fix: handle ContextVar detach errors during task cancellation

Refactors context manager usage in asynchronous generators to gracefully
handle ValueError during cleanup, preventing crashes during task cancellation.

PiperOrigin-RevId: 902183602

Author: google-genai-bot

src/google/adk/agents/base_agent.py

@@ -16,6 +16,7 @@
 
 import inspect
 import logging
+import sys
 from typing import Any
 from typing import AsyncGenerator
 from typing import Awaitable
@@ -285,7 +286,9 @@ async def run_async(
       Event: the events generated by the agent.
     """
 
-    with tracer.start_as_current_span(f'invoke_agent {self.name}') as span:
+    cm = tracer.start_as_current_span(f'invoke_agent {self.name}')
+    span = cm.__enter__()
+    try:
       ctx = self._create_invocation_context(parent_context)
       tracing.trace_agent_invocation(span, self, ctx)
       if event := await self._handle_before_agent_callback(ctx):
@@ -302,6 +305,23 @@ async def run_async(
 
       if event := await self._handle_after_agent_callback(ctx):
         yield event
+    except BaseException:
+      try:
+        cm.__exit__(*sys.exc_info())
+      except ValueError:
+        logger.warning(
+            'Failed to detach context during generator cleanup, likely due to'
+            ' cancellation.'
+        )
+      raise
+    else:
+      try:
+        cm.__exit__(None, None, None)
+      except ValueError:
+        logger.warning(
+            'Failed to detach context during generator cleanup, likely due to'
+            ' cancellation.'
+        )
 
   @final
   async def run_live(

src/google/adk/flows/llm_flows/base_llm_flow.py

@@ -18,6 +18,7 @@
 import asyncio
 import inspect
 import logging
+import sys
 from typing import AsyncGenerator
 from typing import Optional
 from typing import TYPE_CHECKING
@@ -1168,7 +1169,9 @@ async def _call_llm_async(
   ) -> AsyncGenerator[LlmResponse, None]:
 
     async def _call_llm_with_tracing() -> AsyncGenerator[LlmResponse, None]:
-      with tracer.start_as_current_span('call_llm') as span:
+      cm = tracer.start_as_current_span('call_llm')
+      span = cm.__enter__()
+      try:
         # Runs before_model_callback inside the call_llm span so
         # plugins observe the same span as after/error callbacks.
         if response := await self._handle_before_model_callback(
@@ -1261,6 +1264,23 @@ async def _call_llm_with_tracing() -> AsyncGenerator[LlmResponse, None]:
                   llm_response = altered
 
               yield llm_response
+      except BaseException:
+        try:
+          cm.__exit__(*sys.exc_info())
+        except ValueError:
+          logger.warning(
+              'Failed to detach context during generator cleanup, likely due to'
+              ' cancellation.'
+          )
+        raise
+      else:
+        try:
+          cm.__exit__(None, None, None)
+        except ValueError:
+          logger.warning(
+              'Failed to detach context during generator cleanup, likely due to'
+              ' cancellation.'
+          )
 
     async with Aclosing(_call_llm_with_tracing()) as agen:
       async for event in agen:

src/google/adk/runners.py

@@ -543,7 +543,9 @@ async def _run_with_trace(
         new_message: Optional[types.Content] = None,
         invocation_id: Optional[str] = None,
     ) -> AsyncGenerator[Event, None]:
-      with tracer.start_as_current_span('invocation'):
+      cm = tracer.start_as_current_span('invocation')
+      span = cm.__enter__()
+      try:
         session = await self._get_or_create_session(
             user_id=user_id,
             session_id=session_id,
@@ -627,6 +629,23 @@ async def execute(ctx: InvocationContext) -> AsyncGenerator[Event]:
               self.session_service,
               skip_token_compaction=invocation_context.token_compaction_checked,
           )
+      except BaseException:
+        try:
+          cm.__exit__(*sys.exc_info())
+        except ValueError:
+          logger.warning(
+              'Failed to detach context during generator cleanup, likely due to'
+              ' cancellation.'
+          )
+        raise
+      else:
+        try:
+          cm.__exit__(None, None, None)
+        except ValueError:
+          logger.warning(
+              'Failed to detach context during generator cleanup, likely due to'
+              ' cancellation.'
+          )
 
     async with Aclosing(_run_with_trace(new_message, invocation_id)) as agen:
       async for event in agen:

tests/unittests/tools/BUILD

@@ -31,3 +31,17 @@ pytest_test(
         "//third_party/py/pytest_asyncio",
     ],
 )
+
+pytest_test(
+    name = "test_context_cancellation",
+    srcs = ["test_context_cancellation.py"],
+    args = [
+        "-p",
+        "pytest_asyncio.plugin",
+    ],
+    deps = [
+        "//third_party/py/google/adk",
+        "//third_party/py/google/genai",
+        "//third_party/py/pytest_asyncio",
+    ],
+)

tests/unittests/tools/test_context_cancellation.py

@@ -0,0 +1,85 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for ContextVar error handling during cancellation."""
+
+import asyncio
+import logging
+from typing import AsyncGenerator
+from unittest import mock
+
+from google.adk.agents.base_agent import BaseAgent
+from google.adk.agents.invocation_context import InvocationContext
+from google.adk.events.event import Event
+from google.adk.sessions.in_memory_session_service import InMemorySessionService
+from google.adk.telemetry.tracing import tracer
+from google.genai import types
+import pytest
+from typing_extensions import override
+
+
+class MinimalAgent(BaseAgent):
+
+  @override
+  async def _run_async_impl(
+      self, ctx: InvocationContext
+  ) -> AsyncGenerator[Event, None]:
+    yield Event(
+        author=self.name,
+        content=types.Content(parts=[types.Part(text='Hello')]),
+    )
+
+
+class FailingCM:
+
+  def __enter__(self):
+    return mock.Mock()
+
+  def __exit__(self, exc_type, exc_val, exc_tb):
+    raise ValueError('Mocked ContextVar error')
+
+
+@pytest.mark.asyncio
+async def test_run_async_handles_context_var_error(
+    caplog: pytest.LogCaptureFixture,
+):
+  agent = MinimalAgent(name='test_agent')
+
+  with mock.patch.object(
+      tracer, 'start_as_current_span', return_value=FailingCM()
+  ):
+
+    session_service = InMemorySessionService()
+    session = await session_service.create_session(
+        app_name='test', user_id='user'
+    )
+    ctx = InvocationContext(
+        invocation_id='inv_id',
+        agent=agent,
+        session=session,
+        session_service=session_service,
+    )
+
+    events = []
+    with caplog.at_level(logging.WARNING):
+      async for event in agent.run_async(ctx):
+        events.append(event)
+
+    assert len(events) == 1
+    assert events[0].content.parts[0].text == 'Hello'
+
+    assert any(
+        'Failed to detach context during generator cleanup' in record.message
+        for record in caplog.records
+    )