Agent skill
fastapi-backend
FastAPI best practices for building production Python backends. Use when building REST APIs, async endpoints, Pydantic models, dependency injection, middleware, or any FastAPI server.
Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/other/other/fastapi-backend-cohen-liel-hivemind
SKILL.md
FastAPI Backend Patterns
Project Structure
app/
main.py # FastAPI app, lifespan, middleware
config.py # Pydantic Settings, env vars
models/ # SQLAlchemy ORM models
schemas/ # Pydantic request/response schemas
routers/ # APIRouter per domain (auth, users, posts)
services/ # Business logic (no DB or HTTP here)
deps.py # Shared dependencies (get_db, get_current_user)
database.py # Engine, SessionLocal, Base
Core Patterns
App setup with lifespan
from contextlib import asynccontextmanager
from fastapi import FastAPI
@asynccontextmanager
async def lifespan(app: FastAPI):
# startup
await database.connect()
yield
# shutdown
await database.disconnect()
app = FastAPI(lifespan=lifespan)
Dependency injection
# deps.py
async def get_db() -> AsyncGenerator[AsyncSession, None]:
async with AsyncSessionLocal() as session:
try:
yield session
await session.commit()
except Exception:
await session.rollback()
raise
async def get_current_user(
token: str = Depends(oauth2_scheme),
db: AsyncSession = Depends(get_db)
) -> User:
...
Router pattern
router = APIRouter(prefix="/users", tags=["users"])
@router.get("/{user_id}", response_model=UserResponse)
async def get_user(user_id: int, db: AsyncSession = Depends(get_db)):
user = await db.get(User, user_id)
if not user:
raise HTTPException(status_code=404, detail="User not found")
return user
Error handling
from fastapi import Request
from fastapi.responses import JSONResponse
@app.exception_handler(ValueError)
async def value_error_handler(request: Request, exc: ValueError):
return JSONResponse(status_code=400, content={"detail": str(exc)})
Pydantic schemas
class UserCreate(BaseModel):
email: EmailStr
password: str = Field(min_length=8)
name: str = Field(max_length=100)
class UserResponse(BaseModel):
id: int
email: str
name: str
model_config = ConfigDict(from_attributes=True)
Rules
- Always use async def for endpoints
- Validate ALL input with Pydantic (never trust raw dicts)
- Use HTTPException with specific status codes (not 500 for everything)
- Never put business logic in route handlers — use service layer
- Use response_model to control what's returned (never expose passwords)
- Add CORS middleware for browser clients
- Use Depends() for database sessions — never create sessions manually
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