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/5xx only — see doErrorResponse.
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.