Does Flask Work With Auth.js?
Auth.js is designed for JavaScript frameworks and doesn't natively support Flask, but you can use them together by treating Flask as a backend API and running Auth.js in a separate frontend application.
Quick Facts
How Flask Works With Auth.js
Auth.js is fundamentally a client-side and Node.js authentication library designed for frameworks like Next.js, SvelteKit, and Nuxt. Flask is a Python WSGI framework, so they operate in different runtime environments. To use them together, you'll architect your application as a decoupled system: Flask handles your backend API (database, business logic, protected endpoints), while Auth.js runs in a separate JavaScript frontend (Next.js, SvelteKit, or vanilla setup) managing user sessions and authentication flows. Auth.js communicates with Flask via REST/GraphQL APIs, typically storing JWT tokens or session identifiers that Flask validates on incoming requests. This approach requires setting up CORS properly, coordinating session/token formats between the two systems, and ensuring your Flask endpoints validate Auth.js-issued credentials. The developer experience involves working across two separate codebases and managing deployment of both services independently, which adds operational complexity but provides clean separation of concerns.
Best Use Cases
Flask API with Auth.js JWT Validation
pip install flask flask-cors pyjwtfrom flask import Flask, request, jsonify
from flask_cors import CORS
import jwt
from functools import wraps
app = Flask(__name__)
CORS(app)
SECRET_KEY = 'your-auth-js-secret'
def verify_token(f):
@wraps(f)
def decorated_function(*args, **kwargs):
token = request.headers.get('Authorization', '').replace('Bearer ', '')
try:
payload = jwt.decode(token, SECRET_KEY, algorithms=['HS256'])
request.user = payload
except jwt.InvalidTokenError:
return jsonify({'error': 'Invalid token'}), 401
return f(*args, **kwargs)
return decorated_function
@app.route('/api/protected', methods=['GET'])
@verify_token
def protected_route():
return jsonify({'message': f"Hello {request.user['email']}"})
if __name__ == '__main__':
app.run()Known Issues & Gotchas
Auth.js expects a Node.js callback endpoint (/api/auth/*) which Flask can't natively provide
Fix: Run Auth.js in a separate Node.js service or frontend application; have it call your Flask API for custom logic instead of trying to integrate Auth.js directly into Flask
Session/token format mismatch between Auth.js (JWT/cookies) and Flask authentication expectations
Fix: Explicitly configure Auth.js callbacks to generate tokens in a format Flask can validate; use PyJWT on the Flask side to verify Auth.js-issued JWTs with matching secrets
CORS and credential sharing issues when Auth.js frontend and Flask API are on different domains
Fix: Configure Flask with flask-cors, set sameSite and secure cookie policies carefully, and ensure Auth.js is configured with correct API base URLs
Auth.js callback events (like signIn) can't directly trigger Flask logic without additional API calls
Fix: Use Auth.js callbacks to make POST requests to Flask endpoints for side effects like user creation or logging
Alternatives
- •Flask-Login + Flask-SQLAlchemy with custom session management (purely Python-based, simpler if staying monolithic)
- •Auth0 + Flask (third-party provider handles auth, Flask validates tokens via Auth0 SDK)
- •FastAPI + Auth.js (Python async framework that feels more modern and integrates better with external auth systems)
- •Django + Djoser DRF + separate JavaScript frontend (more batteries-included than Flask, with better built-in auth patterns)
Resources
Related Compatibility Guides
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