NOT_FOUND
404 Not Found. A detail route with a key that has no record ({"detail": "Not found."}), or a path that is not routed. On /api/v1/lapse/status/<patent_id>/ an unknown patent answers the same {"detail": "Not found."}, and a v that never existed answers {"error": true} with X-Status-Reason-Code NOT_FOUND. On /api/v1/structure/claims/<patent_id>/ (and on /api/v1/structure/brief/<patent_id>/ and /api/v1/structure/counts/<patent_id>/) a patent the data in service does not have, and a patent it has without claim text in the long-text file (a grant after that file's last grant date), answer {"error": true} with X-Status-Reason-Code NOT_FOUND and the reason in X-Status-Reason. Those answers carry NOT_FOUND as a header; on the others it is the problem details code.
All codes: https://patentref.io/docs/errors/