ERR_Q
400 Bad Request. The q, f, s or o parameter is invalid: malformed JSON in q, f, s or o; no q on a list endpoint other than the lookup endpoints (the cpc, uspc, ipc and wipo lists); more than one top-level key in q; an unknown field; a field outside the endpoint; a sort on a text field without a keyword form; q or s on the lapse group (it is returned with f only); 'offset' in o; a query over the cost cap (100 units, 64 criteria, depth 8); q over 16 KB (16,384 bytes) in a POST body. The cost cap also refuses _contains, a range or an empty _begins on a long-text field (claim_text, summary_text, description_text, draw_desc_text) without patent_id in the same _and: no index serves them, so they would read the whole table; use _text_any, _text_all or _text_phrase there. The X-Status-Reason header carries the upstream text. Two size limits answer before the API and carry no X-Status-Reason-Code: a request body over 64,000 bytes answers 413 with an empty body, and a GET whose request line (the method, the path with its query string and the protocol) is over 4,094 bytes answers 400 with an HTML page, so a large q goes in a POST body.
All codes: https://patentref.io/docs/errors/