---

name: backend-api description: Build FastAPI REST APIs with CORS, JWT auth, Pydantic validation, async endpoints, and proper error handling. Use when creating API endpoints, middleware, or backend services.

FastAPI Backend API Development

Core Pattern

Copyfrom fastapi import FastAPI, HTTPException, Depends
from fastapi.middleware.cors import CORSMiddleware
from pydantic import BaseModel, Field
import os

app = FastAPI(title="API")

# CORS from environment
app.add_middleware(
    CORSMiddleware,
    allow_origins=os.getenv("CORS_ORIGINS", "").split(","),
    allow_credentials=os.getenv("CORS_ALLOW_CREDENTIALS") == "true",
    allow_methods=["*"],
    allow_headers=["*"],
)

class Request(BaseModel):
    field: str = Field(..., min_length=1)

@app.post("/api/{user_id}/endpoint")
async def endpoint(user_id: str, req: Request, current_user: str = Depends(verify_jwt)):
    if user_id != current_user:
        raise HTTPException(status_code=403, detail="User ID mismatch")
    return {"status": "ok"}

Error Handling

• Use HTTPException with proper status codes (401, 403, 404, 500)
• Provide user-friendly error messages
• Always use async for I/O operations

Testing

Copyfrom fastapi.testclient import TestClient
client = TestClient(app)
response = client.post("/api/user123/endpoint", json={"field": "value"})
assert response.status_code == 200