last_resort_not_supported· reason_code· applies to service_call, feature_discoveryA KeyPackage claim requested a last-resort fallback from a server that does not advertise `ak.feature.mls_last_resort_keypackage.v1`. The server MUST continue to fail closed on an empty single-use pool and MUST NOT return a `last_resort=true` package. See zh/crypto-media/encryption-and-audit.md §2.6.2.