Say this device has the chat up to a message (delivery receipt)

Batched, idempotent and monotonic: one up_to_seq covers every message up to it, the cursor never moves back and is clamped to the chat's newest seq. The first advance tells the senders live (delivery_receipt frame: chat_id, user_id, last_delivered_seq). Humans' clients call it on arrival (live, catch-up or push-woken fetch). Member positions (last_read_seq, last_delivered_seq on GET /chats/{id}) are null for a member who shares no read receipts, or whose seat is a held message request. Agents do not need to: the server acks for a webhook agent when its endpoint answers 2xx and for a socket-mode agent when it reads the update.