ParallelAgent on Python 3.10 can raise `RuntimeError: aclose(): asynchronous generator is already running` after one sub-agent fails

[talvaknin744]

On Python 3.10, google.adk.agents.ParallelAgent uses _merge_agent_run_pre_3_11() instead of asyncio.TaskGroup. If one sub-agent async generator raises,
_merge_agent_run_pre_3_11() cancels sibling processing tasks in finally but does not await those tasks before ParallelAgent._run_async_impl() closes each sub-agent
generator.

That can raise a second cleanup exception:

RuntimeError: aclose(): asynchronous generator is already running

This can mask or pollute the original error, for example a normal Pydantic validation failure from a structured-output sub-agent.

Environment

• Python: 3.10.12
• google-adk: 1.26.0
• litellm: 1.82.1
• OS: Linux
• Agent shape: SequentialAgent containing a ParallelAgent with multiple LlmAgent children using output_schema and output_key

Minimal repro

import asyncio
from google.adk.agents.parallel_agent import _merge_agent_run_pre_3_11

async def slow_agent():
    try:
        await asyncio.sleep(10)
        yield "slow-event"
    finally:
        await asyncio.sleep(0.1)

async def failing_agent():
    await asyncio.sleep(0.01)
    raise ValueError("simulated sub-agent validation failure")
    yield "unreachable"

async def main():
    agent_runs = [slow_agent(), failing_agent()]

    try:
        async for event in _merge_agent_run_pre_3_11(agent_runs):
            print("event", event)
    except Exception as exc:
        print("merge raised:", type(exc).__name__, exc)

    for i, agen in enumerate(agent_runs):
        try:
            await agen.aclose()
            print("closed", i)
        except Exception as exc:
            print("close raised:", i, type(exc).__name__, exc)

asyncio.run(main())

## Actual output

merge raised: ValueError simulated sub-agent validation failure
close raised: 0 RuntimeError aclose(): asynchronous generator is already running
closed 1

## Expected behavior

ADK should propagate the original sub-agent failure cleanly and close/cancel sibling sub-agent runs without raising aclose(): asynchronous generator is already running.

## Suspected root cause

In _merge_agent_run_pre_3_11(), the finally block cancels processing tasks:

finally:
  for task in tasks:
    task.cancel()

but does not await their cancellation. Then ParallelAgent._run_async_impl() closes the same async generators:

finally:
  for sub_agent_run in agent_runs:
    await sub_agent_run.aclose()

If a task is still inside async for on a sibling generator, aclose() races that still-running generator.

## Suggested fix

After canceling tasks in _merge_agent_run_pre_3_11(), await them before the parent closes sub-agent generators:

finally:
  for task in tasks:
    task.cancel()
  if tasks:
    await asyncio.gather(*tasks, return_exceptions=True)

In a local simulation, this preserved the original ValueError and allowed both async generators to close cleanly.

[llalitkumarrr]

Hello @talvaknin744 ,

We highly recommend upgrading to a more recent version of Python and trying the latest version of the google-adk (version 1.29.0).

[talvaknin744]

Hi @llalitkumarrr, I tried google-adk==1.29.0 as requested.

On Python 3.10.12, the minimal repro still fails with the same cleanup race:

google-adk 1.29.0
merge raised: ValueError simulated sub-agent validation failure
close raised: 0 RuntimeError aclose(): asynchronous generator is already running
closed 1

I also checked Python 3.11.13 with google-adk==1.29.0. That path uses _merge_agent_run / asyncio.TaskGroup, raises an ExceptionGroup for the original sub-agent failure, and
closes both generators without the aclose() RuntimeError.

So upgrading to google-adk==1.29.0 alone does not fix the Python 3.10 compatibility path. Moving to Python 3.11 avoids the failure because ADK uses the TaskGroup path there,
but the Python 3.10 _merge_agent_run_pre_3_11() implementation still appears affected.

I also inspected the v1.29.0 source for parallel_agent.py; _merge_agent_run_pre_3_11() still cancels tasks with task.cancel() and does not await the cancelled tasks before the
parent closes the sub-agent async generators.

Let me know if there's a specific patch you'd like me to test for the 3.10 path!

[llalitkumarrr]

Hello @talvaknin744 ,

We would also recommend upgrading Python to a more recent version as it would be more compatible with the new ADK version. If you have the flexibility kindly upgrade your Python version.
Also, please check the associated PR to see if it resolves this issue.

[adk-bot]

This issue has been automatically marked as stale because it has not had recent activity for 7 days after a maintainer requested clarification. It will be closed if no further activity occurs within 7 days.

[adk-bot]

This has been automatically closed because it has been marked as stale for over 7 days.