unsupported_protocol_version· code· HTTP 422· scope bothA syntactically well-formed protocol-family bootstrap discriminator is not supported by the receiver. For v1 ServiceDescribe and equivalent ping surfaces, a protocol_version string other than the canonical value 1.0 makes the complete service unusable before capability intersection or route caching. Missing or non-string values remain schema_violation. A binding that implements Arkret-Protocol-Version header or equivalent media-type negotiation returns this code for an unsupported requested version. Dual-registered as a top-level service code and a reason_code. See zh/overview/current-contract.md §1 and §4.