asyncio will garbage-collect your fire-and-forget task

I wanted chat sessions to name themselves. When a conversation starts, kick off a cheap, separate model call built from the first prompt, let it run in the background, and drop the title into the sidebar the moment it's ready — so it shows up seconds in and lands even if the main turn later pauses or the client disconnects. Classic fire-and-forget: create_task, don't await.

It worked. Mostly. Every so often a conversation just never got a title, with no error and no pattern I could pin down.

The cause is a genuine asyncio footgun, and it's in the docs if you go looking: the event loop keeps only a weak reference to a task. If nothing in your code holds a reference to it, the garbage collector is free to collect it mid-flight — and your background work just quietly stops. await and gather hold references, which is why you never notice this until the first time you truly fire-and-forget.

The fix is three lines: keep a strong reference, and drop it when the task finishes.

_background: set[asyncio.Task] = set()   # strong refs; the loop only keeps weak ones
 
def spawn(coro) -> None:
    task = asyncio.create_task(coro)
    _background.add(task)                 # <- this is what keeps it alive
    task.add_done_callback(_background.discard)

Two more things I got wrong on the way, both about "cosmetic work must never break the real work":

• The task must never raise. A missing title is cosmetic; it must not bubble up as a failure on the stream the user is actually watching. So it swallows everything except CancelledError (which it re-raises, so cooperative cancellation still works) and falls back to the first line of the prompt.
• Generate from the prompt alone. My first version built the title from the prompt and the finished answer, so it couldn't run until the turn completed — and a turn that paused for approval never got one. Decoupling it from the assistant's reply is what let it run early and survive.

The reframe I keep: "fire-and-forget" in asyncio is really "fire, hold a reference, and forget." The garbage collector has no idea your background job matters unless something is still pointing at it.