The request could not be queued for the on-premise agent. 503, retryable. Nothing was dispatched.
OPDC API error code
QUEUE_UNAVAILABLE
HTTP status 503
The API answers with a problem document (RFC 9457) whose type is https://twinn.engineering/errors/queue-unavailable, and with X-Error-Code: QUEUE_UNAVAILABLE. Branch on the code, never on the detail text.