RuntimeError: Task attached to a different loop

Root cause

Background task created during FastAPI app startup in a session-scoped async fixture before pytest-asyncio reset the event loop for the individual test. The dispatcher stored a Task/Future bound to the old loop, then the test awaited it through the ASGI client on a new function loop.

Minimal fix

Make the FastAPI app/client fixture function-scoped, provide a function-scoped event_loop if the project overrides it, and explicitly run/await lifespan startup inside the same fixture loop before yielding the AsyncClient. Avoid creating dispatcher tasks at import time or in session-scoped fixtures.