fix(session): unwrap ExceptionGroup in BaseSession.__aexit__

ROOT CAUSE:
BaseSession's task group raises ExceptionGroup wrapping real errors
with CancelledError from cancelled tasks.

CHANGES:
- Modified __aexit__ to unwrap ExceptionGroup before propagating
- Real errors now propagate cleanly to callers

IMPACT:
- Callers can catch specific exceptions directly

FILES MODIFIED:
- src/mcp/shared/session.py: Added exception unwrapping in __aexit
- tests/shared/test_session_exception_group.py: Added test

Author: peter-luminova

src/mcp/shared/session.py

@@ -223,12 +223,23 @@ async def __aexit__(
         exc_val: BaseException | None,
         exc_tb: TracebackType | None,
     ) -> bool | None:
+        from mcp.shared.exceptions import unwrap_task_group_exception
+
         await self._exit_stack.aclose()
         # Using BaseSession as a context manager should not block on exit (this
         # would be very surprising behavior), so make sure to cancel the tasks
         # in the task group.
         self._task_group.cancel_scope.cancel()
-        return await self._task_group.__aexit__(exc_type, exc_val, exc_tb)
+
+        # Exit the task group and unwrap any ExceptionGroup
+        try:
+            return await self._task_group.__aexit__(exc_type, exc_val, exc_tb)
+        except BaseException as e:
+            # Unwrap ExceptionGroup to get only the real error
+            unwrapped = unwrap_task_group_exception(e)
+            if unwrapped is not e:
+                raise unwrapped
+            raise
 
     async def send_request(
         self,

tests/shared/test_session_exception_group.py

@@ -0,0 +1,51 @@
+"""Test that BaseSession unwraps ExceptionGroups properly."""
+from __future__ import annotations
+
+import anyio
+import pytest
+
+from mcp.shared.session import BaseSession
+
+
+class _TestSession(BaseSession):
+    """Test implementation of BaseSession."""
+
+    @property
+    def _receive_request_adapter(self):
+        from pydantic import TypeAdapter
+
+        return TypeAdapter(dict)
+
+    @property
+    def _receive_notification_adapter(self):
+        from pydantic import TypeAdapter
+
+        return TypeAdapter(dict)
+
+
+@pytest.mark.anyio
+async def test_session_propagates_real_error_not_exception_group() -> None:
+    """Test that real errors propagate unwrapped from session task groups."""
+    # Create streams
+    read_sender, read_stream = anyio.create_memory_object_stream()
+    write_stream, write_receiver = anyio.create_memory_object_stream()
+
+    try:
+        session = _TestSession(
+            read_stream=read_stream,
+            write_stream=write_stream,
+            read_timeout_seconds=None,
+        )
+
+        # The session's receive loop will start in __aenter__
+        # If it fails with ExceptionGroup, we want only the real error
+        with pytest.raises(ConnectionError, match="connection failed"):
+            async with session:
+                # Raise a connection error to trigger exception group behavior
+                raise ConnectionError("connection failed")
+
+    finally:
+        await read_sender.aclose()
+        await read_stream.aclose()
+        await write_stream.aclose()
+        await write_receiver.aclose()