Fastify
High-performance Node.js web framework focused on developer experience and speed. Fastify is significantly faster than Express (~2-3x throughput) using JSON Schema validation for requests/responses that also drives serialization. TypeScript types built-in, plugin system with encapsulation, and async-first design. Becoming the preferred Express replacement for performance-critical Node.js services.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
JSON Schema validation strips unknown fields from responses — prevents data leakage. @fastify/helmet for security headers. Rate limiting via @fastify/rate-limit. Better defaults than Express but still requires explicit security plugin setup.
⚡ Reliability
Best When
You need Express-like simplicity with 2-3x better performance, TypeScript support, and JSON Schema validation for Node.js APIs.
Avoid When
You're heavily invested in Express-specific middleware or need NestJS-level structure.
Use Cases
- • Build high-throughput REST APIs in Node.js where Express performance is insufficient
- • Use JSON Schema validation for request body, params, and response serialization to auto-generate faster output
- • Create TypeScript-first APIs with Fastify's built-in types and type inference for route handlers
- • Build plugin-based modular applications using Fastify's encapsulated plugin system
- • Serve APIs with automatic Swagger/OpenAPI documentation via @fastify/swagger from JSON Schema definitions
Not For
- • Teams with heavy Express ecosystem dependency — not all Express middleware is compatible with Fastify
- • Applications needing Django/NestJS-level structure — Fastify is still a microframework; NestJS provides more opinionated structure
- • Simple prototyping where Express familiarity is more valuable than performance
Interface
Authentication
Framework with no built-in auth — use @fastify/jwt, @fastify/auth, or Passport.js adapters.
Pricing
Free and open source.
Agent Metadata
Known Gotchas
- ⚠ Fastify's plugin encapsulation means decorators and hooks registered inside a plugin scope are NOT available outside — use fastify-plugin() wrapper to expose decorations to parent scope
- ⚠ JSON Schema for request validation is REQUIRED for Fastify's fast serialization — omitting schema means serialization falls back to JSON.stringify() which is slower and less safe
- ⚠ Response serialization uses JSON Schema — returning properties not in the schema are silently omitted; this is a feature (data stripping) but can cause confusing missing fields in responses
- ⚠ Fastify 5 dropped Node.js 16 support and changed some plugin APIs — plugins using Fastify 4 patterns may need updates for Fastify 5 compatibility
- ⚠ Route handler return values are auto-serialized — returning non-serializable values (class instances, circular refs) causes FastifyError; always return plain objects
- ⚠ Hooks (onRequest, preHandler, etc.) must call done() or return a promise — forgetting done() causes requests to hang indefinitely
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for Fastify.
Scores are editorial opinions as of 2026-03-06.