RuntimeError: Task attached to a different loop

Root cause

Background publisher task was created during FastAPI startup from a session-scoped async fixture, then pytest-asyncio reset/replaced the running loop for the test function. The request handler awaited a task bound to the stale loop.

Minimal fix

Change the FastAPI app/client fixture to function scope, avoid creating background tasks at import or session setup time, and explicitly run/await application startup inside the same event loop used by each test. For example, use a function-scoped ASGITransport with LifespanManager(app) or call startup before yielding the AsyncClient, then shut down after the test.