RuntimeError: Task attached to a different loop

Root cause

Background dispatcher task and asyncio.Queue were initialized by a session-scoped FastAPI app/client fixture before pytest-asyncio replaced the event loop for the individual test, so request handling awaited a Future bound to the stale loop.

Minimal fix

Make the async client/app fixture function-scoped, create startup resources inside the active test loop, and explicitly await FastAPI lifespan startup/shutdown in the fixture, e.g. use LifespanManager(app) with httpx.AsyncClient(ASGITransport(app=app), base_url='http://test') inside a function-scoped async fixture.