operation_selector_required· code· HTTP 400· scope bothEvery canonical Arkret HTTP or TUS request MUST carry exactly one Arkret-Operation selector naming the exact locally advertised operation_id before body parsing, including endpoint families with one candidate. A missing selector fails with operation_selector_required. Endpoint uniqueness, payload shape, SDK version, defaults, and fallback MUST NOT replace the selector. WebSocket open frames carry the operation_id required by their frame schema.