stream_tail_missing· reason_code· applies to client_syncOne authorized commit stream cannot be continued from the caller's durable cursor: the tail range between that cursor and the stream's current head is unavailable to this service (retention or history floor reached, or the durable range is otherwise not servable). It is per-stream and scoped to exactly one stream_ref, so the client re-acquires the snapshot and tail for that stream alone and MUST NOT reset other Realm / Circle / Sidecar streams, the account baseline or to-device ACKs. It is NOT the same as an empty tail: an authorized stream that is simply at its head returns zero items and `accepted`. emitting this code for a stream the caller may not read, or for one that does not exist, would turn that anti-enumeration bucket into an oracle. Only a stream the caller is currently authorized to read may be reported with it. See zh/sync/client-sync.md section 3.