Clerk + Fastify: clerkPlugin() registration, ESM, and getAuth(request)

Clerk on Fastify

Project shape

The Clerk Fastify guide assumes ECMAScript modules: "type": "module" in package.json. Mixing require() with the ESM-only imports is the first failure.

Env vars (CLERK_PUBLISHABLE_KEY, CLERK_SECRET_KEY) must be available before any Clerk module initializes. Run with node --env-file=.env, or load your env loader before importing @clerk/fastify. Same rule as Express: Clerk reads keys at import time.

Register the plugin, then protect routes

import Fastify from 'fastify'
import { clerkClient, clerkPlugin, getAuth } from '@clerk/fastify'

const fastify = Fastify({ logger: true })
fastify.register(clerkPlugin)

fastify.get('/protected', async (request, reply) => {
  const { isAuthenticated, userId } = getAuth(request)
  if (!isAuthenticated) {
    return reply.code(401).send({ error: 'User not authenticated' })
  }
  const user = await clerkClient.users.getUser(userId)
  return reply.send({ message: 'ok', user })
})

Notes:

• clerkPlugin() is a Fastify plugin. Register it before the routes that need auth; Fastify plugin encapsulation means registration order and scope matter.
• getAuth(request) (not req) returns the Auth object. Same as Express, it returns state, it does not send the 401 for you.
• The docs show registering the plugin for all routes, with per-route scoping covered in the clerkPlugin() reference for when only some routes need auth.

Checklist

• fastify.register(clerkPlugin) with no call parens: it is a plugin function, not middleware factory output.
• TypeScript: getAuth(request) is typed against the Fastify request; do not pass a raw Node req.
• As with Express, clerkClient.users.getUser() is a Backend API call per invocation; cache or avoid in hot paths.

Clerk's Fastify quickstart requires ESM and env before Clerk imports, registers clerkPlugin for all routes, and protects routes with getAuth(request) plus a manual 401. Verified live at https://clerk.com/docs/fastify/getting-started/quickstart.md on 2026-09-26.