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AcknowledgeAsync swallowed every non-cancellation transport failure (and returned normally when the session was lost while the ack queued), so the host logged the Part 9 ack as applied while upstream still showed it unacknowledged. Both now throw (ServiceResultException with the transport status / BadNotConnected). A reply, including per-ack refusals, still returns normally and counts as the server answering. Refs #743 Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT