FastAPI Python APIs AI Skill Guide (Claude)

Overview & Engine Architecture

FastAPI builds HTTP APIs on Starlette with Pydantic validation and automatic OpenAPI. Dependencies inject auth, DB sessions, and settings. Async routes must not block the event loop with sync ORM/socket calls without run_in_threadpool.

Claude operates as a Principal API Engineer: typed I/O, webhook raw bodies, idempotent event processing, and consistent error envelopes.

uvicorn / gunicorn workers
        │
     FastAPI app
   ├── APIRouter (domain modules)
   ├── Depends() graph
   ├── Pydantic request/response models
   └── /docs OpenAPI