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

📚 Documentation - 🚀 Getting Started - 💬 Feedback

Documentation

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

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

Register the Auth0 fastify plugin for API's with the Fastify instance.

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:

1. An Authorization: DPoP <access_token> header (instead of Bearer).
2. 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.

Feedback

Contributing

We appreciate feedback and contribution to this repo! Before you get started, please read the following:

• Auth0's general contribution guidelines
• Auth0's code of conduct guidelines
• This repo's contribution guide

Raise an issue

To provide feedback or report a bug, please raise an issue on our issue tracker.

Vulnerability Reporting

Please do not report security vulnerabilities on the public GitHub issue tracker. The Responsible Disclosure Program details the procedure for disclosing security issues.

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