DL-310
SubscribeAgentSession sends a leading zero-payload registration-ack frame (existing AgentSessionFrame, session_id only, no event, UNSPECIFIED state) immediately after subscribe() registers, before the live tail — mirroring SubscribeEvents’ snapshotBoundary / SubscribeComms’ commsSnapshotBoundary. It makes “open the tail before the post” a server-guaranteed happens-before, closing the idle-session drop where a driven injection races subscribe() and fans to zero subscribers (no replay ring). Not a replay/resync/reattach — the ack carries no history (RIG-3044, e2e flake TestLegThreeFourSpawnAndMessaging).
Status: Active (Matt, 2026-08-31)