Problem

When a mobile client loses its WebSocket mid-run (backgrounding, network handoff), events emitted while disconnected are gone: the gateway keeps no replay buffer, and gap detection today only broadcasts an observational "seq gap" error. Clients recover by refetching full history — the iOS app additionally runs a post-send polling ladder (1.5s→90s) to catch late messages. Recovery is slow, bandwidth-heavy, and still misses in-flight streaming state.

Proposal

Additive, no protocol version bump:

1. Gateway: optional afterSeq cursor on chat.history — return only messages after the given per-run/session sequence point (existing seq fields in the chat event schema; offset pagination unchanged when the param is absent).
2. Clients (iOS, Android): track the last applied seq per session; on reconnect, fetch the delta via afterSeq and reconcile instead of refetching everything. Fall back to full history against gateways that don't support the param (App Store app vs. older gateway is a real skew). iOS can then retire most of the post-send polling ladder.

Impact

Fast, cheap recovery after backgrounding — the top item on the mobile "reconnect recovery" workstream — and less gateway load from full-history refetches.