RemoteA2aAgent in task mode hangs Runner.run_async() when the caller stops at its error event #7918

🔴 Required Information

Describe the Bug:
In task mode, RemoteA2aAgent._run_async_impl yields the finish_task and agent state events from its finally block. Most failure paths (authentication or initialization failure, no parts to send, a request interceptor returning an event, an unfinished stream, a request error) set should_release_task_control before they yield the error event. If the generator is closed while it is paused at that error event, the finally block yields again and Python raises RuntimeError: async generator ignored GeneratorExit.

This happens when the caller stops reading Runner.run_async() at that event, with break or by cancelling the task. The Runner then cancels the root node, and the cancellation closes the agent's generator. The RuntimeError replaces the cancellation:

• NodeRunner handles it as a node failure. It logs Node execution failed with exception with the traceback below, and on_agent_error_callback plugins receive the RuntimeError.
• NodeRunner then enqueues an error event for that failure. Nothing reads the event queue anymore, so the remote agent's node task waits forever. Closing Runner.run_async() never finishes, and the caller's break or cancel() hangs.

With abort_signal passed to run_async(), the run still closes, but the log and the plugin callback happen the same way.

Steps to Reproduce:

1. Run the code below. Nothing listens on 127.0.0.1:9, so the remote agent fails to initialize.

Expected Behavior:
The run closes after break, and no agent error is reported.

stopping at: Failed to initialize remote A2A agent: ...
closed

Observed Behavior:
closed is never printed and the script does not exit. Before it hangs, this is logged:

Node execution failed with exception
Traceback (most recent call last):
  File ".../google/adk/agents/base_agent.py", line 448, in _run
    yield event
GeneratorExit

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".../google/adk/workflow/_node_runner.py", line 170, in run
    await self._execute_node(ctx, node_input)
  ...
  File ".../google/adk/agents/base_agent.py", line 446, in _run
    async with Aclosing(impl_fn(ctx)) as agen:
  ...
RuntimeError: async generator ignored GeneratorExit

The plugin also prints on_agent_error_callback: RuntimeError('async generator ignored GeneratorExit').

Environment Details:

• ADK Library Version (pip show google-adk): 2.13.0 (also on main at 9a5117b)
• Desktop OS: macOS
• Python Version (python -V): 3.14.7

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (the repro uses a stub model)

🟡 Optional Information

Additional Context:
Calling _run_async_impl directly shows the cause: after the first (error) event, aclose() raises the RuntimeError in task mode and returns normally when mode is not set.

Only the paths that set should_release_task_control before yielding are affected. A close while the response stream is being read, or after finish_task is received, is fine.

Minimal Reproduction Code:

import asyncio

from google.adk.agents import LlmAgent
from google.adk.agents.remote_a2a_agent import RemoteA2aAgent
from google.adk.apps import App
from google.adk.models import BaseLlm
from google.adk.models import LlmResponse
from google.adk.plugins import BasePlugin
from google.adk.runners import InMemoryRunner
from google.adk.utils.context_utils import Aclosing
from google.genai import types


class CallRemote(BaseLlm):

  async def generate_content_async(self, llm_request, stream=False):
    yield LlmResponse(
        content=types.Content(
            role="model",
            parts=[types.Part.from_function_call(name="remote", args={})],
        )
    )


class PrintAgentErrors(BasePlugin):

  async def on_agent_error_callback(self, *, agent, callback_context, error):
    print("on_agent_error_callback:", repr(error))


async def main():
  remote = RemoteA2aAgent(
      name="remote",
      description="Remote agent",
      agent_card="http://127.0.0.1:9/.well-known/agent-card.json",
      mode="task",
  )
  root = LlmAgent(
      name="root", model=CallRemote(model="call-remote"), sub_agents=[remote]
  )
  runner = InMemoryRunner(
      app=App(
          name="app",
          root_agent=root,
          plugins=[PrintAgentErrors(name="print_agent_errors")],
      )
  )
  session = await runner.session_service.create_session(
      app_name="app", user_id="user"
  )

  async with Aclosing(
      runner.run_async(
          user_id="user",
          session_id=session.id,
          new_message=types.Content(
              role="user", parts=[types.Part(text="start")]
          ),
      )
  ) as events:
    async for event in events:
      if event.author == "remote" and event.error_message:
        print("stopping at:", event.error_message)
        break
  print("closed")


asyncio.run(main())

How often has this issue occurred?:

• Always (100%)