fix: Fix error swallowing in MCP session context

Co-authored-by: Xuan Yang <xygoogle@google.com>
PiperOrigin-RevId: 936815704

Author: xuanyang15

src/google/adk/tools/mcp_tool/session_context.py

@@ -36,6 +36,34 @@
 _T = TypeVar('_T')
 
 
+def _format_exception(exc: BaseException | None) -> str:
+  """Formats an exception into a readable string representation.
+
+  This handles `ExceptionGroup` (by flattening inner exceptions) and optionally
+  extracts HTTP response bodies for network-related errors, truncating them
+  to 1000 characters to prevent log/context overflow.
+
+  Args:
+    exc: The exception to format.
+
+  Returns:
+    A formatted string representing the exception and its pertinent details.
+  """
+  if exc is None:
+    return 'None'
+  if hasattr(exc, 'exceptions') and getattr(exc, 'exceptions'):
+    return ' | '.join(_format_exception(e) for e in exc.exceptions)
+  if hasattr(exc, 'response') and exc.response is not None:
+    try:
+      response_text = exc.response.text
+      if len(response_text) > 1000:
+        response_text = response_text[:1000] + '... [truncated]'
+      return f'{exc} (Response: {response_text})'
+    except Exception:
+      pass
+  return str(exc)
+
+
 class SessionContext:
   """Represents the context of a single MCP session within a dedicated task.
 
@@ -143,7 +171,8 @@ def _retrieve_exception(t: asyncio.Task):
 
     if self._task.done() and self._task.exception():
       raise ConnectionError(
-          f'Failed to create MCP session: {self._task.exception()}'
+          'Failed to create MCP session:'
+          f' {_format_exception(self._task.exception())}'
       ) from self._task.exception()
 
     # Pre-fix code returned `self._session` here directly (typed as
@@ -186,7 +215,7 @@ async def _run_guarded(self, coro: Coroutine[Any, Any, _T]) -> _T:
       # Close the coroutine to avoid "was never awaited" warnings.
       coro.close()
       raise ConnectionError(
-          f'MCP session task has already terminated: {exc}'
+          f'MCP session task has already terminated: {_format_exception(exc)}'
       ) from exc
 
     coro_task = asyncio.ensure_future(coro)
@@ -212,7 +241,9 @@ async def _run_guarded(self, coro: Coroutine[Any, Any, _T]) -> _T:
       pass
 
     exc = self._task.exception() if not self._task.cancelled() else None
-    raise ConnectionError(f'MCP session connection lost: {exc}') from exc
+    raise ConnectionError(
+        f'MCP session connection lost: {_format_exception(exc)}'
+    ) from exc
 
   async def close(self):
     """Signal the context task to close and wait for cleanup."""

tests/unittests/tools/mcp_tool/test_session_context.py

@@ -23,7 +23,9 @@
 
 from google.adk.features import FeatureName
 from google.adk.features._feature_registry import temporary_feature_override
+from google.adk.tools.mcp_tool.session_context import _format_exception
 from google.adk.tools.mcp_tool.session_context import SessionContext
+import httpx
 from mcp import ClientSession
 import pytest
 
@@ -896,3 +898,43 @@ async def test_no_extra_none_check_when_flag_off(self):
           assert result is not None
         finally:
           await session_context.close()
+
+
+class TestFormatException:
+  """Test suite for _format_exception helper."""
+
+  def test_format_exception_normal(self):
+    exc = ValueError('normal error')
+    assert _format_exception(exc) == 'normal error'
+
+  def test_format_exception_http_status_error(self):
+    request = httpx.Request('GET', 'http://test')
+    response = httpx.Response(403, request=request, text='Forbidden access')
+    exc = httpx.HTTPStatusError(
+        '403 Forbidden', request=request, response=response
+    )
+
+    formatted = _format_exception(exc)
+    assert '403 Forbidden' in formatted
+    assert 'Forbidden access' in formatted
+
+  def test_format_exception_group(self):
+    class MockExceptionGroup(Exception):
+
+      def __init__(self, message, exceptions):
+        super().__init__(message)
+        self.exceptions = exceptions
+
+    request = httpx.Request('GET', 'http://test')
+    response = httpx.Response(403, request=request, text='Forbidden access')
+    exc1 = httpx.HTTPStatusError(
+        '403 Forbidden', request=request, response=response
+    )
+    exc2 = ValueError('another error')
+
+    eg = MockExceptionGroup('Group', [exc1, exc2])
+    formatted = _format_exception(eg)
+
+    assert '403 Forbidden' in formatted
+    assert 'Forbidden access' in formatted
+    assert 'another error' in formatted