FastAPI BackgroundTasks runs after request-scoped DB sessions close, needs its own session

FastAPI's BackgroundTasks fire AFTER the response is sent and AFTER request-scoped dependencies with yield have run their cleanup, so a Session opened by a with Session(engine) as session: generator dependency is already closed by the time the background task runs. Closing over self.session from a service whose lifetime is tied to the request will surface as ResourceClosedError or stale-state errors when the task body issues queries. Compounding this: sync background tasks run in Starlette's threadpool, and SQLAlchemy Session objects are not safe to share across threads even if they were still open. Verified 2026-04 with FastAPI + SQLModel + sync BackgroundTasks.add_task.

For any work scheduled via FastAPI BackgroundTasks, do NOT close over the request-scoped session. Pass primitives (ids, strings) into the task and have the task open its own short-lived with Session(engine) as session: context. To audit existing code, trace whether the dep that opens the session uses yield, those are request-scoped and unsafe to capture into a task closure. Verify the failure shape with a TestClient test: TestClient runs background tasks before returning the response, so a "session is closed" failure surfaces immediately rather than only in production.