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Design 25 §2 builds enrolment around carrying the reply subject in the payload, because a JetStream consumer claims the transport Reply field for its own ack address. The whole handshake rests on it, so it is checked: the caller asked for _INBOX.LCr3M83q... and the consumer saw $JS.ACK.PROBE.probe_consumer... The design was right, and the workaround is necessary rather than defensive. Worth having as a test rather than a note: if a future server version stopped doing this, enrolment would keep working and the reason for the payload field would quietly become folklore.