Koa vs Fastify in 2026: Middleware Architecture Compared

TL;DR

Fastify is the better choice for new projects in 2026. Koa (~2.8M weekly downloads) is the elegant, minimal successor to Express with async/await support. Fastify (~3.5M) is faster, has better TypeScript, and is more actively developed. Koa's main advantage is its onion-layer middleware model — elegant for complex middleware composition. If you're starting fresh, use Fastify. If you have an existing Koa codebase, there's no urgent reason to migrate.

Key Takeaways

Koa: ~2.8M weekly downloads — Fastify: ~3.5M (npm, March 2026)
Fastify is ~3x faster than Koa in benchmarks
Koa's onion model is genuinely elegant for middleware composition
Fastify has better TypeScript — generics for request/response typing
Koa is unmaintained compared to Fastify — fewer releases, smaller core team

Koa's Core Idea: The Onion Model

Koa's middleware model is its claim to fame. Unlike Express's linear middleware (next()), Koa uses async generators that "wrap" the request:

// Koa's onion model — middleware runs in and out
const app = new Koa();

// This middleware wraps ALL subsequent middleware
app.use(async (ctx, next) => {
  const start = Date.now();
  console.log(`→ ${ctx.method} ${ctx.url}`);

  await next(); // Runs all downstream middleware

  const ms = Date.now() - start;
  console.log(`← ${ctx.status} ${ms}ms`);
  // ctx.body is now set from the route handler below
});

app.use(async (ctx, next) => {
  try {
    await next();
  } catch (err) {
    ctx.status = err.status || 500;
    ctx.body = { error: err.message };
    ctx.app.emit('error', err, ctx);
  }
});

// Route
app.use(async (ctx) => {
  ctx.body = { hello: 'world' };
});

The execution order is:

Middleware 1 (start) → 2. Middleware 2 (start) → 3. Route handler → 2. Middleware 2 (end) → 1. Middleware 1 (end)

This makes timing, error handling, and response transformation much cleaner than Express-style linear middleware.

Fastify's Plugin Model

// Fastify uses a plugin/hook model
import Fastify from 'fastify';

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

// Hooks are the equivalent of Koa middleware
app.addHook('onRequest', async (request, reply) => {
  request.startTime = Date.now();
});

app.addHook('onSend', async (request, reply, payload) => {
  const ms = Date.now() - request.startTime;
  reply.header('X-Response-Time', `${ms}ms`);
  return payload;
});

// Error handler
app.setErrorHandler((error, request, reply) => {
  reply.status(error.statusCode || 500).send({
    error: error.message
  });
});

Fastify's hooks are more explicit than Koa's middleware but serve the same purpose. The difference: Fastify separates "plugin registration" (encapsulated scope) from "hooks" (cross-cutting concerns).

TypeScript Comparison

// Koa with TypeScript — needs @types/koa, still loose
import Koa, { Context } from 'koa';

const app = new Koa();
app.use(async (ctx: Context) => {
  // ctx.request.body is any — needs koa-bodyparser + @types
  const body = ctx.request.body; // any
  ctx.body = { received: body };
});

// Fastify — generics for full type safety
import Fastify from 'fastify';
import { Type } from '@sinclair/typebox';

const app = Fastify();

app.post<{
  Body: { name: string; email: string }
  Reply: { id: string }
}>(
  '/users',
  {
    schema: {
      body: Type.Object({
        name: Type.String(),
        email: Type.String({ format: 'email' }),
      }),
    },
  },
  async (request, reply) => {
    // request.body is typed as { name: string; email: string }
    const user = await db.user.create({ data: request.body });
    return reply.status(201).send({ id: user.id }); // typed
  }
);

Fastify's generics are more ergonomic than Koa's type system, especially for request/response body typing.

Performance

Framework	Req/s	Notes
Fastify	~230K	JSON schema validation via ajv
Koa	~85K	No built-in validation
Express	~80K	Baseline

The Fastify advantage comes largely from:

JSON Schema validation compiled to ajv (not evaluated at runtime)
Fast-json-stringify for response serialization
Optimized routing algorithm (radix trie)

Router Comparison

Both support basic routing, but neither has built-in routing as sophisticated as Express Router. Common solutions:

// Koa: use koa-router
import Router from '@koa/router';
const router = new Router();

router.get('/users/:id', async (ctx) => {
  ctx.body = await getUser(ctx.params.id);
});

app.use(router.routes()).use(router.allowedMethods());

// Fastify: built-in routing with full TypeScript
app.get<{ Params: { id: string } }>('/users/:id', async (req) => {
  return getUser(req.params.id);
});

Fastify's routing is built-in and typed. Koa requires @koa/router as a dependency.

When to Choose

Choose Koa when:

Maintaining an existing Koa codebase
You love the onion middleware model and want it explicitly
Your team knows Koa and learning Fastify isn't worth the switch
You're building something where the minimal surface area of Koa is appealing

Choose Fastify when:

Starting a new Node.js API project
TypeScript type safety matters
Performance matters (microservices, high-throughput)
You want a more active community and ecosystem

Migrate Koa → Fastify when:

Performance is a blocking issue
TypeScript type safety is causing bugs
You're doing a significant refactor anyway

Compare Koa and Fastify package health on PkgPulse.