Next.js App Router (Auth0Client): getAccessToken returns encrypted JWE (alg: dir) instead of JWT (RS256) despite correct Audience

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Hi everyone,

I am facing a persistent issue integrating a Next.js 15+ (App Router) frontend with a Python (FastAPI) backend.

The Problem: Even though I have configured the audience correctly in my Auth0Client instantiation, the auth0.getAccessToken() method in my Next.js API route returns an encrypted token (alg: dir, JWE) instead of the expected signed JWT (alg: RS256).

My Python backend rejects this token because it expects a standard RS256 Access Token to validate against the JWKS.

My Stack:

  • SDK: @auth0/nextjs-auth0 (Latest version using App Router setup)

  • Frontend: Next.js 15 (App Router)

  • Backend: Python / FastAPI

  • Flow: Authorization Code Flow

Configuration:

  1. Dashboard Settings:

    • API Identifier: https://vision-api

    • Signing Algorithm: RS256

    • JSON Web Encryption (JWE): Disabled (I double-checked this).

    • Application (M2M) is authorized for this API.

  2. Next.js Implementation (src/lib/auth0.ts): I am instantiating the client manually to use in the App Router:

    TypeScript

    import { Auth0Client } from "@auth0/nextjs-auth0/server";

export const auth0 = new Auth0Client({
  // domain, clientID, secret are loaded from env
  authorizationParams: {
    scope: 'openid profile email offline_access',
    audience: 'https://vision-api', // <--- Explicitly set here
  },
});

  3. The Proxy Route (src/app/api/proxy/route.ts):

    TypeScript

    import { auth0 } from '@/lib/auth0';

export async function GET(req: NextRequest) {
  try {
    const tokenResponse = await auth0.getAccessToken();
    const accessToken = tokenResponse.accessToken || tokenResponse.token;

    // DEBUG LOGGING
    console.log("Token Header:", decodeHeader(accessToken)); 
    // OUTPUT: { "alg": "dir", "enc": "A256GCM", ... } -> THIS IS THE ISSUE
  } catch (error) {
    // ...
  }
}

  4. The Auth Route (src/app/api/auth/[auth0]/route.ts):

    TypeScript

    import { auth0 } from '@/lib/auth0';
export const GET = auth0.handleAuth();

What I have verified (Debugging):

  1. Manual CURL Test (Success): If I request a token manually using client_credentials via terminal:

    Bash

    curl --request POST --url ... --data '{"audience":"https://vision-api", ...}'

    I receive a correct RS256 JWT. I can pass this token to my Python backend and it works perfectly (HTTP 200). This proves the API and Auth0 Dashboard configuration are correct.

  2. Browser Network Tab: When checking the /authorize request initiated by the Next.js app, I suspect the audience parameter might not be passing through correctly, causing Auth0 to issue an opaque/encrypted session token instead of an API Access Token.

The Question: Why does @auth0/nextjs-auth0 return an opaque/encrypted JWE (alg: dir) even when:

  1. audience is set in authorizationParams.

  2. JWE is disabled in the API settings in the Dashboard.

  3. I am requesting a token for a Custom API.

How can I force the SDK to retrieve the decrypted RS256 Access Token so I can pass it to my external backend?

Any help is appreciated.

3

Hi @GuiEzz

Welcome to the Auth0 Community!

Can you try to go to the Auth0 Dashboard-> Settings (Tenant setting)-> API Authorization Settings-> Default Audience ->" Set it as same as your api audience“.

Let me know if this does the trick to you or not!

Kind Regards,
Nik

4

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