ReadonlyreasonAn optional machine-readable reason, rendered into the response body alongside the message. DECLARED HERE rather than sniffed structurally at the mapper so the channel is part of the contract a subclass opts into, and so the mapper needs no import of any subclass to render it (see the header for why that import must not exist).
ReadonlystatusThe response status, 4xx or 5xx; any other status answers 500.
A start carried a key onto a deployment whose host never wired the reservation store.
503, and never a silent pass-through. Honouring the request without a reservation would answer an exactly-once REQUEST with at-least-once BEHAVIOUR, which is worse than refusing: the caller would have every reason to believe a retry is safe, and no way to find out it is not. The condition is a wiring fault an operator fixes, so it reads as transient rather than as the caller's mistake.