Node16",
    "outDir": "./dist",
    "esModuleInterop": true,
    "strict": true
  }
}

Set environment variables:

echo "AI_GATEWAY_API_KEY=your-api-key" > .env

Basic Streaming

Stream AI responses with Fastify:

import { streamText } from 'ai';
import Fastify from 'fastify';
import 'dotenv/config';

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

fastify.post('/api/chat', async function (request, reply) {
  const { prompt } = request.body as { prompt: string };

  const result = streamText({
    model: 'openai/gpt-4o',
    prompt: prompt || 'Tell me a fun fact',
  });

  reply.header('Content-Type', 'text/plain; charset=utf-8');
  return reply.send(result.toUIMessageStream());
});

fastify.listen({ port: 8080 }, (err) => {
  if (err) throw err;
  console.log('Server running on http://localhost:8080');
});

Run the server:

pnpx tsx src/index.ts

Test with curl:

curl -X POST http://localhost:8080/api/chat \
  -H "Content-Type: application/json" \
  -d '{"prompt": "Why is the sky blue?"}'

UI Message Stream

Stream messages compatible with useChat:

import { streamText, convertToModelMessages, UIMessage } from 'ai';
import Fastify from 'fastify';
import cors from '@fastify/cors';

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

await fastify.register(cors, {
  origin: true,
});

fastify.post('/api/chat', async function (request, reply) {
  const { messages } = request.body as { messages: UIMessage[] };

  const result = streamText({
    model: 'openai/gpt-4o',
    messages: await convertToModelMessages(messages),
  });

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

fastify.listen({ port: 8080 });

Text-Only Stream

Stream plain text responses:

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

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

fastify.post('/api/generate', async function (request, reply) {
  const { prompt } = request.body as { prompt: string };

  const result = streamText({
    model: 'openai/gpt-4o',
    prompt,
  });

  reply.header('Content-Type', 'text/plain; charset=utf-8');
  return reply.send(result.textStream);
});

fastify.listen({ port: 8080 });

Custom Data Streaming

Send custom data with AI responses:

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

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

fastify.post('/api/chat', async function (request, reply) {
  const { prompt } = request.body as { prompt: string };

  const stream = createUIMessageStream({
    execute: async ({ writer }) => {
      writer.write({ type: 'start' });

      writer.write({
        type: 'data-custom',
        data: {
          timestamp: new Date().toISOString(),
          requestId: Math.random().toString(36),
        },
      });

      const result = streamText({
        model: 'openai/gpt-4o',
        prompt,
      });

      writer.merge(result.toUIMessageStream({ sendStart: false }));
    },
    onError: error => {
      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 });

Tool Calling

Implement AI tools with Fastify:

import {
  streamText,
  convertToModelMessages,
  tool,
  UIMessage,
} from 'ai';
import Fastify from 'fastify';
import { z } from 'zod';

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

fastify.post('/api/chat', async function (request, reply) {
  const { messages } = request.body as { messages: UIMessage[] };

  const result = streamText({
    model: 'openai/gpt-4o',
    messages: await convertToModelMessages(messages),
    tools: {
      getWeather: tool({
        description: 'Get current weather for a city',
        inputSchema: z.object({
          city: z.string().describe('City name'),
        }),
        execute: async ({ city }) => {
          const weatherData = {
            city,
            temperature: 72,
            condition: 'Sunny',
            humidity: 65,
          };
          return weatherData;
        },
      }),
      searchWeb: tool({
        description: 'Search the web for information',
        inputSchema: z.object({
          query: z.string(),
        }),
        execute: async ({ query }) => {
          // Implement web search
          return {
            query,
            results: [
              { title: 'Result 1', url: 'https://example.com/1' },
              { title: 'Result 2', url: 'https://example.com/2' },
            ],
          };
        },
      }),
    },
  });

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

fastify.listen({ port: 8080 });

Structured Output

Generate structured JSON with schema validation:

import { generateObject, Output } from 'ai';
import Fastify from 'fastify';
import { z } from 'zod';

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

const recipeSchema = z.object({
  name: z.string(),
  ingredients: z.array(z.string()),
  steps: z.array(z.string()),
  prepTime: z.number(),
  cookTime: z.number(),
});

fastify.post('/api/recipe', async function (request, reply) {
  const { dish } = request.body as { dish: string };

  const { object: recipe } = await generateObject({
    model: 'openai/gpt-4o',
    prompt: `Generate a recipe for ${dish}`,
    output: Output.object({
      schema: recipeSchema,
    }),
  });

  return reply.send(recipe);
});

fastify.listen({ port: 8080 });

Schema Validation

Use Fastify’s built-in schema validation:

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

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

fastify.post(
  '/api/chat',
  {
    schema: {
      body: {
        type: 'object',
        required: ['prompt'],
        properties: {
          prompt: { type: 'string', minLength: 1, maxLength: 1000 },
          temperature: { type: 'number', minimum: 0, maximum: 2 },
        },
      },
      response: {
        200: {
          type: 'object',
          properties: {
            text: { type: 'string' },
          },
        },
      },
    },
  },
  async function (request, reply) {
    const { prompt, temperature } = request.body as {
      prompt: string;
      temperature?: number;
    };

    const result = streamText({
      model: 'openai/gpt-4o',
      prompt,
      temperature,
    });

    return reply.send(result.toUIMessageStreamResponse());
  },
);

fastify.listen({ port: 8080 });

Plugins

Rate Limiting Plugin

import fp from 'fastify-plugin';
import rateLimit from '@fastify/rate-limit';

export default fp(async (fastify) => {
  await fastify.register(rateLimit, {
    max: 100,
    timeWindow: '15 minutes',
  });
});

Authentication Plugin

import fp from 'fastify-plugin';

export default fp(async (fastify) => {
  fastify.decorate('authenticate', async (request, reply) => {
    const apiKey = request.headers['x-api-key'];

    if (!apiKey || apiKey !== process.env.API_KEY) {
      reply.code(401).send({ error: 'Unauthorized' });
    }
  });
});

Use plugins:

import Fastify from 'fastify';
import ratelimitPlugin from './plugins/ratelimit';
import authPlugin from './plugins/auth';

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

await fastify.register(ratelimitPlugin);
await fastify.register(authPlugin);

fastify.post(
  '/api/chat',
  { onRequest: [fastify.authenticate] },
  async (request, reply) => {
    // Protected route
  },
);

fastify.listen({ port: 8080 });

Error Handling

Global error handler:

import Fastify from 'fastify';

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

fastify.setErrorHandler((error, request, reply) => {
  request.log.error(error);

  reply.status(error.statusCode || 500).send({
    error: error.message,
    statusCode: error.statusCode || 500,
  });
});

fastify.post('/api/chat', async (request, reply) => {
  try {
    // AI logic
  } catch (error) {
    throw error;
  }
});

fastify.listen({ port: 8080 });

Production Setup

Project Structure

my-ai-server/
├── src/
│   ├── index.ts
│   ├── routes/
│   │   ├── chat.ts
│   │   └── generate.ts
│   ├── plugins/
│   │   ├── auth.ts
│   │   └── ratelimit.ts
│   └── schemas/
│       └── chat.ts
├── .env
├── package.json
└── tsconfig.json

Routes as Plugins

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

const chatRoutes: FastifyPluginAsync = async (fastify) => {
  fastify.post('/chat', async (request, reply) => {
    const { prompt } = request.body as { prompt: string };

    const result = streamText({
      model: 'openai/gpt-4o',
      prompt,
    });

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

export default chatRoutes;

import Fastify from 'fastify';
import chatRoutes from './routes/chat';
import generateRoutes from './routes/generate';

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

await fastify.register(chatRoutes, { prefix: '/api' });
await fastify.register(generateRoutes, { prefix: '/api' });

fastify.listen({ port: 8080 });

Build Scripts

{
  "scripts": {
    "dev": "tsx watch src/index.ts",
    "build": "tsc",
    "start": "node dist/index.js",
    "test": "vitest"
  }
}

Deployment

Docker

FROM node:20-alpine

WORKDIR /app

COPY package*.json ./
RUN npm install --production

COPY dist ./dist

EXPOSE 8080

CMD ["npm", "start"]

Environment Variables

AI_GATEWAY_API_KEY=your-key
API_KEY=your-auth-key
PORT=8080
NODE_ENV=production

Performance Tips

Enable HTTP/2: For better streaming performance
Use Async/Await: Properly handle promises
Connection Pooling: Reuse database connections
Caching: Cache frequent responses
Compression: Use for non-streaming routes

Best Practices

Use Plugins: Modular architecture
Schema Validation: Validate all inputs
Logging: Use Fastify’s built-in logger
Error Handling: Centralized error handling
TypeScript: Full type safety
Testing: Write tests for routes

Example Repository

View the complete example: github.com/vercel/ai/examples/fastify

Guides

Frameworks

Fastify with AI SDK

Fastify with AI SDK

Why Fastify?

Prerequisites

Quick Start

Basic Streaming

UI Message Stream

Text-Only Stream

Custom Data Streaming

Tool Calling

Structured Output

Schema Validation

Plugins

Rate Limiting Plugin

Authentication Plugin

Error Handling

Production Setup

Project Structure

Routes as Plugins

Build Scripts

Deployment

Docker

Environment Variables

Performance Tips

Best Practices

Example Repository

Resources

Guides

Frameworks

​Fastify with AI SDK

​Why Fastify?

​Prerequisites

​Quick Start

​Basic Streaming

​UI Message Stream

​Text-Only Stream

​Custom Data Streaming

​Tool Calling

​Structured Output

​Schema Validation

​Plugins

​Rate Limiting Plugin

​Authentication Plugin

​Error Handling

​Production Setup

​Project Structure

​Routes as Plugins

​Build Scripts

​Deployment

​Docker

​Environment Variables

​Performance Tips

​Best Practices

​Example Repository

​Resources

Fastify with AI SDK

Why Fastify?

Prerequisites

Quick Start

Basic Streaming

UI Message Stream

Text-Only Stream

Custom Data Streaming

Tool Calling

Structured Output

Schema Validation

Plugins

Rate Limiting Plugin

Authentication Plugin

Error Handling

Production Setup

Project Structure

Routes as Plugins

Build Scripts

Deployment

Docker

Environment Variables

Performance Tips

Best Practices

Example Repository

Resources