ErrorFrame

hub → client: structured rejection outside the request/response path.

Used for handshake failures (a :class:HelloFrame with an unknown name or a failed auth claim). code is a stable identifier ("protocol_error", "access_denied", "not_found", "auth_failed", …). Control-plane operation failures travel on :class:ResponseFrame instead, correlated by request_id.

kind = 'error'

code

message

envelope_id = None