README.md

June 22, 2026 · View on GitHub

The Auth0 Fastify-API SDK is a library for protecting API's in Fastify applications.

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📚 Documentation - 🔎 Features - 🚀 Getting Started - 💬 Feedback

Documentation

Examples - examples for your different use cases.
Docs Site - explore our docs site and learn more about Auth0.

Features

Jump straight to the capability you need.

Feature	What it does
Quick start	Protect an API with `fastify.register` in a few lines
Protect an API route (`requireAuth`)	Require a valid bearer access token in a preHandler
Read token claims (`request.user`)	Access claims extracted from the verified token
Custom token / user type	Type your custom claims via module augmentation
DPoP (proof-of-possession)	Bind access tokens to a client key pair (RFC 9449)
On-Behalf-Of Token Exchange	Exchange the caller's token for a downstream API token
Multiple Custom Domains (MCD)	Accept tokens from multiple issuer domains of one tenant
Custom `fetch`	Swap in your own fetch (proxies, retries, instrumentation)
Discovery cache	Control caching of discovery metadata and signing keys

Getting Started

1. Install the SDK

npm i @auth0/auth0-fastify-api

This library requires Node.js 20 LTS and newer LTS versions.

3. Register the Auth0 Fastify plugin for APIs

import fastifyAuth0Api from '@auth0/auth0-fastify-api';

const fastify = Fastify({
  logger: true,
});

fastify.register(fastifyAuth0Api, {
  domain: '<AUTH0_DOMAIN>',
  audience: '<AUTH0_AUDIENCE>',
});

The AUTH0_DOMAIN can be obtained from the Auth0 Dashboard once you've created an API. The AUTH0_AUDIENCE is the identifier of the API that is being called. You can find this in the API section of the Auth0 dashboard.

Protecting API Routes

In order to protect an API route, you can use the SDK's requireAuth() method in a preHandler:

fastify.register(() => {
  fastify.get(
    '/protected-api',
    {
      preHandler: fastify.requireAuth(),
    },
    async (request: FastifyRequest, reply) => {
      return `Hello, ${request.user.sub}`;
    }
  );
});

The SDK exposes the claims, extracted from the token, as the user property on the FastifyRequest object. In order to use a custom user type to represent custom claims, you can configure the Token type in a module augmentation:

declare module '@auth0/auth0-fastify-api' {
  interface Token {
    id: number;
    name: string;
    age: number;
  }
}

Doing so will change the user type on the FastifyRequest object automatically:

fastify.register(() => {
  fastify.get(
    '/protected-api',
    {
      preHandler: fastify.requireAuth(),
    },
    async (request: FastifyRequest, reply) => {
      return `Hello, ${request.user.name}`;
    }
  );
});

Important

The above is to protect API routes by the means of a bearer token, and not server-side rendering routes using a session.

DPoP (Demonstration of Proof-of-Possession)

DPoP binds access tokens to a specific client's key pair, preventing stolen tokens from being replayed by attackers. The SDK supports DPoP with three modes:

allowed (default): accepts both Bearer and DPoP-bound tokens.
required: only DPoP-bound tokens are accepted; Bearer tokens are rejected.
disabled: DPoP is ignored; Bearer-only behavior.

import fastifyAuth0Api from '@auth0/auth0-fastify-api';

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

fastify.register(fastifyAuth0Api, {
  domain: '<AUTH0_DOMAIN>',
  audience: '<AUTH0_AUDIENCE>',
  dpop: { mode: 'required' },
});

When DPoP is enabled, clients send:

An Authorization: DPoP <access_token> header (instead of Bearer).
A DPoP header containing a proof JWT tied to the request method and URL.

The SDK automatically extracts the DPoP proof from the request, validates it against the access token's cnf.jkt claim, and verifies that the proof matches the current HTTP method and URL.

For the full configuration reference and error handling details, see the DPoP section in EXAMPLES.md.

On-Behalf-Of Token Exchange

Use fastify.auth0Client.getTokenOnBehalfOf() when your Fastify API needs to call a downstream API on behalf of the same user, such as in an MCP server. The method exchanges the incoming access token for a new one scoped to the downstream API while preserving the user's identity.

getTokenOnBehalfOf() requires a confidential client. Register the plugin with clientId and clientSecret (or clientAssertionSigningKey for client assertion authentication).

import fastifyAuth0Api from '@auth0/auth0-fastify-api';

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

fastify.register(fastifyAuth0Api, {
  domain: '<AUTH0_DOMAIN>',
  audience: '<AUTH0_AUDIENCE>',
  clientId: '<AUTH0_CLIENT_ID>',
  clientSecret: '<AUTH0_CLIENT_SECRET>',
});

fastify.register(() => {
  fastify.post(
    '/schedule-meeting',
    {
      preHandler: fastify.requireAuth(),
    },
    async (request: FastifyRequest) => {
      // request.getToken() returns the raw JWT without the `Bearer ` prefix.
      const obo = await fastify.auth0Client!.getTokenOnBehalfOf(request.getToken()!, {
        audience: 'https://calendar-api.example.com',
        scope: 'calendar:read calendar:write',
      });

      const response = await fetch('https://calendar-api.example.com/meetings', {
        method: 'POST',
        headers: { authorization: `Bearer ${obo.accessToken}` },
        body: JSON.stringify(request.body),
      });

      return response.json();
    }
  );
});

For a full walkthrough including error handling, act claim inspection, and the downstream verifier pattern, see the On-Behalf-Of Token Exchange section in EXAMPLES.md.

This project is licensed under the MIT license. See the LICENSE file for more info.

README.md

Documentation

Features

Getting Started

1. Install the SDK

3. Register the Auth0 Fastify plugin for APIs

Protecting API Routes

DPoP (Demonstration of Proof-of-Possession)

On-Behalf-Of Token Exchange

Feedback

Contributing

Raise an issue

Vulnerability Reporting

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