unknown_field· reason_code· applies to event_envelopeCurrent parser rejected an Event carrying a top-level or payload field not declared by the closed schema for its kind (including removed / renamed fields in artifacts/registry/forbidden-wire-fields.json). Distinct from `schema_violation` in that it pinpoints an unrecognized field rather than a constraint violation on a known field. See zh/overview/current-contract.md §1.