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.
The caller's own key, pointed at a different workflow or agent than the one it reserved. 409, and it DOES name the reservation's target: the caller owns this key, so telling it what the key already means is telling it about its own state, and a caller that reused a key by accident needs exactly that to find the bug.