FastAPI

Builds, debugs, and hardens FastAPI services: async endpoints, Pydantic models, dependency injection, auth, and deployment. Use when writing or reviewing a FastAPI app, router, or schema; when the API hangs or every route slows because a sync driver or CPU work blocks the event loop; when a request returns an unexpected 422, a POST becomes a 307, or a 500 arrives with no CORS headers; when migrating pydantic v1 to v2 (`model_dump`, `from_attributes`, `field_validator`); when `Depends` runs too often, `yield` cleanup misfires, or dependency overrides do not take; when SQLAlchemy raises `QueuePool limit` or `MissingGreenlet`, or connections leak across workers; when TestClient, httpx `AsyncClient`, or pytest-asyncio fail with a closed event loop; when uvicorn or gunicorn workers, graceful shutdown, `root_path`, or proxy headers need deciding; or when background tasks, WebSockets, SSE, uploads, or the OpenAPI docs misbehave. Not for Django, Flask, or general Python — `django`, `flask`, `py`.