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

Anti-Pattern	Why It's Bad	Correct Approach
Not using schema validation	Loses performance advantage	Define schemas for routes
Missing fastify-plugin wrapper	Encapsulation issues	Use fp() for shared plugins
Using app.listen() in tests	Slow, port conflicts	Use app.inject()
Ignoring response schema	Slower serialization	Define response schemas

Quick Troubleshooting

Issue	Likely Cause	Solution
"Cannot call reply.send twice"	Multiple sends	Return after first reply.send()
Plugin not accessible	Missing fastify-plugin	Wrap with fp()
Schema not working	Not registered	Add schema: {} to route options
Type errors with schemas	Provider missing	Use withTypeProvider<TypeBoxTypeProvider>()

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

Metric	Target
Response time (p99)	< 50ms
Error rate (5xx)	< 0.1%
Request throughput	> 10k/s

Reference Documentation

• Schema Validation
• Plugins