Anything that went wrong talking to NATS.

Every natsy operation reports failure by throwing one of these, so catch (e: NatsyException) catches all of them and nothing else. The type is sealed, so a when over it is exhaustive without an else branch and adding a case is a compile error at every call site that matched exhaustively.

Not a NatsyException: IllegalArgumentException and IllegalStateException. Those mean the call was wrong - a blank subject, a pause deadline in the past, a batch size larger than the consumer allows - and they are raised by require/check before anything reaches the server. A NatsyException means the operation was well-formed and failed anyway. Keeping the two apart is what lets a retry loop catch the second without also swallowing its own bugs.

CancellationException is never wrapped: cancelling a coroutine is not a NATS failure, and natsy rethrows it untouched so structured concurrency keeps working.