auth0-fastapi-api

Prerequisites

• FastAPI application (Python 3.9+)

• Auth0 API resource configured (not an Application — must be an API)

• If you don't have Auth0 set up yet, use the auth0-quickstart skill first

Quick Start Workflow

1. Install SDK

pip install auth0-fastapi-api python-dotenv

2. Create Auth0 API

You need an API (not Application) in Auth0.

STOP — ask the user before proceeding.

Ask exactly this question and wait for their answer before doing anything else:

"How would you like to create the Auth0 API resource?

• Automated — I'll run Auth0 CLI scripts that create the resource and write the exact values to your .env automatically.

• Manual — You create the API yourself in the Auth0 Dashboard (or via auth0 apis create) and provide me the Domain and Audience.

Which do you prefer? (1 = Automated / 2 = Manual)"

Do NOT proceed to any setup steps until the user has answered. Do NOT default to manual.

If the user chose Automated, follow the Setup Guide for complete CLI scripts. The automated path writes .env for you — skip Step 3 below and proceed directly to Step 4.

If the user chose Manual, follow the Setup Guide (Manual Setup section) for full instructions. Then continue with Step 3 below.

Quick reference for manual API creation:

# Using Auth0 CLI
auth0 apis create \
 --name "My FastAPI API" \
 --identifier https://my-api.example.com

Or create manually in Auth0 Dashboard → Applications → APIs

3. Configure Environment

Create .env:

AUTH0_DOMAIN=your-tenant.us.auth0.com
AUTH0_AUDIENCE=https://your-api.example.com

AUTH0_DOMAIN is your Auth0 tenant domain (without https://). AUTH0_AUDIENCE is the API identifier you set when creating the API resource in Auth0.

4. Initialize Auth0

import os
from fastapi import FastAPI, Depends
from fastapi_plugin import Auth0FastAPI
from dotenv import load_dotenv

load_dotenv()

app = FastAPI()

auth0 = Auth0FastAPI(
 domain=os.getenv("AUTH0_DOMAIN"),
 audience=os.getenv("AUTH0_AUDIENCE"),
)

Create one Auth0FastAPI instance per application and reuse it across routes. Never hardcode the domain or audience — always use environment variables.

5. Protect Routes

# Require any valid access token
@app.get("/api/private")
async def private(claims: dict = Depends(auth0.require_auth())):
 return {"user": claims["sub"]}

# No authentication required
@app.get("/api/public")
async def public():
 return {"message": "Public endpoint"}

The require_auth() dependency validates the Bearer token, verifies the issuer and audience, and returns the decoded JWT claims.

Error responses:

• 400 invalid_request — Missing or malformed Authorization header

• 401 invalid_token — Expired token, invalid signature, wrong issuer/audience

• 403 insufficient_scope — Valid token but missing required scopes

• 500 internal_server_error — Unexpected errors

Response body format: {"detail": {"error": "...", "error_description": "..."}}

6. Protect Routes with Scope Checks

# Requires the read:messages scope
@app.get("/api/messages")
async def get_messages(claims: dict = Depends(auth0.require_auth(scopes="read:messages"))):
 return {"messages": []}

# Requires both read:data and write:data scopes
@app.post("/api/data")
async def write_data(claims: dict = Depends(auth0.require_auth(scopes=["read:data", "write:data"]))):
 return {"created": True}

require_auth(scopes=...) checks the scope claim in the JWT. All specified scopes must be present (AND logic). Missing scopes return 403.

7. Access Token Claims

The decoded JWT claims are returned directly from the dependency:

@app.get("/api/profile")
async def profile(claims: dict = Depends(auth0.require_auth())):
 return {
 "sub": claims["sub"], # user ID
 "scope": claims.get("scope"), # granted scopes
 }

Key claims:

• claims["sub"] — user/client ID

• claims["scope"] — space-separated granted scopes

• claims["iss"] — issuer (your Auth0 domain URL)

• claims["aud"] — audience

• claims["exp"] — expiration timestamp

• claims["iat"] — issued-at timestamp

8. Protect Routes Without Needing Claims

@app.get("/api/protected", dependencies=[Depends(auth0.require_auth())])
async def protected():
 return {"message": "You need a valid access token to see this."}

9. Test the API

# No token — expect 401
curl http://localhost:8000/api/private

# With a valid access token
curl http://localhost:8000/api/private \
 -H "Authorization: Bearer YOUR_ACCESS_TOKEN"

Get a test token via Client Credentials flow or Auth0 Dashboard → APIs → Test tab.

Common Mistakes

Mistake Fix Hardcoding domain or audience in source Always read from environment variables — never embed credentials in code Using python-jose or PyJWT directly Not needed; auth0-fastapi-api handles all validation via JWKS Manually parsing Authorization header The SDK extracts and validates the token automatically Calling jwt.decode() manually The SDK verifies tokens against the JWKS endpoint — do not verify yourself Using fastapi-users for Auth0 JWT validation That package is for user management, not Auth0 JWT verification Created an Application instead of an API in Auth0 Must create an API resource (Applications → APIs) — an Application doesn't issue access tokens with the right audience Passing domain as full URL with https:// domain should be the bare domain, e.g. my-tenant.us.auth0.com, not https://my-tenant.us.auth0.com Using an ID token instead of an access token Must use the access token for API auth — ID tokens are for the client app, not for API authorization Not configuring CORS for SPA clients Add CORSMiddleware to allow requests from your frontend origin os.getenv() returns None silently Ensure python-dotenv is installed and load_dotenv() is called before Auth0FastAPI() initialization — or use os.environ[] to fail fast