Browser Use Version

0.13.1 where I hit it; the code paths are the same in 0.13.10 and main at 302d8fc, which pin bubus==1.5.6

Bug Description, Steps to Reproduce, Screenshots

Agent.run() can end with asyncio.CancelledError: Event handler browser_use.browser.watchdog_base.DOMWatchdog.on_BrowserStateRequestEvent#...(...) was interrupted because of a parent timeout, raised in a task that nobody cancelled (asyncio.current_task().cancelling() == 0). CancelledError is a BaseException, so Agent.step's except Exception (agent/service.py#L1080-L1082) and the caller's except Exception both miss it. I hit this in a production app. It comes from get_browser_state_summary (via step -> _prepare_context), where event.event_result(raise_if_none=True, raise_if_any=True) re-raises the stored CancelledError of DOMWatchdog.on_BrowserStateRequestEvent after it ran inline inside BrowserSession.on_NavigateToUrlEvent.

The cause is in bubus 1.5.6 (main has the same code):

1. A handler that awaits a child event processes the queued events of every EventBus in the process inside its own task (models.py#L303-L313); a BrowserStateRequestEvent queued by an agent can run inline inside a navigate handler.
2. When that navigate handler hits its timeout, the inline handler is cancelled with it, and bubus stores a new CancelledError("... interrupted because of a parent timeout") as that event's result (service.py#L1162-L1174).
3. If the interrupted handler had dispatched a child (DOMWatchdog dispatches a ScreenshotEvent), the child's completion later marks the event complete, and event_result() re-raises the stored CancelledError to the awaiter (models.py#L507-L516); the loop meant to ignore errors while collecting results only catches Exception (models.py#L487-L492). Without a child, the event never completes and the awaiter gets a bare TimeoutError after event_timeout.

get_browser_state_summary handles TimeoutError since 0.13.10 (session.py#L1633-L1635), but not this CancelledError.

Draining every bus is also the root cause of #5509. browser-use/bubus#34 drains only the awaited chain and fixes the repro below; browser-use/bubus#30 does not. The repro uses bubus alone (one loop, one bus, no browser) with browser-use's event names.

Failing Python Code

import asyncio
from importlib.metadata import version

from bubus import BaseEvent, EventBus


class NavigateToUrlEvent(BaseEvent[None]):
	event_timeout: float | None = 0.5


class NavigationCompleteEvent(BaseEvent[None]):
	pass


class BrowserStateRequestEvent(BaseEvent[str]):
	event_timeout: float | None = 5.0


class ScreenshotEvent(BaseEvent[str]):
	pass


async def main() -> None:
	print('bubus', version('bubus'))
	bus = EventBus(name='Session')

	async def on_navigate(event: NavigateToUrlEvent) -> None:
		# awaiting a child inside a handler makes bubus run other queued events inline, in this task
		await bus.dispatch(NavigationCompleteEvent())

	async def on_navigation_complete(event: NavigationCompleteEvent) -> None:
		pass

	async def on_state_request(event: BrowserStateRequestEvent) -> str:
		bus.dispatch(ScreenshotEvent())  # like DOMWatchdog, which dispatches a ScreenshotEvent
		await asyncio.sleep(2)  # longer than the navigate handler's 0.5 s timeout
		return 'state'

	async def on_screenshot(event: ScreenshotEvent) -> str:
		return 'png'

	bus.on(NavigateToUrlEvent, on_navigate)
	bus.on(NavigationCompleteEvent, on_navigation_complete)
	bus.on(BrowserStateRequestEvent, on_state_request)
	bus.on(ScreenshotEvent, on_screenshot)

	bus.dispatch(NavigateToUrlEvent())
	state_request = bus.dispatch(BrowserStateRequestEvent())
	try:
		print('result:', await state_request.event_result(raise_if_any=True, raise_if_none=True))
	except Exception as e:
		print('caught by except Exception:', type(e).__name__, e)
	except asyncio.CancelledError as e:
		print(f'CancelledError in a task with cancelling()={asyncio.current_task().cancelling()}: {e}')
	await bus.stop(clear=True)


asyncio.run(main())

LLM Model

None (no agent or LLM involved in the repro)

Operating System & Browser Versions

macOS, Python 3.11; the repro launches no browser

Full DEBUG Log Output

bubus 1.5.6
CancelledError in a task with cancelling()=0: Event handler __main__.on_state_request#9888(?▶ BrowserStateRequestEvent#89dc 🏃) was interrupted because of a parent timeout

Same on bubus main de1519b and on the head of browser-use/bubus#30. On the head of browser-use/bubus#34 it prints result: state.

Expected: Agent.run() does not raise CancelledError unless its own task was cancelled; a state request interrupted by another handler's timeout surfaces as an ordinary step error.

Possible fixes: merge and release browser-use/bubus#34 and bump the pin. Until then, get_browser_state_summary could handle a CancelledError caught while asyncio.current_task().cancelling() == 0 like its TimeoutError. In bubus, storing a non-cancellation error for the interrupted handler (the TODO at service.py#L1167 mentions InterruptedError) would also stop the re-raise.