Every response of a matched route carries the header X-Hubby-Call-Id, the id of the call in Hubby's records. A request refused before it reaches a route carries no call id: an unknown path (404, code not_found), a method the path does not take (405, code method_not_allowed), or a query string PHP cannot parse whole (400, code malformed_query_string). Those two refusals, 405 and 400, can answer any path and are not listed per operation. Every 4xx refusal body carries a message and a code to branch on. A 5xx can carry a message alone: the issue call's 500 (POST /v1/bookings/{booking}/items/{item}/package) has no code; retry it with the same Idempotency-Key.