Fix #915: Resolve AnyIO cancellation scope RuntimeError in ClientSessionGroup on connection disconnects

Author: RJ Lopez

src/mcp/client/session_group.py

@@ -165,10 +165,11 @@ async def __aexit__(
         if self._owns_exit_stack:
             await self._exit_stack.aclose()
 
-        # Concurrently close session stacks.
-        async with anyio.create_task_group() as tg:
-            for exit_stack in self._session_exit_stacks.values():
-                tg.start_soon(exit_stack.aclose)
+        # Sequentially close session stacks to preserve AnyIO task contexts.
+        # Concurrent teardown spawns task groups that cross cancel scopes, leading
+        # to RuntimeError: Attempted to exit cancel scope in a different task.
+        for exit_stack in list(self._session_exit_stacks.values()):
+            await exit_stack.aclose()
 
     @property
     def sessions(self) -> list[mcp.ClientSession]: