• The chat canvas reads from the normalized conversation store as the only live source of truth. Derived history, sidebar rows, or legacy chatItems mirrors may reconcile into that store, but must not replace live/pending/streaming state wholesale.
• Conversation identity is canonicalized by conversationKey = threadId ?? sessionId ?? pendingClientId. RPC acknowledgements may remap aliases from an optimistic key to a real session/thread key, but the UI must not fork a second independent conversation.
• Sending a message must write a client message id, optimistic user message, and thinking placeholder immediately. The outbox state machine is pending -> sent -> streaming -> complete/failed; an RPC ack only means the backend accepted the turn, not that streaming is complete.
• Socket replay/live events may append or reconcile entities by sequence, message id, turn id, and client message id. They must not silently switch the user's selected conversation, reorder the active thread, or overwrite a live pending turn.
• History hydration is per-turn reconciliation. It may confirm completed turns and fill gaps, but it must preserve current live, pending, and streaming items unless the matching client message id, turn id, or message id proves replacement is correct.
• Development tracing must cover submit intent, queued state, RPC start, ack, socket receive, reducer apply, stream seen, selected conversation render, and reconciliation or failure. Render traces must stay summarized; never log the full visible message list on every render.

CodexNext must preserve Codex app-server turn/item semantics. Do not collapse official app-server ThreadItem data into flat chat text as the integration boundary.

• Status: completed - Protocol schemas use official turn fields: itemsView, status, error, startedAt, completedAt, and durationMs are required on app-server turns.
• Status: completed - Protocol schemas require app-server item id and type, and expose item render classification for user, assistant, process, and metadata items.
• Status: completed - Agent event adaptation records app-server item lifecycle, reasoning deltas, MCP progress, and process output as structured local events.
• Status: completed - Historical thread/read / thread/turns/list data and realtime app-server notifications enter the same normalized turn store before chat rendering. Refresh, cold switching, replay, and live streaming must project from that store instead of maintaining separate history/live UI paths.
• Status: completed - Submit reconciliation uses historical CodexThreadTurn.items to confirm submitted user input and later assistant or command responses. It must not decide delivery state from legacy flat LocalCodexHistoryMessage[] pages.
• Status: completed - Web chat state no longer exposes flat message hydration helpers. History hydration and older-page prepending must use hydrateSessionFromTurns / prependSessionHistoryTurns with app-server turn data.
• Status: completed - History hydration guards read normalized page state (historyPages[sourceKey]) through chat-state selectors. They must not scan ChatItem projection ids such as history-${sessionId}-....
• Status: completed - History turn hydration and prepend reconciliation preserve local live items from the selected conversation projection only. They must not fall back to global workspace.chatItems.
• Status: completed - Local submit state, thinking feedback, ack binding, agent errors, legacy assistant/command/diff deltas, and outbox recovery are represented as turn/items first. ChatItem is a projection for the current renderer, not a business-state write target for those flows.
• Status: completed - TurnGroup is a read-only projection derived from normalized turns. It classifies userMessage as user input, process item types as process, agentMessage as answer, and carries status/timing without writing back to the store.
• Status: completed - The chat canvas receives TurnGroup projections as the primary render input. Legacy ChatItem[] is only a projection fallback at the renderer boundary and must not become a second live source of truth.
• Status: completed - Chat rendering uses ChatRenderItem as the renderer DTO. TurnGroupItem must not carry chatItem; render items are derived at the UI boundary and must never be written back to the turn store.
• Status: completed - Chat header titles and the summary sheet read the selected conversation's TurnGroup projection. They must not accept ChatItem[] fallback inputs.
• Status: completed - Sidebar session rows receive precomputed titles from per-session TurnGroup projections. Sidebar grouping must not inspect global chatItems.
• Status: completed - Development render traces and turn completion checks read TurnGroup / normalized turn selectors. They must stay summary-only and must not scan global workspace.chatItems.
• Status: completed - Completed turns with process items render a turn-level process summary such as 已处理 5m 58s; running or failed turns keep process, approval, and error rows visible. Assistant answer items remain expanded and are not hidden inside the process summary.
• Status: completed - High-content blocks use a shared thin CollapsibleBlock wrapper. Markdown, code highlighting, diff parsing, and virtualization continue to use the installed render stack (react-markdown, remark-gfm, rehype-highlight, and @tanstack/react-virtual) instead of a custom renderer or a new dependency.
• Status: completed - Model selection remains part of the start/resume/turn path. Schema or adapter work must not drop the selected model when switching or sending.

Cold conversation switching is a product-critical path. The implemented architecture is local-first: clicking a conversation commits selection immediately, renders normalized in-memory or persisted cache first, and lets network history refresh run in the background.

• Selection should commit in under 50 ms. Do not block the click path on getCodexHistoryTurns, listSessions, event replay, or history hydration.
• Show the best local state first: in-memory conversation, persisted conversation cache, or a lightweight thread skeleton using sidebar title and preview metadata. Avoid empty waits for unopened conversations.
• Persist a bounded recent normalized turn cache per conversation in IndexedDB, separate from the outbox and legacy ChatItem projections. The outbox is for unsent/in-flight recovery; it is not enough for fast cold thread switching.
• Revalidate stale conversations in the background and merge by message id, turn id, and client message id. Users should not experience "loading the whole history" as the primary interaction.
• Prefetch visible sidebar threads, pinned threads, and recent threads during idle time with a small concurrency budget.
• Large histories must use virtualized rendering or an equivalent windowing strategy. The chat canvas must not render hundreds of Markdown/code-highlighted messages in one synchronous pass.
• Development render logs must remain summary-only: selected key, message count, latest sequence, status counts, and latest item metadata. Do not log every visible message on each render.

All seven cold conversation switching requirements are implemented:

• Status: completed - Conversation selection is local-first and must not await getCodexHistoryTurns, listSessions, replay, or history hydration.
• Status: completed - The chat surface renders the best local state first: normalized in-memory conversation, persisted conversation cache, or a lightweight thread skeleton.
• Status: completed - Recent conversation bodies are persisted in bounded IndexedDB cache as normalized turnOrder / turns data, separate from the outbox recovery layer. ChatItem[] is only a renderer fallback projection.
• Status: completed - Network history refresh runs as background stale-while-revalidate and reconciles by message id, turn id, and client message id.
• Status: completed - Sidebar history prefetch runs during idle time for visible, pinned, and recent threads with bounded concurrency.
• Status: completed - Large chat histories use virtualized rendering so the UI does not synchronously render hundreds of Markdown/code-highlighted messages.
• Status: completed - Development render logs are summary-only and must not write the full visible message list per render.

• Development-only Next.js route indicators must not cover mobile composer controls. Keep dev overlays away from the bottom-left composer action area.
• Mobile chat states must prioritize the conversation viewport. Loading and empty states should be lightweight; they must not take over the screen like a desktop card.
• Sidebar thread titles must be readable summaries. Terminal output, build logs, stack traces, and prompt noise should be collapsed into the user command or the most useful diagnostic line.
• Fresh browsers with no localStorage must recover relay devices after Web session bootstrap. They should not strand the user on "connect device" when the relay already has online devices.
• Sidebar action controls must remain discoverable: thread rows expose pin and archive with concise aria-label/title text, and mobile keeps the actions accessible without hover.
• These UX rules are additive to the conversation state/performance guardrails above. Do not fix visual polish by weakening normalized conversation state, optimistic outbox, reconciliation, cache-first switching, virtualization, or summary-only dev traces.