auth0-fastify
Use when adding session-based login, logout, or protected routes to a Fastify web application. Integrates @auth0/auth0-fastify — use even if the user says "add login to my Fastify app". For Fastify APIs validating Bearer tokens, use auth0-fastify-api instead.
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/auth0/agent-skills --skill auth0-fastify- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads auth0-fastify automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
Auth0 Fastify Integration
Add authentication to Fastify web applications using @auth0/auth0-fastify.
Prerequisites
- Fastify application (v5.x or newer)
- Node.js 20 LTS or newer
- Auth0 account and application configured
- If you don't have Auth0 set up yet, use the
auth0-quickstartskill first
When NOT to Use
- Single Page Applications - Use
auth0-react,auth0-vue, orauth0-angularfor client-side auth - Next.js applications - Use
auth0-nextjsskill which handles both client and server - Mobile applications - Use
auth0-react-nativefor React Native/Expo - Stateless APIs - Use
@auth0/auth0-fastify-apiinstead for JWT validation without sessions - Microservices - Use JWT validation for service-to-service auth
Quick Start Workflow
1. Install SDK
npm install @auth0/auth0-fastify fastify @fastify/view ejs dotenv
2. Configure Environment
Create .env:
AUTH0_DOMAIN=your-tenant.auth0.com
AUTH0_CLIENT_ID=your-client-id
AUTH0_CLIENT_SECRET=your-client-secret
SESSION_SECRET=<openssl-rand-hex-64>
APP_BASE_URL=http://localhost:3000
Generate secret: openssl rand -hex 64
3. Configure Auth Plugin
Create your Fastify server (server.js):
import 'dotenv/config';
import Fastify from 'fastify';
import fastifyAuth0 from '@auth0/auth0-fastify';
import fastifyView from '@fastify/view';
import ejs from 'ejs';
const fastify = Fastify({ logger: true });
// Register view engine
await fastify.register(fastifyView, {
engine: { ejs },
root: './views',
});
// Configure Auth0 plugin
await fastify.register(fastifyAuth0, {
domain: process.env.AUTH0_DOMAIN,
clientId: process.env.AUTH0_CLIENT_ID,
clientSecret: process.env.AUTH0_CLIENT_SECRET,
appBaseUrl: process.env.APP_BASE_URL,
sessionSecret: process.env.SESSION_SECRET,
});
fastify.listen({ port: 3000 });
This automatically creates:
/auth/login- Login endpoint/auth/logout- Logout endpoint/auth/callback- OAuth callback
4. Add Routes
// Public route
fastify.get('/', async (request, reply) => {
const session = await fastify.auth0Client.getSession({ request, reply });
return reply.view('views/home.ejs', {
isAuthenticated: !!session,
});
});
// Protected route
fastify.get('/profile', {
preHandler: async (request, reply) => {
const session = await fastify.auth0Client.getSession({ request, reply });
if (!session) {
return reply.redirect('/auth/login');
}
}
}, async (request, reply) => {
const user = await fastify.auth0Client.getUser({ request, reply });
return reply.view('views/profile.ejs', { user });
});
5. Test Authentication
Start your server:
node server.js
Visit http://localhost:3000 and test the login flow.
Common Mistakes
| Mistake | Fix |
|---|---|
| Forgot to add callback URL in Auth0 Dashboard | Add /auth/callback path to Allowed Callback URLs (e.g., http://localhost:3000/auth/callback) |
| Missing or weak SESSION_SECRET | Generate secure 64-char secret with openssl rand -hex 64 and store in .env |
| App created as SPA type in Auth0 | Must be Regular Web Application type for server-side auth |
| Session secret exposed in code | Always use environment variables, never hardcode secrets |
| Wrong appBaseUrl for production | Update APP_BASE_URL to match your production domain |
| Not awaiting fastify.register | Fastify v4+ requires awaiting plugin registration |
Related Skills
auth0-quickstart- Basic Auth0 setupauth0-migration- Migrate from another auth providerauth0-mfa- Add Multi-Factor Authenticationauth0-cli- Manage Auth0 resources from the terminal
Quick Reference
Plugin Options:
domain- Auth0 tenant domain (required)clientId- Auth0 client ID (required)clientSecret- Auth0 client secret (required)appBaseUrl- Application URL (required)sessionSecret- Session encryption secret (required, min 64 chars)audience- API audience (optional, for calling APIs)
Client Methods:
fastify.auth0Client.getSession({ request, reply })- Get user sessionfastify.auth0Client.getUser({ request, reply })- Get user profilefastify.auth0Client.getAccessToken({ request, reply })- Get access tokenfastify.auth0Client.logout(options, { request, reply })- Logout user
Common Use Cases:
- Protected routes → Use
preHandlerto check session (see Step 4) - Check auth status →
!!session - Get user info →
getUser({ request, reply }) - Call APIs →
getAccessToken({ request, reply })
References
FAQ
What does the auth0-fastify skill do?
Use when adding session-based login, logout, or protected routes to a Fastify web application. Integrates @auth0/auth0-fastify — use even if the user says "add login to my Fastify app". For Fastify APIs validating Bearer tokens, use auth0-fastify-api instead.
How do I install auth0-fastify?
Run npx skills add https://github.com/auth0/agent-skills --skill auth0-fastify in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/auth0-fastify/ (personal) or .claude/skills/auth0-fastify/ (project).
Which AI agents can use auth0-fastify?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is auth0-fastify safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains auth0-fastify?
It is published by auth0 in auth0/agent-skills, and it is an official skill from the maker of the technology it covers. It has 555 recorded installs.
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