plat 466

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PLAT-466 — Lighter chat restore (older turns as messages only) and opening at the latest message

Coordination	Value
State	fixed on main; not deployed
Date	2026-10-04
Owner	frontend-chat

Problem

An interactive chat restore replaced the tab with the latest page of durable events (the client asked for 100 rows). One turn with ~50 tool calls fills that page, pushes real messages out, and puts the "Load earlier messages" row right above the latest turn. Fast Cmd+K switching could also leave a chat showing the top of its loaded window.

Owner decisions (2026-10-04)

1. A restore returns the latest turn complete (a running turn too) and every older turn as messages only: no tool call, result, status or thinking rows and no "N tool calls" count. No expand, no inspect. Nothing is deleted; the rows stay in the journal and are simply not returned.
2. A chat that opens or is switched to shows its latest content and follows it while a turn runs or the reader has not scrolled.
3. Tool calls are a debugging aid, not a supported feature, and may be removed from the UI: one frontend switch (utils/toolCallVisibility.ts) decides whether they render; the server compact view does not depend on it.

Design

Opt-in: GET /api/sessions/{id}/events?durable_chat=1&view=messages[&limit=N][&before_sequence=S]. Without view=messages the response is exactly as before (read-only and scheduled-run tabs, Pulse, external APIs, terminal views). With it, limit counts messages (default 40) instead of rows.

Rules (agent_go/internal/events/event_journal_compact.go), over the journal's sequence order:

• A turn starts at a user_message row. A user_message directly after a tool call row is a mid-turn steering message and does not start the latest turn.
• First page: the whole latest turn (every row, capped at 1000 rows from the tail so a runaway turn cannot make the restore unbounded), plus older rows of the kept set until 40 messages are collected, then moved back to the start of that turn. A page therefore never begins between a user message and its reply. 40 messages is about 20 exchanges: enough to read back, light enough to open instantly; the whole latest turn is what the reader is looking at.
• Kept set for older turns: user messages, assistant messages (transcript chunk, unified_completion, agent_end, conversation_end, background_agent_completed), agent_error / conversation_error, and every question, approval and feedback row with its resolution (coding_agent_question, plan_approval, request_human_feedback, blocking_human_feedback, human_feedback_resolved), so an unanswered one stays answerable. Dropped: tool_call_*, live-input receipts, background agent started/terminated, status.
• before_sequence pages apply the same cut over the compact view only and return ~50 messages (the frontend passes 50). has_more is true only when a kept row exists below the page's first row; oldest_sequence is the cursor.
• latest_sequence/the live tail are unchanged, so SSE and polling continue from the journal tip exactly as before.

Frontend: hydrateTabEvents restores compactly only for an interactive chat tab (not view-only, execution or bot tabs), with compact: false for terminal views (TerminalCenter, restoredTerminal, useFormattedTranscriptHydration). The tab's pagination records compact: true so "Load earlier messages" pages the same filtered view.

Position fix (components/useTranscriptScroll.ts): a reading position or following=false is saved only if the reader used a scroll gesture or the transcript has been mounted for 700 ms (TRANSCRIPT_SETTLE_MS); the saved state carries deliberate, and a state that is not deliberate is reset to "follow the latest message" when read. A new chat or one with a running turn therefore opens at the bottom; a deliberate scroll-up is kept; sending a message clears it. "Load earlier messages" is shown only once the transcript has settled.

Done

• Backend: compact journal read, opt-in on the events endpoint, Go tests (event_journal_compact_test.go, polling_test.go).
• Frontend: restore opt-in (hydrateTabEvents, getRecentChatEvents), compact "Load earlier messages" paging (pagination carries compact), the single tool-call visibility switch, the position fix and its tests (sessionRestore.compact.test.ts, useTranscriptScroll.dom.test.tsx).

Left

• Not verified in a browser (a chat with a very long running turn, fast Cmd+K A->B->A->B).
• Not deployed.