mcp-sse-parsing-streamable-http-client-call-tool-hangs-when-receiv-db4b94c6

THEN Add a custom message_handler to the ClientSession that cancels the task group on ValidationError to prevent indefinite hang. Alternatively, sanitize tool outputs to avoid characters like \x85 that trigger httpx line truncation.

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

# When creating ClientSession:
client_session = ... # from streamable_http
handler = partial(exception_handler, session=client_session)
ClientSession(streams[0], streams[1], message_handler=handler)