mcp-sse-error-handling-client-hangs-indefinitely-when-server-sends-trunca-eff7acd4

THEN Add a custom message_handler to the ClientSession that listens for ValidationError and cancels the task group to break the hang. Implement an exception_handler that calls session._task_group.cancel_scope.cancel() on ValidationError, and pass it via the message_handler parameter of ClientSession.

async def exception_handler(session, exc):
    await anyio.lowlevel.checkpoint()
    if isinstance(exc, ValidationError):
        session._task_group.cancel_scope.cancel()

sess_exception_handler = partial(exception_handler, session=client_session)
client_session = ClientSession(streams[0], streams[1], message_handler=sess_exception_handler)