Fastify Core Knowledge

Full Reference: See advanced.md for WebSocket authentication, room management, heartbeat patterns, message validation, and Redis scaling.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: fastify for comprehensive documentation.

Basic Setup

import Fastify from 'fastify';
import cors from '@fastify/cors';

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

await app.register(cors);
await app.register(userRoutes, { prefix: '/api/users' });

app.listen({ port: 3000, host: '0.0.0.0' });

Route with Schema Validation

import { FastifyPluginAsync } from 'fastify';
import { Type, Static } from '@sinclair/typebox';

const UserSchema = Type.Object({
  name: Type.String(),
  email: Type.String({ format: 'email' }),
});

type User = Static<typeof UserSchema>;

const routes: FastifyPluginAsync = async (app) => {
  app.post<{ Body: User }>('/', {
    schema: {
      body: UserSchema,
      response: { 201: UserSchema }
    }
  }, async (request, reply) => {
    const user = await db.users.create(request.body);
    reply.status(201).send(user);
  });
};

Plugins

import fp from 'fastify-plugin';

// Custom plugin with fastify-plugin
export default fp(async (app) => {
  app.decorate('db', db);
  app.addHook('onRequest', async (request) => {
    request.startTime = Date.now();
  });
});

app.register(myPlugin);

Error Handling

import { FastifyError, FastifyReply, FastifyRequest } from 'fastify';

export class AppError extends Error {
  constructor(public statusCode: number, message: string) {
    super(message);
  }
}

app.setErrorHandler((error: FastifyError, request, reply) => {
  request.log.error({ err: error }, 'Request error');

  if (error.validation) {
    return reply.status(400).send({ error: 'Validation failed', details: error.validation });
  }

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

Health Checks

app.get('/health', async () => ({ status: 'healthy' }));

app.get('/ready', async (request, reply) => {
  try {
    await app.pg.query('SELECT 1');
    return { status: 'ready', database: 'connected' };
  } catch (error) {
    reply.status(503);
    return { status: 'not ready' };
  }
});

Testing

import { test } from 'vitest';
import { buildApp } from '../src/app';

test('GET /api/users returns users', async () => {
  const app = await buildApp();

  const response = await app.inject({
    method: 'GET',
    url: '/api/users',
  });

  expect(response.statusCode).toBe(200);
  await app.close();
});

WebSocket Setup

import websocket from '@fastify/websocket';

await app.register(websocket);

app.get('/ws', { websocket: true }, (socket, req) => {
  socket.on('message', (data) => {
    const message = JSON.parse(data.toString());
    handleMessage(socket, message);
  });
});

When NOT to Use This Skill

• Enterprise Architecture - Use NestJS for DI and modular design
• Simple CRUD APIs - Use Express if performance is not critical
• Edge Runtimes - Use Hono for Cloudflare Workers
• Deno Projects - Use Oak or Fresh instead

Anti-Patterns

Quick Troubleshooting

Production Checklist

• TypeBox schema validation
• Helmet security headers
• Rate limiting configured
• Structured logging (Pino)
• Request ID tracing
• Custom error handler
• Health/readiness endpoints
• Graceful shutdown
• Database connection pooling

Monitoring Metrics

Reference Documentation

• Schema Validation
• Plugins