Does FastAPI Work With Auth.js?
FastAPI and Auth.js can work together, but they require careful architectural planning since Auth.js is designed for JavaScript frameworks and FastAPI is a Python backend.
Quick Facts
How FastAPI Works With Auth.js
FastAPI and Auth.js aren't naturally paired because Auth.js is a JavaScript library designed to work with frameworks like Next.js, SvelteKit, and Nuxt, while FastAPI is a Python backend framework. However, they can integrate effectively in a decoupled architecture where FastAPI serves as your API backend and Auth.js handles authentication in a JavaScript frontend. Auth.js can be configured to call FastAPI endpoints for custom credential verification, session management, and user data retrieval. The typical pattern involves Auth.js managing the authentication UI and session tokens, then sending those tokens to FastAPI in Authorization headers for protected API calls. You'll need to implement JWT validation in FastAPI using libraries like `python-jose` and manage CORS carefully to allow your frontend to communicate securely. The main advantage is leveraging Auth.js's battle-tested auth UI components while keeping your backend in Python, though you lose some of the integrated provider ecosystems that exist for Next.js-based setups.
Best Use Cases
FastAPI with Auth.js JWT Validation
pip install fastapi python-jose python-dotenv pydanticfrom fastapi import FastAPI, Depends, HTTPException
from fastapi.security import HTTPBearer, HTTPAuthCredentials
from jose import JWTError, jwt
from datetime import datetime
import os
app = FastAPI()
security = HTTPBearer()
SECRET_KEY = os.getenv("AUTH_SECRET")
async def verify_token(credentials: HTTPAuthCredentials = Depends(security)):
try:
payload = jwt.decode(
credentials.credentials,
SECRET_KEY,
algorithms=["HS256"]
)
email: str = payload.get("email")
if email is None:
raise HTTPException(status_code=401, detail="Invalid token")
return email
except JWTError:
raise HTTPException(status_code=401, detail="Invalid token")
@app.get("/api/protected")
async def protected_route(email: str = Depends(verify_token)):
return {"message": f"Hello {email}"}Known Issues & Gotchas
Auth.js session tokens may not automatically sync with FastAPI's token validation strategy
Fix: Explicitly configure Auth.js JWT callbacks to match your FastAPI JWT claims structure, and validate tokens in FastAPI middleware using the same secret/algorithm
CORS preflight requests fail when sending Authorization headers from Auth.js to FastAPI
Fix: Configure FastAPI's CORSMiddleware to allow Authorization headers and set proper credentials=true in your fetch calls
Auth.js providers are configured for JavaScript-based callbacks, making custom FastAPI integration awkward
Fix: Use Auth.js's credentials provider or OAuth providers, then validate/enrich user data in FastAPI after token verification
Session refresh logic may cause race conditions between Auth.js client-side token expiry and FastAPI backend validation
Fix: Implement refresh token rotation in Auth.js callbacks that coordinate with FastAPI's token endpoint
Alternatives
- •NextAuth.js + Next.js API Routes: Full JavaScript ecosystem with built-in integration, best if you're already using Next.js
- •Passport.js + Express.js: Mature Node.js auth solution with extensive strategy support, lighter than Auth.js for backend-heavy apps
- •Keycloak + FastAPI: Enterprise-grade identity provider with both FastAPI and JavaScript client libraries for complete separation of concerns
- •Django + django-allauth: If you prefer Django over FastAPI, django-allauth provides comprehensive auth with similar Auth.js-like features
Resources
Related Compatibility Guides
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