SSE and client stream

Endpoint

GET /api/sessions/:id/events?after=<seq>

• Server assigns monotonic seq per session.
• connected payload: { type, id, seq, ringStart } (see sse-protocol.js).
• Live frames: slim agent events (slimSseEvent in hub/http).

Resume policy

Implemented in shouldReplayRing / client ChatStream:

Situation	Behavior
Cold start, gap, or empty UI	No ring replay → client REST snapshot (GET …/messages) then drain buffer
Hot resume (after in ring)	Replay seq > after, then live
Duplicate / old seq	Client drops

Client merge (chat-stream.ts)

• Optimistic user ids c:* replaced when server user message_start arrives
• Streaming assistant updates one slot until stable id
• toolResult keyed by toolCallId
• agent_settled / errors clear turn phase

Do not invent “merge last message of same role”.

Tests that lock this

• server/sse-protocol.test.js
• web/src/lib/chat-stream.test.ts
• server/stabilize.test.js

Change protocol only with these green.