Merge origin/master into feature/tui-first-run-welcome

.agents/notes/implemented/feature/2026-06-30-session-store-fork-api.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/feature/2026-06-30-session-store-fork-api.md
-2026-06-30-session-store-fork-api.md: 5342deba8ca879026d32ee1420cb3c0fdf67c500
-2026-06-30-session-store-fork-api.zh.md: 51a3e0ce50aff10a9812c91d24dc6e78a56c43ba
+2026-06-30-session-store-fork-api.md: 69ff85e1f137f4f263bf951af0a3f655411c606a
+2026-06-30-session-store-fork-api.zh.md: 3304a6f384c9004b3572c95881f832a4aa21b77c

.agents/notes/implemented/feature/2026-06-30-session-store-fork-api.md

@@ -28,6 +28,12 @@ class SessionStore extends Service {
 
 An empty prefix is forkable; any non-empty boundary must be a safe existing sequence outside an open turn. Typed errors distinguish missing sources, stale objects, duplicate child ids, invalid boundaries, and prefixes ending during execution. Broader log validation and crash repair remain with their existing owners.
 
+### Host and browser adaptation
+
+The Host `session.fork` RPC accepts `atSeq` as an anchor within the desired turn rather than as the store's inclusive safe boundary. It selects the first `turn/end` at or after that anchor; an omitted or past-end anchor selects the last completed turn. An anchor already in the log but not followed by a matching `turn/end` returns `fork-unavailable` and never falls back to an earlier turn, so a message action cannot silently omit the clicked message.
+
+The Host creates the child through the agent registry with the selected seed and lineage, and pre-publication setup installs the latest logged provider, model, and reasoning target before the child can run. It then attaches the child to the source Workspace. An attachment failure returns `workspace-attach-failed` with the already-published child id; the client reconciles that child into its summary list before surfacing the error. The Session-row action uses the last completed turn, while a message action supplies its event seq; both open the child after success, and lineage expansion makes it visible beneath the source.
+
 ## Alternatives considered
 
 **Separate `ctx.sessionFork` service.** This was the first implementation, but review showed it overfit the capability-seam pattern. The code had no swappable backend, no extra event surface, no independent ownership lifecycle, and no durable behavior beyond `ctx.sessions.create({ seed, meta })`. Keeping a separate package would make callers discover and install a second service just to perform policy around a session-store primitive.
@@ -40,4 +46,4 @@ An empty prefix is forkable; any non-empty boundary must be a safe existing sequ
 
 The public surface stays small and discoverable: live session branching is part of `ctx.sessions`, next to `create({ seed })`, rather than a standalone service or a two-step helper pair. Persistence continues to work through existing `session/created` and `session/flush` behavior: a forked child starts life with seeded events, so existing backends persist that seed once and preserve `parentSession` / `seedLength` in the header.
 
-The v1 scope still excludes ACP `session/fork`, unloaded persisted-session forking, model-facing tools, and subagent refactors. If a future ACP method is added, it should advertise the capability only after it has protocol and snapshot coverage; this Agent Note adds no ACP wire behavior, so no ACP snapshot is required. Fork-child replay remains covered by the existing [seed-boundary testing Agent Note](../testing/2026-06-22-fork-child-replay-seed-boundary.md), while this API gets focused `dsh-session` unit tests plus JSONL persistence coverage.
+The v1 scope still excludes ACP `session/fork`, unloaded persisted-session forking, model-facing tools, and subagent refactors. If a future ACP method is added, it should advertise the capability only after it has protocol and snapshot coverage; this Agent Note adds no ACP wire behavior, so no ACP snapshot is required. Fork-child replay remains covered by the existing [seed-boundary testing Agent Note](../testing/2026-06-22-fork-child-replay-seed-boundary.md); focused store, Host, carrier, and client tests pin the boundary and reconciliation contracts, while the real Chromium scenario pins the assembled message action and lineage tree.

.agents/notes/implemented/feature/2026-06-30-session-store-fork-api.zh.md

@@ -28,6 +28,12 @@ class SessionStore extends Service {
 
 空前缀可以被 fork；任何非空边界都必须是位于开放轮次之外且安全、已存在的序号。类型化的错误区分源缺失、对象陈旧、子 id 重复、边界无效和前缀结束于执行过程中等情况。更广泛的日志校验与崩溃恢复仍由其现有的负责方处理。
 
+### Host 与浏览器适配
+
+Host 的 `session.fork` RPC 接受 `atSeq`，并将其视为所需轮次内的锚点，而非 store 中包含该序号的安全边界。它选择该锚点处或其后的首个 `turn/end`；锚点省略或超过末尾时，选择最后一个已完成轮次。若锚点已在日志中，但从该锚点起找不到匹配的 `turn/end`，则返回 `fork-unavailable`，绝不回退到更早的轮次，因此消息操作不会静默遗漏所点击的消息。
+
+Host 通过 agent（智能体）注册表，以选定的种子和谱系创建子会话；发布前 setup 会先安装日志中最新的提供方、模型和推理（reasoning）目标，子会话才能运行。随后，Host 将子会话附加到源 Workspace。若附加失败，则返回 `workspace-attach-failed` 及已发布的子会话 id；客户端先将该子会话对账到摘要列表，再向调用方报告错误。Session 行操作使用最后一个已完成轮次，消息操作则提供其事件 seq；两者都会在成功后打开子会话，展开谱系后可在源会话下看到它。
+
 ## 曾考虑的替代方案
 
 **独立的 `ctx.sessionFork` 服务。** 这是最初的实现，但评审表明它过度套用了 capability-seam 模式。代码没有可替换的后端、没有额外的事件面、没有独立的所有权生命周期，也没有超出 `ctx.sessions.create({ seed, meta })` 的持久化行为。保留独立包会迫使调用方为了在会话存储原语之上执行一层策略而去发现并安装第二个服务。
@@ -40,4 +46,4 @@ class SessionStore extends Service {
 
 公开接口保持精简且易于发现：活跃会话分支是 `ctx.sessions` 的一部分，紧邻 `create({ seed })`，而非一个独立服务或一对两步辅助函数。持久化继续通过现有的 `session/created` 和 `session/flush` 行为运作：fork 出的子会话以种子事件开始生命，因此现有后端只需持久化该种子一次，并在 header 中保存 `parentSession`／`seedLength`。
 
-v1 范围仍然排除 ACP（Agent Client Protocol） `session/fork`、对未加载的已持久化会话的 fork、面向模型的工具，以及 subagent 重构。如果未来添加 ACP 方法，应在具备协议与快照覆盖后才广播该能力；本 Agent Note 不添加任何 ACP 协议行为，因此不需要 ACP 快照。fork 子会话的回放仍由现有的[种子边界测试 Agent Note](../testing/2026-06-22-fork-child-replay-seed-boundary.md) 覆盖，而本 API 则获得专门的 `dsh-session` 单元测试加 JSONL 持久化覆盖。
+v1 范围仍然排除 ACP（Agent Client Protocol） `session/fork`、对未加载的已持久化会话的 fork、面向模型的工具，以及 subagent 重构。如果未来添加 ACP 方法，应在具备协议与快照覆盖后才广播该能力；本 Agent Note 不添加任何 ACP 协议行为，因此不需要 ACP 快照。fork 子会话的回放仍由现有的[种子边界测试 Agent Note](../testing/2026-06-22-fork-child-replay-seed-boundary.md) 覆盖；store、Host、载体与客户端的专项测试固定边界和对账契约，真实 Chromium 场景则固定组装后的消息操作与谱系树。

.agents/notes/implemented/feature/2026-07-25-session-list-browsing-and-manual-order.i18n.yaml

@@ -1,6 +1,6 @@
 # Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
-#   pnpm run verify-translation-pairing --write
-2026-07-25-session-list-browsing-and-manual-order.md: 586995bf459aeaee88672863977f7acf2a7061a3
-2026-07-25-session-list-browsing-and-manual-order.zh.md: 432d5167a57d30bc04a0b4faf213e4341f07bd2f
+#   pnpm run verify-translation-pairing --write .agents/notes/implemented/feature/2026-07-25-session-list-browsing-and-manual-order.md
+2026-07-25-session-list-browsing-and-manual-order.md: 831aa53e532a75392690c330837482bb0f9c32b1
+2026-07-25-session-list-browsing-and-manual-order.zh.md: 9ad074d59c13585aa4fca46ae4d40e2deb15cde6

.agents/notes/implemented/feature/2026-07-25-session-list-browsing-and-manual-order.md

@@ -12,14 +12,14 @@ Two existing mechanisms stood in the way. First, the host durably promoted the a
 
 ## Decision
 
-### Flat view and viewing state
+### Flat rows and viewing state
 
-The group-by menu offers two modes, WorkSpace / In one list. Flat mode renders every session (fork children included) as a top-level row, strictly newest-first by `updatedAt`, with no parent/child adjacency; the Intent placeholder renders as the first row. The mode choice persists in the browser (`dsh.workspace.view`) across reloads.
+The group-by menu offers two modes, WorkSpace / In one list. WorkSpace mode renders peer session rows within each group in the manual order from `WorkspaceView.sessionIds`; In one list combines every session and sorts them strictly newest-first by `updatedAt`. Neither mode projects `parentId` into a list hierarchy; fork lineage remains session data only. [Web session fork actions](2026-07-27-web-session-fork-actions.md) define the complete fork behavior. The mode choice persists in the browser (`dsh.workspace.view`) across reloads.
 
 ### Row interactions
 
 - Session rows show a detail card after a 500ms hover dwell (full title / relative time / status line; the status line has only running/idle until the wire grows a status field). The card and the row menu are mutually exclusive: no card while a menu is open or a drag is in flight.
-- Session-row … menu: Rename / Fork session / Delete session, visual-only this iteration; workspace-header … menu: Rename (wired) / Delete workspace (visual-only). Menus close when the pointer leaves them.
+- Session-row … menu: Rename / Fork session / Delete session; Rename and Fork are wired, while Delete remains visual-only. The workspace-header … menu's Rename / Delete workspace actions are both wired. Menus close when the pointer leaves them.
 - Supporting primitives: `Menu` gains label entries, danger rows, and `closeOnPointerLeave`; a new `HoverCard` (portaled placement, open delay, disabled guard).
 
 ### workspace.rename
@@ -30,7 +30,7 @@ The group-by menu offers two modes, WorkSpace / In one list. Flat mode renders e
 
 The `session/event` → `touchSession` activity-pinning chain is deleted wholesale; the workspace account order is now manually owned — new sessions prepend at attach, and explicit reordering goes through `workspace.insertSessionBefore({ workspaceId, sessionId, beforeSessionId? })` (DOM insertBefore semantics: with an anchor it inserts before it, omitted appends to the end). The entity throws a typed `WorkspaceMoveInvalidError` only for unaccounted session/anchor ids; the handler maps exactly that to the business code `workspace-move-invalid`, while storage failures stay internal.
 
-The UI is HTML5 drag on root rows inside a group (workspace grouping only, outside search; fork children ride with their parent and are not draggable). Order authority stays entirely host-side: drop only sends the RPC, the client performs zero local reordering, and the view refreshes from the response upsert and the changed frame; a failed move changes nothing. The client's upsert rejects snapshots older (`updatedAt`) than the installed projection so a late unary response cannot roll back a newer frame.
+The UI is HTML5 drag on session rows inside a group (workspace grouping only, outside search; fork children and their source sessions are ordered independently). Order authority stays entirely host-side: drop only sends the RPC, the client performs zero local reordering, and the view refreshes from the response upsert and the changed frame; a failed move changes nothing. The client's upsert rejects snapshots older (`updatedAt`) than the installed projection so a late unary response cannot roll back a newer frame.
 
 ### Shell/region split
 
@@ -46,15 +46,15 @@ ui-sidebar shrinks to the column-geometry shell: brand row, fold state machine,
 
 **Keep the rename dialog in ui-sidebar (smallest change)** — that is the problem itself: workspace-domain dialogs scattered in a borrowed slot, with each addition (the Delete confirmation is coming) repeating the cross-package wiring. Review first considered moving only the rename modal; the ruling was to give the whole browsing region to ui-workspace and leave the shell geometry-only.
 
-**Keep parent/child adjacency in flat mode** — contradicts strict recency (a child newer than its parent's sibling cannot slot adjacently), and the flat view's purpose is dropping the hierarchy; flattening fully and disabling drag in flat mode (no persistence carrier) is more consistent.
+**Nest sessions by fork lineage in WorkSpace mode** — nesting makes the current child visible only while its ancestors are expanded and limits in-group manual ordering to root nodes; `parentId` is lineage data, not a list-navigation structure. Flattening all sessions into peer rows lets each row be opened, searched, and ordered independently; In one list still disables drag because it has no workspace persistence carrier.
 
 ## Consequences
 
 - Manual order is the sole authority over the workspace account: an order the user arranges is never scrambled by activity; the cost is losing float-to-top-on-activity, whose signal now rides the row status dot and time label. The `WorkspaceView.sessionIds` wire contract is reworded to the manual-order semantics.
 - The two-fact shell/region contract funnels every future workspace-domain feature (Delete confirmation, cross-group moves, Ungrouped adoption) into the single ui-workspace package; ui-sidebar no longer evolves with session-list features.
 - Flat mode supports neither reordering nor a create-in-workspace entry point (switching back to grouped view is required) — an accepted scope reduction.
-- Wiring the three session-menu items and workspace Delete, and growing the wire status enum, remain future iterations.
+- Wiring session Delete and growing the wire status enum remain future iterations.
 
 ## Testing
 
-Package-level suites cover the derivations (deriveGroups/deriveFlat), row components, both apply registrations and passthroughs, host entity move semantics, and the rename/insertSessionBefore RPC implementations with their fixture stubs; the `apps/web` keyless snapshots regress the assembled application; delivery acceptance additionally runs a 12-item playwright (chromium headless) checklist (grouped default, flat switch and persistence, hover-card appearance and suppression, both menus, the full rename chain, drag persistence) and drives the real host over the wire for rename success / duplicate rejection / `workspace-move-invalid`.
+Package-level suites cover the derivations (deriveGroups/deriveFlat), peer session rows, both apply registrations and passthroughs, host entity move semantics, and the rename/insertSessionBefore RPC implementations with their fixture stubs; the `apps/web` keyless snapshots regress the assembled application and pin that a fork does not introduce session expansion controls.

.agents/notes/implemented/feature/2026-07-25-session-list-browsing-and-manual-order.zh.md

@@ -12,14 +12,14 @@ Status: implemented
 
 ## Decision
 
-### 平铺视图与浏览态
+### 平铺行与浏览态
 
-group-by 菜单提供 WorkSpace / In one list 两种模式。平铺模式把所有 session(含 fork 子)一律作为顶层行,严格按 `updatedAt` 新→旧排序,不保持父子相邻;Intent 占位行渲染在列表首行。模式选择持久化在浏览器(`dsh.workspace.view`),刷新保持。
+group-by 菜单提供 WorkSpace / In one list 两种模式。WorkSpace 模式按 `WorkspaceView.sessionIds` 的手动序在各组内展示同级 session 行；In one list 把所有 session 合并后严格按 `updatedAt` 新→旧排序。两种模式都不把 `parentId` 投影成列表层级，fork 谱系只保留为 session 数据；完整 fork 行为由 [Web session fork 操作](2026-07-27-web-session-fork-actions.md)定义。模式选择持久化在浏览器(`dsh.workspace.view`),刷新保持。
 
 ### 行交互
 
 - session 行悬停 500ms 出详情卡(全名/相对时间/状态行;状态本期只有 running/idle 两态,枚举扩展待 wire 增补 status 字段)。卡片与行菜单互斥:菜单开启或拖拽进行中不出卡。
-- session 行 … 菜单:Rename / Fork session / Delete session,本期纯视觉;workspace 组头 … 菜单:Rename(已接线)/ Delete workspace(纯视觉)。菜单鼠标移出即关。
+- session 行 … 菜单:Rename / Fork session / Delete session，其中 Rename 与 Fork 已接线，Delete 仍为纯视觉；workspace 组头 … 菜单的 Rename / Delete workspace 均已接线。菜单鼠标移出即关。
 - 支撑件:`Menu` 新增 label 条目、danger 行、`closeOnPointerLeave`;新增 `HoverCard`(portal 定位、开启延时、disabled 守卫)。
 
 ### workspace.rename
@@ -30,7 +30,7 @@ group-by 菜单提供 WorkSpace / In one list 两种模式。平铺模式把所
 
 `session/event` → `touchSession` 活动置顶链整体删除;workspace 账本序改为纯手动拥有——新 session attach 时前插,显式重排走 `workspace.insertSessionBefore({ workspaceId, sessionId, beforeSessionId? })`(DOM insertBefore 语义:锚给了插锚前,缺省 append 到末尾)。实体只对不在账的 session/锚抛类型化的 `WorkspaceMoveInvalidError`,handler 仅把它映射为业务码 `workspace-move-invalid`,存储故障保持 internal。
 
-UI 为组内 root 行的 HTML5 拖拽(仅 workspace 分组、非搜索态;fork 子随父不单独拖)。顺序权威完全在 host:drop 只发 RPC,client 零本地重排,视图靠响应体 upsert 与 changed 帧刷新;失败即无事发生。client 的 upsert 拒绝比已装载投影更旧(`updatedAt`)的快照,防迟到的一元响应回滚更新的帧。
+UI 为组内 session 行的 HTML5 拖拽(仅 workspace 分组、非搜索态；fork 子与源会话一样独立排序)。顺序权威完全在 host:drop 只发 RPC,client 零本地重排,视图靠响应体 upsert 与 changed 帧刷新;失败即无事发生。client 的 upsert 拒绝比已装载投影更旧(`updatedAt`)的快照,防迟到的一元响应回滚更新的帧。
 
 ### 壳/区域切分
 
@@ -46,15 +46,15 @@ ui-sidebar 缩为列几何壳:品牌行、折叠状态机、New Session、Settin
 
 **rename 对话框留在 ui-sidebar(最小改动)** —— 正是问题本身:workspace 域的对话框散落在借来的坑里,每加一个(Delete 确认框将至)都重演跨包接线。评审中先议了「只挪 rename Modal」的中间态,最终裁定整个浏览区域归 ui-workspace,壳只留几何。
 
-**平铺模式保持父子相邻成组** —— 与「严格按时间」矛盾(子新于兄则插不进相邻位),且平铺本意就是取消层级;拉平并禁用平铺下的拖拽(无持久化载体)更一致。
+**WorkSpace 模式按 fork 谱系嵌套 session** —— 嵌套会让当前子会话依赖祖先展开态才能可见，也让组内手动序只能移动根节点；`parentId` 是 lineage 数据，不是列表导航结构。所有 session 拍平成同级行后，每行都可独立打开、搜索与排序；In one list 仍因没有 workspace 持久化载体而禁用拖拽。
 
 ## Consequences
 
 - 手动序是唯一的 workspace 账本序权威:用户排好的顺序不再被活动打乱;代价是「最近活跃浮到最上」的行为消失,活跃感知转由行内状态点与时间标签承担。`WorkspaceView.sessionIds` 的 wire 契约随之改为手动序措辞。
 - 壳/区域两事实契约把 workspace 域的后续功能(Delete 确认、跨组移动、Ungrouped 收编)全部收进 ui-workspace 单包;ui-sidebar 不再随 session 列表功能演进。
 - 平铺模式不支持排序与分组入口(建到指定 workspace 需切回分组视图),是拍板接受的范围收窄。
-- session 菜单三项与 workspace Delete 的功能接线、状态枚举扩 wire,留待后续迭代。
+- session Delete 的功能接线与状态枚举扩 wire,留待后续迭代。
 
 ## Testing
 
-包级用例覆盖派生(deriveGroups/deriveFlat)、行组件、两处 apply 注册与透传、host 实体移位语义、rename/insertSessionBefore 的 RPC 实现与 fixture 桩;`apps/web` keyless snapshot 回归覆盖装配后的应用;交付验收另以 playwright(chromium headless)过 12 项清单(分组默认、平铺切换与持久化、hover 卡出现与抑制、双菜单、rename 全链、拖拽落盘),并对真 host 直打 wire 验证 rename 成功/重名拒绝/`workspace-move-invalid` 三径。
+包级用例覆盖派生(deriveGroups/deriveFlat)、同级 session 行、两处 apply 注册与透传、host 实体移位语义、rename/insertSessionBefore 的 RPC 实现与 fixture 桩；`apps/web` keyless snapshot 回归覆盖装配后的应用，并钉住 fork 后没有 session 展开控件。

.agents/notes/implemented/feature/2026-07-27-web-session-fork-actions.i18n.yaml

@@ -0,0 +1,6 @@
+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# side as of the last confirmed-consistent state. Both languages carry equal authority;
+# after editing either side, bring the other along and re-record with:
+#   pnpm run verify-translation-pairing --write .agents/notes/implemented/feature/2026-07-27-web-session-fork-actions.md
+2026-07-27-web-session-fork-actions.md: b5dc7e820de069a68b38ed87c7d29ffbdb4867bc
+2026-07-27-web-session-fork-actions.zh.md: 774cd74d69eb02d43ca01c8ec7ba1cf94c24b1dc

.agents/notes/implemented/feature/2026-07-27-web-session-fork-actions.md

@@ -0,0 +1,33 @@
+# Agent Note: Web session fork actions
+
+Status: implemented
+
+English | [中文](2026-07-27-web-session-fork-actions.zh.md)
+
+## Problem
+
+The Session store already provides a fork primitive that creates a child session from a completed-turn prefix, but the Web client has no unified interaction contract. The Session-row menu can express only “branch from the latest completed turn,” while message IconActions need to express “branch from the turn containing this message”; if the two entry points independently interpret the boundary, switching, and failure behavior, the same user action acquires two sets of semantics. Nesting a fork child beneath its source session also makes the newly selected child visible only while its ancestors are expanded and weakens the workspace manual-order model.
+
+## Decision
+
+The Web Session-row menu and message IconActions share the client runtime's `sessions.fork` action. A Session row passes `{ sessionId, increaseTitle: true }`, so it forks at the source session's last completed turn; a user message or settled assistant content message passes `{ sessionId, atSeq: node.seq, increaseTitle: true }`, so it forks at the turn containing that event. Only the client consumes `increaseTitle`: after adding the child session to its local list, the client increments a trailing `(N)` or `（N）` in the source session's persisted title without changing bracket style, appends ` (1)` to an unnumbered title, and skips the rename when no persisted title exists; the Host fork request still contains only `sessionId` and the optional `atSeq`. The caller opens the child only after the rename succeeds; a fork or rename failure leaves the source session and current selection unchanged, while a child created before a rename failure remains in the list.
+
+`forkAt(seq)` touches the session service only in ui-conversation's apply injection layer; message components report only the event `seq`. Session rows likewise initiate the operation only through ui-workspace's injected callback. Neither presentation package owns session mutation state or duplicates the host's boundary evaluation.
+
+Session lineage is not projected into a list hierarchy. WorkSpace mode displays source sessions and all fork children as peer rows in the manual order from `WorkspaceView.sessionIds`; every row can be opened, searched, and dragged independently. In one list mode continues to sort strictly by `updatedAt`; the Ungrouped group also sorts by recency when no workspace ledger is available. `parentId` remains available for lineage, tool presentation, and later queries, but does not control session-list visibility.
+
+## Alternatives considered
+
+**Wire only the Session-row menu.** Rejected: at a message, the user has already selected more precise context; forcing them back to the list can only degrade the boundary to the latest completed turn, while the visible message branch icon would remain non-responsive.
+
+**Allow branching only from user messages.** Rejected: settled assistant content also has a stable event `seq`, and the host places it in its containing completed turn; making only one of two visually identical branch buttons work would create an invisible behavioral difference.
+
+**Nest fork children beneath their source by `parentId`.** Rejected: lineage is not navigation ownership; nesting requires automatic ancestor expansion to reveal the current item and prevents children from participating in the workspace's peer manual order.
+
+**Call the session service directly from message components.** Rejected: client components must not touch `ctx` or business services; injected callbacks keep mutation in the apply world and leave components driven purely by props.
+
+## Consequences
+
+Users can create forks from Session rows, user messages, or settled assistant content messages; all three entry points ultimately use the same runtime/host operation. Message entry points preserve the exact event boundary, while the list entry point preserves the “latest completed turn” shortcut. Successive fork titles increment through `(1)`, `(2)`, and so on instead of repeatedly appending `(1)`; titles with fullwidth parentheses retain that style. Every fork child immediately appears as an ordinary peer row, so the list no longer needs session expansion state, recursive nodes, or twist controls.
+
+Fork and child-rename failures stay silent and preserve the source selection, preventing a derivation action from disrupting the current reading position; this tradeoff also means the UI does not yet expose a failure reason or retry entry point. Package tests separately pin the two message `seq` paths, title increments, and the peer-list derivation; `apps/web/tests/message-actions.e2e.ts` exercises assistant-message branching and Session-row menu branching through the assembled application.

.agents/notes/implemented/feature/2026-07-27-web-session-fork-actions.zh.md

@@ -0,0 +1,33 @@
+# Agent Note: Web session fork 操作
+
+Status: implemented
+
+[English](2026-07-27-web-session-fork-actions.md) | 中文
+
+## Problem
+
+Session store 已提供按完成轮前缀创建子会话的 fork 原语，但 Web 端没有一份统一的交互契约。Session 行菜单只能表达「从最新完成轮分支」，消息 IconActions 还需要表达「从这条消息所在轮分支」；如果两处各自解释边界、切换与失败行为，同一个用户动作会形成两套语义。把 fork 子会话嵌套在源会话下还会让新选中的子会话依赖祖先展开态才能看见，并削弱 workspace 的手动排序模型。
+
+## Decision
+
+Web 的 session 行菜单与消息 IconActions 共用 client runtime 的 `sessions.fork` 操作。Session 行传 `{ sessionId, increaseTitle: true }`，因此在源会话最后一个已完成轮次处分支；用户消息与已定稿 assistant 内容消息传 `{ sessionId, atSeq: node.seq, increaseTitle: true }`，因此在包含该事件的轮次处分支。`increaseTitle` 只由 client 消费：子会话进入本地列表后，client 把源会话持久化标题尾部的 `(N)` 或 `（N）` 递增并保留括号样式，无编号时追加 ` (1)`，没有持久化标题时不改名；Host fork 请求仍只有 `sessionId` 与可选的 `atSeq`。改名成功后调用方才打开子会话；fork 或改名失败时保持源会话与当前选择不变，改名失败时已创建的子会话仍留在列表中。
+
+`forkAt(seq)` 只在 ui-conversation 的 apply 注入层接触 session 服务，消息组件只回传事件 `seq`。Session 行同理只通过 ui-workspace 的注入回调发起操作；两个呈现包都不持有 session mutation 状态，也不复制 host 的边界求值。
+
+Session lineage 不投影成列表层级。WorkSpace 模式按 `WorkspaceView.sessionIds` 的手动序把源会话与所有 fork 子会话显示为同级行，每行都可独立打开、搜索和拖拽；In one list 模式继续按 `updatedAt` 严格排序；Ungrouped 组在没有 workspace 账本时也按 recency 排序。`parentId` 仍用于 lineage、工具呈现和后续查询，但不控制 session 列表可见性。
+
+## Alternatives considered
+
+**只接 session 行菜单。** 否决：用户在消息处已经选择了更精确的上下文，强迫其回到列表只能退化为最新完成轮，且已展示的消息分支图标会成为无响应控件。
+
+**只允许用户消息分支。** 否决：已定稿 assistant 内容同样有稳定事件 `seq`，host 会把它归入所属完成轮；让两个外观相同的分支按钮只有一个可用会制造不可见的行为差异。
+
+**按 `parentId` 把 fork 子会话嵌套在源会话下。** 否决：lineage 不是导航所有权；嵌套要求自动展开祖先才能看见当前项，并让子会话无法参与 workspace 的同级手动排序。
+
+**由消息组件直接调用 session 服务。** 否决：client 组件不得接触 `ctx` 或业务服务；注入回调让 mutation 留在 apply 世界，组件保持纯 props。
+
+## Consequences
+
+用户可从 session 行、用户消息或已定稿 assistant 内容消息创建分支，三处最终走同一个 runtime/host 操作；消息点位保留精确事件边界，列表点位保留「最新完成轮」快捷语义。连续 fork 的标题按 `(1)`、`(2)` 递增，而不是重复追加 `(1)`；全角括号标题保持全角样式。所有 fork 子会话立即作为普通同级行出现，列表不再需要 session 展开状态、递归节点或 twist 控件。
+
+Fork 与子会话改名失败都保持静默并保留源选择，避免一个派生操作破坏当前阅读位置；该取舍也意味着 UI 暂不提供失败原因或重试入口。Package tests 分别钉住两种消息 `seq`、标题递增与同级列表派生，`apps/web/tests/message-actions.e2e.ts` 通过装配后的应用执行 assistant 消息分支与 session 行菜单分支。

.agents/notes/implemented/feature/2026-07-29-web-message-icon-actions-and-clock.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/feature/2026-07-29-web-message-icon-actions-and-clock.md
-2026-07-29-web-message-icon-actions-and-clock.md: e79662056792c3ab413468ad038dec40455be767
-2026-07-29-web-message-icon-actions-and-clock.zh.md: 72d3b4e0cda19438f2f46fd402b3b76de3726ae5
+2026-07-29-web-message-icon-actions-and-clock.md: ccc9b127d2fcf276fa415ec0ede8b062e45f3df5
+2026-07-29-web-message-icon-actions-and-clock.zh.md: fb4b82a22478b2de38755368ec48a5dca420e79d

.agents/notes/implemented/feature/2026-07-29-web-message-icon-actions-and-clock.md

@@ -12,7 +12,7 @@ The web chat user bubble already had copy / branch / edit IconActions but no clo
 
 **User bubbles prepend a date-aware local clock to the existing IconActions row; finalized assistant *content* nodes (non-empty text blocks) append a copy / branch / clock row with `margin-top: 16px`; both seats stay visible whenever mounted and re-format at the next local midnight.**
 
-Both seats format `node.time` through `formatMessageClock`: same calendar day → `HH:mm`, earlier this year → `M月D日 HH:mm`, other years → `YYYY年M月D日 HH:mm`. `useCalendarDay` is a component-local day tick (timeout to the next local midnight) so memoized rows re-render when the calendar day changes without a new framework hook. `MessageItem` places the label before copy (figma `388:20051`). `AssistantMarkdown` places it after branch (figma `43:32997`) only when `streaming` is false, the event time is known, and the node has non-empty text content; Think-only nodes and the streaming tail omit the row. Copy writes joined text blocks. Branch stays a chrome stub. Clipboard write and the clock helpers live in `message-chrome.ts`. The assembled surface is pinned by `apps/web/tests/message-actions.e2e.ts` (cold-seeded history + aria golden); aria normalization collapses every clock shape to `{{clock}}`.
+Both seats format `node.time` through `formatMessageClock`: same calendar day → `HH:mm`, earlier this year → `M月D日 HH:mm`, other years → `YYYY年M月D日 HH:mm`. `useCalendarDay` is a component-local day tick (timeout to the next local midnight) so memoized rows re-render when the calendar day changes without a new framework hook. `MessageItem` places the label before copy (figma `388:20051`). `AssistantMarkdown` places it after branch (figma `43:32997`) only when `streaming` is false, the event time is known, and the node has non-empty text content; Think-only nodes and the streaming tail omit the row. Copy writes joined text blocks. Both message rows pass their event's `seq` to the same fork callback; [Web session fork actions](2026-07-27-web-session-fork-actions.md) define the real mutation contract. Clipboard write and the clock helpers live in `message-chrome.ts`. The assembled surface is pinned by `apps/web/tests/message-actions.e2e.ts` (cold-seeded history + aria golden); aria normalization collapses every clock shape to `{{clock}}`.
 
 ## Alternatives considered
 
@@ -22,10 +22,10 @@ Both seats format `node.time` through `formatMessageClock`: same calendar day
 
 **Hover-reveal the action row on hover-capable pointers.** Rejected: once the row exists it should stay discoverable; opacity hiding made the chrome easy to miss and required parent hover selectors that duplicated the mount gate.
 
-**Wire branch to a real session fork.** Rejected for this change: same rationale as the archived [user IconActions note](../../archived/feature/2026-07-27-user-message-icon-actions.md) — the mutation path is unspecified; the button reserves the design seat.
+**Let the IconActions decision also define session fork semantics.** Rejected: this note owns only message chrome, clocks, and mount gating; boundary selection, failure behavior, and switching semantics belong to the separate [Web session fork actions](2026-07-27-web-session-fork-actions.md), keeping presentation components from becoming a second home for session mutation.
 
 **Publish the calendar day through a chat store or inject hook.** Rejected: the day tick is presentation-only local state with no cross-entry consumers; a component-local timeout matches the client rule that behavioral hooks may own state that does not subscribe to an external source.
 
 ## Consequences
 
-Settled assistant content answers expose copy and the event clock as soon as the row mounts; Think-only nodes stay chrome-free; branch stays a stub. User and assistant clocks share the same day/year widening rules and refresh after midnight without a message mutation. Per-message paging remains a deferred footer seat in the package README. Package tests pin the three clock shapes, the midnight widen, and the content-only assistant gate; the web e2e scenario pins the assembled IconActions chrome.
+Settled assistant content answers expose copy, branch, and the event clock as soon as the row mounts; Think-only nodes stay chrome-free. User and assistant clocks share the same day/year widening rules and refresh after midnight without a message mutation. Per-message paging remains a deferred footer seat in the package README. Package tests pin the three clock shapes, the midnight widen, the content-only assistant gate, and the respective event `seq` values passed by the user and assistant branch buttons; the web e2e scenario pins the assembled IconActions chrome.

.agents/notes/implemented/feature/2026-07-29-web-message-icon-actions-and-clock.zh.md

@@ -12,7 +12,7 @@ Web 聊天的用户气泡已有复制／分支／编辑 IconActions，但没有
 
 **用户气泡在既有 IconActions 行前追加感知日期的本地时钟；已定稿的 assistant *内容*节点（非空 text 块）在正文下追加带 `margin-top: 16px` 的复制／分支／时钟；两边只要挂载就保持可见，并在下一个本地午夜重新格式化。**
 
-两边都通过 `formatMessageClock` 格式化 `node.time`：同一日历日 → `HH:mm`，同年更早 → `M月D日 HH:mm`，跨年 → `YYYY年M月D日 HH:mm`。`useCalendarDay` 是组件本地的日刻度（定时到下一个本地午夜），因此 memo 行在日历日变化时会重渲染，且不新增框架 hook。`MessageItem` 把标签放在复制之前（figma `388:20051`）。`AssistantMarkdown` 把它放在分支之后（figma `43:32997`），且仅在 `streaming` 为 false、已知事件时间、且节点含非空 text 内容时渲染；纯 Think 节点与流式尾部省略该行。复制写入拼接后的 text 块。分支仍是 chrome stub。剪贴板写入与时钟辅助函数放在 `message-chrome.ts`。组装面由 `apps/web/tests/message-actions.e2e.ts`（冷 seed 历史 + aria golden）钉住；aria 归一化把每种时钟形态折叠为 `{{clock}}`。
+两边都通过 `formatMessageClock` 格式化 `node.time`：同一日历日 → `HH:mm`，同年更早 → `M月D日 HH:mm`，跨年 → `YYYY年M月D日 HH:mm`。`useCalendarDay` 是组件本地的日刻度（定时到下一个本地午夜），因此 memo 行在日历日变化时会重渲染，且不新增框架 hook。`MessageItem` 把标签放在复制之前（figma `388:20051`）。`AssistantMarkdown` 把它放在分支之后（figma `43:32997`），且仅在 `streaming` 为 false、已知事件时间、且节点含非空 text 内容时渲染；纯 Think 节点与流式尾部省略该行。复制写入拼接后的 text 块。两种消息行都把自己的事件 `seq` 交给同一个 fork 回调；真实 mutation 契约由 [Web session fork 操作](2026-07-27-web-session-fork-actions.md)定义。剪贴板写入与时钟辅助函数放在 `message-chrome.ts`。组装面由 `apps/web/tests/message-actions.e2e.ts`（冷 seed 历史 + aria golden）钉住；aria 归一化把每种时钟形态折叠为 `{{clock}}`。
 
 ## 曾考虑的方案
 
@@ -22,10 +22,10 @@ Web 聊天的用户气泡已有复制／分支／编辑 IconActions，但没有
 
 **在具备 hover 能力的指针上用 hover 才揭示操作行。** 否决：行一旦存在就应保持可发现；用 opacity 隐藏容易漏看，且需要父级 hover 选择器重复挂载门控。
 
-**把分支接到真实的会话 fork。** 本次否决：与已归档的[用户 IconActions 笔记](../../archived/feature/2026-07-27-user-message-icon-actions.md)同一理由——变更路径尚未规定；按钮只预留设计座位。
+**由 IconActions 决策同时定义 session fork 语义。** 否决：本笔记只拥有消息 chrome、时钟与挂载门控；边界选择、失败行为和切换语义属于独立的 [Web session fork 操作](2026-07-27-web-session-fork-actions.md)，避免展示组件成为 session mutation 的第二正家。
 
 **通过 chat store 或 inject hook 发布日历日。** 否决：日刻度只是展示层本地状态，没有跨入口消费者；组件本地 timeout 符合「行为 hook 可拥有不订阅外部源的状态」这一客户端规则。
 
 ## 后果
 
-已定稿的 assistant 内容回答在行挂载后立刻暴露复制与事件时钟；纯 Think 节点不带 chrome；分支仍为 stub。用户与 assistant 时钟共用同一套跨天／跨年加宽规则，并在午夜后无需消息变更即可刷新。逐消息分页仍是包 README 中的暂缓 footer 座位。包级测试钉住三种时钟形态、午夜加宽与 assistant 仅内容门控；Web e2e 场景钉住组装后的 IconActions chrome。
+已定稿的 assistant 内容回答在行挂载后立刻暴露复制、分支与事件时钟；纯 Think 节点不带 chrome。用户与 assistant 时钟共用同一套跨天／跨年加宽规则，并在午夜后无需消息变更即可刷新。逐消息分页仍是包 README 中的暂缓 footer 座位。包级测试钉住三种时钟形态、午夜加宽、assistant 仅内容门控，以及 user/assistant 分支按钮各自传递的事件 `seq`；Web e2e 场景钉住组装后的 IconActions chrome。

apps/web/tests/message-actions.e2e.ts

@@ -8,6 +8,7 @@ import { fileURLToPath } from 'node:url'
 import type { Browser, Page } from 'playwright'
 import { chromium } from 'playwright'
 import { afterAll, beforeAll, describe, expect, it, onTestFailed } from 'vitest'
+import { SessionId } from '@deepseek-ai/dsh-session'
 import {
   assertFixtureInventory, captureStableAria, compareOrRefreshGolden, fixtureUserPrompts,
   launchWebScaffold, seedSession, watchConsole, webSnapshotMode, type WebScaffold,
@@ -19,6 +20,7 @@ const SNAPSHOT_DIR = fileURLToPath(new URL('./snapshots/message-actions', import
 // a new recording (workspace-management / sidebar-scrollbar pattern).
 const SEED = fileURLToPath(new URL('./snapshots/seeded-history/seed.jsonl', import.meta.url))
 const UI_EXPECTED = join(SNAPSHOT_DIR, 'ui.expected.md')
+const FORK_EXPECTED = join(SNAPSHOT_DIR, 'fork.expected.md')
 const MODE = webSnapshotMode()
 const SEED_ID = 'message-actions-web-e2e'
 
@@ -85,9 +87,58 @@ describe('web e2e: message IconActions and clocks on settled history', () => {
     await compareOrRefreshGolden(UI_EXPECTED, snapshot, MODE)
   })
 
+  it.skipIf(MODE === 'record')('forks through the settled-message and session-row actions', async () => {
+    onTestFailed(() => saveFailureShot(page, 'web-e2e-message-fork'))
+    // Exercise the assistant action specifically; package coverage pins the
+    // user action separately at its own event seq.
+    await page.getByRole('button', { name: '在新对话中分支' }).last().click()
+    await expect.poll(
+      () => scaffold.ctx.agents.list().find(agent => agent.session.header.parentSession === SessionId(SEED_ID)),
+      { timeout: 15_000 },
+    ).toBeDefined()
+    await expect.poll(
+      () => page.locator('[role="treeitem"]').count(),
+      { timeout: 10_000 },
+    ).toBe(3)
+    await expect.poll(
+      () => page.locator('[role="treeitem"][aria-selected="true"]').count(),
+      { timeout: 10_000 },
+    ).toBe(1)
+    // The row action owns a distinct ui-workspace injection from the message
+    // action above, so exercise both through the loaded app before capture.
+    const sourceRow = page.locator('[role="treeitem"][aria-selected="true"]')
+    const rowBox = await sourceRow.boundingBox()
+    if (rowBox === null) throw new Error('fork source row has no layout box')
+    const actionButton = sourceRow.locator('button[aria-label^="Session actions for "]')
+    await sourceRow.hover({ position: { x: rowBox.width - 16, y: rowBox.height / 2 } })
+    await expect.poll(() => actionButton.isVisible(), { timeout: 2_000 }).toBe(true)
+    const buttonBox = await actionButton.boundingBox()
+    if (buttonBox === null) throw new Error('fork source row action has no layout box')
+    await page.mouse.click(buttonBox.x + buttonBox.width / 2, buttonBox.y + buttonBox.height / 2)
+    await page.getByRole('menuitem', { name: 'Fork session' }).click()
+    await expect.poll(
+      () => scaffold.ctx.agents.list().filter(agent => agent.session.header.parentSession !== undefined).length,
+      { timeout: 15_000 },
+    ).toBe(2)
+    await expect.poll(
+      () => page.locator('[role="treeitem"]').count(),
+      { timeout: 10_000 },
+    ).toBe(4)
+    await expect.poll(
+      () => page.locator('[role="treeitem"][aria-selected="true"]').count(),
+      { timeout: 10_000 },
+    ).toBe(1)
+    const tree = await captureStableAria(
+      page,
+      '[role="tree"][aria-label="Sessions"]',
+      scaffold.workspaceCwd,
+    )
+    await compareOrRefreshGolden(FORK_EXPECTED, tree, MODE)
+  })
+
   it.skipIf(MODE === 'record')('issued zero model calls and kept a closed inventory', async () => {
     expect(tripwire.pageErrors).toEqual([])
     expect(tripwire.warnings).toEqual([])
-    await assertFixtureInventory(SNAPSHOT_DIR, ['ui.expected.md'])
+    await assertFixtureInventory(SNAPSHOT_DIR, ['fork.expected.md', 'ui.expected.md'])
   })
 })

apps/web/tests/snapshots/message-actions/fork.expected.md

@@ -0,0 +1,7 @@
+- tree "Sessions":
+  - treeitem "Ungrouped 3 sessions" [expanded]:
+    - img
+    - text: Ungrouped 3 sessions
+  - treeitem "Use the read tool twice (2) now" [selected]
+  - treeitem "Use the read tool twice (1) now"
+  - treeitem "Use the read tool twice 1min"

packages/client/connection/src/client/fixture.ts

@@ -1042,6 +1042,56 @@ export function createFixtureApi(options: FixtureOptions = {}): ApiProxy {
         const appended = logOf(sessionId).at(-1) as SessionEvent
         return ok(request, { title: normalized, seq: appended.seq })
       },
+      fork: (request) => {
+        const { sessionId, atSeq } = request.payload
+        const source = summaryOf(sessionId)
+        if (source === undefined) {
+          return err(request, {
+            code: 'session-not-found',
+            message: `no session ${sessionId}`,
+            details: { sessionId },
+          })
+        }
+        const log = logs.get(sessionId) ?? []
+        const lastSeq = log.at(-1)?.seq ?? -1
+        const anchoredBoundary = atSeq === undefined
+          ? undefined
+          : log.find(e => e.type === 'turn/end' && e.seq >= atSeq)
+        const boundary = anchoredBoundary
+          ?? (atSeq === undefined || atSeq > lastSeq
+            ? log.findLast(e => e.type === 'turn/end')
+            : undefined)
+        if (boundary === undefined) {
+          return err(request, {
+            code: 'fork-unavailable',
+            message: atSeq !== undefined && atSeq <= lastSeq
+              ? `session ${sessionId} has not completed the turn containing event ${String(atSeq)}`
+              : `session ${sessionId} has no completed turn`,
+            details: { sessionId },
+          })
+        }
+        let cut = boundary.seq + 1
+        while (cut < log.length && log[cut]?.type !== 'turn/start') cut++
+        const child: SessionSummary = {
+          sessionId: sid(`fx-${nextSession++}`), updatedAt: Date.now(), running: false, blank: false,
+          parentSessionId: sessionId,
+          ...source.cwd === undefined ? {} : { cwd: source.cwd },
+        }
+        logs.set(child.sessionId, log.slice(0, cut))
+        sessions.push(child)
+        emitHost({
+          type: 'host/session-added', sessionId: child.sessionId, blank: false,
+          parentSessionId: sessionId,
+          ...source.cwd === undefined ? {} : { cwd: source.cwd },
+        })
+        const workspace = workspaces.find(w => w.sessionIds.includes(sessionId))
+        if (workspace !== undefined) {
+          workspace.sessionIds = [child.sessionId, ...workspace.sessionIds]
+          workspace.updatedAt = new Date().toISOString()
+          emitHost({ type: 'host/workspace-changed', workspace: { ...workspace } })
+        }
+        return ok(request, { sessionId: child.sessionId })
+      },
       history: async (request) => {
         const log = logs.get(request.payload.sessionId) ?? []
         // Snapshot at request time, deliver after the transit delay (mirrors a real host under latency).
@@ -1591,6 +1641,7 @@ export class FixtureApiClient extends AbstractApiClient {
       case 'session.models': return this.api.sessions.models(request)
       case 'session.selectModel': return this.api.sessions.selectModel(request)
       case 'session.rename': return this.api.sessions.rename(request)
+      case 'session.fork': return this.api.sessions.fork(request)
       case 'session.prompt': return this.api.sessions.prompt(request)
       case 'session.updateQueue': return this.api.sessions.updateQueue(request)
       case 'session.cancel': return this.api.sessions.cancel(request)

packages/client/connection/tests/fake-api.ts

@@ -46,6 +46,7 @@ export class FakeApiClient implements IApiClient {
   onList: (payload: unknown) => Promise<RpcResponse<{ items: never[] }>> = () => Promise.resolve(ok({ items: [] }))
   onCreate: (payload: unknown) => Promise<RpcResponse<{ sessionId: SessionId }>> = () => Promise.resolve(ok({ sessionId: 'fk-new' as SessionId }))
   onRename: (payload: unknown) => Promise<RpcResponse<{ title: string; seq: number }>> = () => Promise.resolve(ok({ title: 'fk-renamed', seq: 0 }))
+  onFork: (payload: unknown) => Promise<RpcResponse<{ sessionId: SessionId }>> = () => Promise.resolve(ok({ sessionId: 'fk-fork' as SessionId }))
   onHistory: (payload: { sessionId: SessionId; beforeSeq?: number; maxMessages?: number })
   => Promise<RpcResponse<{ events: never[]; hasMore: boolean; modelTarget: ModelTarget }>> =
     () => Promise.resolve(ok({
@@ -99,6 +100,7 @@ export class FakeApiClient implements IApiClient {
     selectModel: (payload: ModelTarget & { sessionId: SessionId }) =>
       this.record('session.selectModel', payload, this.onSelectModel(payload)),
     rename: (payload: unknown) => this.record('session.rename', payload, this.onRename(payload)),
+    fork: (payload: unknown) => this.record('session.fork', payload, this.onFork(payload)),
     prompt: (payload: unknown) => this.record('session.prompt', payload, this.onPrompt(payload)),
     updateQueue: (payload: unknown) => this.record('session.updateQueue', payload, this.onUpdateQueue(payload)),
     cancel: (payload: unknown) => this.record('session.cancel', payload, this.onCancel(payload)),

packages/client/runtime/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/runtime/README.md
-README.md: 766d8516225cd46cb1a3a80c832d1cf55e816140
-README.zh.md: 9b514afca91f604b3e895187de3b5532bf22a692
+README.md: b85deeec92fd4da1f342b5536757692f594853a5
+README.zh.md: 2dbb66c56ad5687fb299fe030d0abfe451004a62

packages/client/runtime/README.md

@@ -28,6 +28,10 @@ SlotsService gives the renderer separate bare observables for `useSessions` and
 
 `SessionManager` retains the latest validated `session/title` control snapshot independently of list and session-instance arrival. Newer event seqs replace older snapshots, title timestamps contribute to list recency, and a subscription baseline discards any retained title beyond its `lastSeq` before the optional folded title arrives. Explicit session removal also clears the retained title. The client-facing `SessionSummary.title` is therefore only the actual durable title; `displayTitle` is always present and falls back through the cwd basename and session id. A cold persisted session keeps that fallback until opening or resuming it causes the host to fold and project its log-backed title. `ISession.rename` settles the `title` projection cell directly from the unary response's `{title, seq}` under the same higher-seq-wins rule — the list row and every `useProjection('title')` reader update ahead of the push frame, whose later replay of the same seq is a no-op.
 
+## Session forking
+
+`ISessions.fork({sessionId, atSeq?, increaseTitle?})` resolves only after the child summary is locally addressable, carrying source lineage and cwd with `blank: false`; callers choose whether to open it. With `increaseTitle: true`, the client renames the child from the source session's persisted title: a trailing `(N)` or `（N）` is incremented without changing bracket style, while any other title gets ` (1)` appended; the rename is skipped when the source has no persisted title, and a rename failure rejects the promise but leaves the created child in place. This option is not sent in the Host fork request. A `workspace-attach-failed` response still identifies a child already published by the Host, so `SessionManager` reconciles that partial success before `SessionForkError` reaches the caller instead of making a retry create a duplicate child.
+
 ## Session model selection
 
 Each resident `Session` owns a `modelSelection` snapshot containing the current provider/model target, provider-grouped directory, provider-local failures, and the `idle`/`loading`/`ready`/`selecting`/`error` state. History establishes or refreshes the current target, opening a selector refreshes the directory, and selection failures preserve the last target and usable groups. Directory and selection operations share a monotonically increasing generation so an older response cannot overwrite a newer selection. A reconnect rebuild restores the target reported by the Host without replacing unchanged selection substructure.

packages/client/runtime/README.zh.md

@@ -28,6 +28,10 @@ SlotsService 分别为 renderer 提供 `useSessions` 与 `useWorkspaces` 的裸
 
 `SessionManager` 独立于列表和 Session 实例到达情况，保留最近一次通过验证的 `session/title` 控制快照。seq 更高的事件会替换旧快照，标题时间戳计入列表新近程度；订阅基线会先丢弃 seq 超过其 `lastSeq` 的任何已保留标题，再接收可选的折叠标题。显式移除 Session 也会清除已保留标题。因此，面向客户端的 `SessionSummary.title` 只包含实际的持久化标题；`displayTitle` 始终存在，并依次回退到 cwd basename 和 Session id。冷态持久化会话会保持该回退值，直到打开或恢复会话，促使主机折叠并投影由日志支撑的标题。`ISession.rename` 用 unary 响应中的 `{title, seq}` 直接结算 `title` 投影格，遵循同一 seq 高者胜规则——列表行和所有 `useProjection('title')` 读者在推送帧到达前即更新；推送帧随后重放同一 seq 时为无操作。
 
+## 会话 fork
+
+`ISessions.fork({sessionId, atSeq?, increaseTitle?})` 只在子会话摘要已能在本地寻址后才完成；该摘要携带源会话的谱系和 cwd，且 `blank: false`，由调用方决定是否打开。`increaseTitle: true` 会在 client 端把源会话的持久化标题改名到子会话：尾部 `(N)` 或 `（N）` 递增并保留括号样式，其余标题追加 ` (1)`；源会话没有持久化标题时跳过改名，改名失败时拒绝 promise 但保留已创建的子会话。该选项不会进入 Host fork 请求。即使响应为 `workspace-attach-failed`，其中仍会标识 Host 已发布的子会话，因此 `SessionManager` 会先将这一部分成功对账，再让 `SessionForkError` 到达调用方，避免重试创建重复的子会话。
+
 ## 会话模型选择
 
 每个常驻 `Session` 都拥有一个 `modelSelection` 快照，其中包含当前提供方/模型目标、按提供方分组的目录、逐提供方失败记录，以及 `idle`／`loading`／`ready`／`selecting`／`error` 状态。历史记录会建立或刷新当前目标，打开选择器会刷新目录；选择失败会保留上一个目标和可用分组。目录与选择操作共用单调递增的代次，因此较旧响应无法覆盖较新的选择。重连重建会恢复 Host 报告的目标，同时不替换未变化的选择子结构。

packages/client/runtime/src/client/contract/sessions.ts

@@ -29,6 +29,17 @@ export interface ISessions {
   open(id: SessionId): void
   /** Clear the current selection into the no-session view state. */
   clear(): void
+  /**
+   * Fork a session from a completed-turn prefix of the source; on resolution
+   * the child is in the list store and `open()` can target it.
+   * @param opts - source session id, the optional event seq anchoring the
+   *   cut (the boundary is the first turn/end at or after it; an in-log
+   *   anchor in an open turn is unavailable rather than clipped backward),
+   *   and whether to increment an inherited durable title before resolving.
+   * @returns the child session id.
+   * @throws when the fork fails, or when a requested child-title rename fails after creation.
+   */
+  fork(opts: { sessionId: SessionId; atSeq?: number; increaseTitle?: boolean }): Promise<SessionId>
   /**
    * Register a per-session standard-props provider (hooks become `use<Name>`
    * selector hooks on the render side; props spread verbatim).

packages/client/runtime/src/client/sessions/manager.ts

@@ -289,6 +289,40 @@ export class SessionManager {
     }
   }
 
+  /**
+   * Contract session.fork; on success merge the child into summaries
+   * immediately (same synchronous-addressability guarantee as create). The
+   * child carries the source's history, so it is never blank; lineage rides
+   * parentSessionId so the list nests it under its source. A child published
+   * before Workspace attachment fails is also reconciled into the list.
+   * @param opts - source session and the optional seq anchoring the cut.
+   * @returns the fork result (the child session id).
+   */
+  async fork(
+    opts: { sessionId: SessionId; atSeq?: number },
+  ): Promise<RpcResult<{ sessionId: SessionId }>> {
+    try {
+      const source = this.summaries.find(s => s.sessionId === opts.sessionId)
+      const { result } = await this.api.sessions.fork({
+        sessionId: opts.sessionId,
+        ...opts.atSeq === undefined ? {} : { atSeq: opts.atSeq },
+      })
+      const childId = result.ok
+        ? result.value.sessionId
+        : workspaceAttachSessionId(result.error)
+      if (childId !== undefined) {
+        this.recordMutation({ kind: 'upsert', summary: {
+          sessionId: childId, updatedAt: Date.now(), running: false, blank: false,
+          parentSessionId: opts.sessionId,
+          ...(source?.cwd !== undefined ? { cwd: source.cwd } : {}),
+        } })
+      }
+      return result
+    } catch (error) {
+      return transportError(error)
+    }
+  }
+
   /**
    * Insert-or-enrich a locally synthesized summary: a new id prepends; an
    * existing entry only gains fields it lacks (the session-added frame and the

packages/client/runtime/src/client/sessions/service.ts

@@ -81,6 +81,22 @@ export class SessionCreateError extends Error {
   }
 }
 
+/** Structured session-fork failure. */
+export class SessionForkError extends Error {
+  override readonly name = 'SessionForkError'
+
+  /**
+   * @param rpcError - Host business or folded transport error.
+   * @param sourceSessionId - the session the fork was cut from.
+   */
+  constructor(
+    readonly rpcError: RpcError,
+    readonly sourceSessionId: SessionId,
+  ) {
+    super(`session fork failed: ${rpcError.code}: ${rpcError.message}`)
+  }
+}
+
 /** Session assembly handle for SessionProvider/inject factories (identity-stable per session). */
 export interface SessionBinding {
   readonly sessionId: SessionId
@@ -121,6 +137,24 @@ function displayTitleOf(title: string | undefined, cwd: string | undefined, id:
   return id
 }
 
+/**
+ * Increment a trailing fork number while preserving its half-width or
+ * full-width parentheses; an unnumbered title starts with ` (1)`.
+ * @param title - source session's durable title.
+ * @returns the title assigned to the fork child.
+ */
+function increasedForkTitle(title: string): string {
+  const ascii = /^(.*?)\((\d+)\)$/u.exec(title)
+  if (ascii?.[1] !== undefined && ascii[2] !== undefined) {
+    return `${ascii[1]}(${BigInt(ascii[2]) + 1n})`
+  }
+  const fullWidth = /^(.*?)（(\d+)）$/u.exec(title)
+  if (fullWidth?.[1] !== undefined && fullWidth[2] !== undefined) {
+    return `${fullWidth[1]}（${BigInt(fullWidth[2]) + 1n}）`
+  }
+  return `${title} (1)`
+}
+
 interface ScopeRecord {
   fiber: Fiber
   ctx: Context
@@ -317,6 +351,42 @@ export class SessionsService implements ISessions {
     return result.value.sessionId
   }
 
+  /**
+   * Fork a session from a completed-turn prefix of the source (same
+   * synchronous-addressability guarantee as {@link SessionsService.create}:
+   * on resolution the child is in the list store and open() can target it).
+   * @param opts - source session id, the optional event seq anchoring the
+   *   cut (the boundary is the first turn/end at or after it; an in-log
+   *   anchor in an open turn is unavailable rather than clipped backward),
+   *   and whether to increment an inherited durable title before resolving.
+   * @returns the child session id.
+   * @throws {SessionForkError} with the source id.
+   * @throws {Error} when a requested child-title rename fails after creation.
+   */
+  async fork(opts: {
+    sessionId: SessionId
+    atSeq?: number
+    increaseTitle?: boolean
+  }): Promise<SessionId> {
+    const sourceTitle = opts.increaseTitle
+      ? this.list.getSnapshot().byId[opts.sessionId]?.title
+      : undefined
+    const result = await this.manager.fork({
+      sessionId: opts.sessionId,
+      ...(opts.atSeq === undefined ? {} : { atSeq: opts.atSeq }),
+    })
+    if (!result.ok) throw new SessionForkError(result.error, opts.sessionId)
+    this.projectList()
+    const childId = result.value.sessionId
+    if (sourceTitle !== undefined) {
+      const child = this.binding(childId)?.session
+      if (child === undefined) throw new Error(`fork child "${childId}" is not locally addressable`)
+      const renamed = await child.rename(increasedForkTitle(sourceTitle))
+      if (!renamed.ok) throw new Error(`fork child rename failed: ${renamed.error.code}: ${renamed.error.message}`)
+    }
+    return childId
+  }
+
   /**
    * Resolve an Agent-scoped context view (use-and-discard).
    * @param id - session id (the agent identity — 1:1 same axis).

packages/client/runtime/tests/fake-api.ts

@@ -64,6 +64,7 @@ export class FakeApiClient implements IApiClient {
   onCreate: (payload: unknown) => Promise<RpcResponse<{ sessionId: SessionId }>> = () => Promise.resolve(ok({ sessionId: 'fk-new' as SessionId }))
   readonly defaultModel: ModelTarget = { provider: 'deepseek', model: 'deepseek-v4-flash' }
   onRename: (payload: unknown) => Promise<RpcResponse<{ title: string; seq: number }>> = () => Promise.resolve(ok({ title: 'fk-renamed', seq: 0 }))
+  onFork: (payload: unknown) => Promise<RpcResponse<{ sessionId: SessionId }>> = () => Promise.resolve(ok({ sessionId: 'fk-fork' as SessionId }))
   onHistory: (payload: { sessionId: SessionId; beforeSeq?: number; maxMessages?: number })
   => Promise<RpcResponse<{ events: never[]; hasMore: boolean }>> =
     () => Promise.resolve(ok({ events: [], hasMore: false }))
@@ -118,6 +119,7 @@ export class FakeApiClient implements IApiClient {
     selectModel: (payload: { provider: string; model: string }) =>
       this.record('session.selectModel', payload, this.onSelectModel(payload)),
     rename: (payload: unknown) => this.record('session.rename', payload, this.onRename(payload)),
+    fork: (payload: unknown) => this.record('session.fork', payload, this.onFork(payload)),
     prompt: (payload: unknown) => this.record('session.prompt', payload, this.onPrompt(payload)),
     updateQueue: (payload: unknown) => this.record('session.updateQueue', payload, this.onUpdateQueue(payload)),
     cancel: (payload: unknown) => this.record('session.cancel', payload, this.onCancel(payload)),

packages/client/runtime/tests/manager.spec.ts

@@ -277,6 +277,23 @@ describe('remaining branches', () => {
     expect(manager.getListSnapshot().items[0]).not.toHaveProperty('cwd')
   })
 
+  it('reconciles a fork child published before workspace attachment fails', async () => {
+    const api = new FakeApiClient()
+    api.onFork = () => Promise.resolve(err({
+      code: 'workspace-attach-failed',
+      message: 'forked but unattached',
+      details: { sessionId: S2, workspaceId: 'w1' },
+    } as never))
+    const manager = new SessionManager(api)
+    const result = await manager.fork({ sessionId: S1 })
+    expect(result).toMatchObject({ ok: false, error: { code: 'workspace-attach-failed' } })
+    expect(manager.getListSnapshot().items).toEqual([expect.objectContaining({
+      sessionId: S2,
+      parentSessionId: S1,
+      blank: false,
+    })])
+  })
+
   it('reconciles a preallocated id after an ordinary transport failure', async () => {
     const api = new FakeApiClient()
     api.onCreate = () => Promise.reject(new Error('response lost'))

packages/client/runtime/tests/sessions-service.spec.ts

@@ -10,7 +10,7 @@ import { Context } from 'cordis'
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import type { SessionId } from '@deepseek-ai/dsh-client-connection/client'
 import { SessionCreateError, SessionsService, scopeOf } from '../src/client/sessions/service.ts'
-import { FakeApiClient, deferred, ok } from './fake-api.ts'
+import { FakeApiClient, deferred, err, ok } from './fake-api.ts'
 
 const sid = (s: string): SessionId => s as SessionId
 
@@ -399,6 +399,69 @@ describe('create', () => {
   })
 })
 
+describe('fork', () => {
+  it.each([
+    ['Roadmap', 'Roadmap (1)'],
+    ['Roadmap (1)', 'Roadmap (2)'],
+    ['计划（1）', '计划（2）'],
+    ['计划 （9）', '计划 （10）'],
+  ])('increments the durable title %j after the child is published', async (sourceTitle, childTitle) => {
+    const b = bench()
+    b.svc.handleMuxEnvelope({
+      rpcId: 'source-title' as never,
+      payload: { type: 'session/projection', sessionId: sid('source'), key: 'title', value: sourceTitle, seq: 2 } as never,
+    })
+    await feedList(b, [{ id: 'source', cwd: '/work' }])
+    b.api.onFork = () => Promise.resolve(ok({ sessionId: sid('child') }))
+    b.api.onRename = (payload) => {
+      const { title } = payload as { title: string }
+      return Promise.resolve(ok({ title, seq: 3 }))
+    }
+
+    await expect(b.svc.fork({
+      sessionId: sid('source'), atSeq: 7, increaseTitle: true,
+    })).resolves.toBe('child')
+
+    expect(b.api.callsOf('session.fork')).toEqual([{ sessionId: 'source', atSeq: 7 }])
+    expect(b.api.callsOf('session.rename')).toEqual([{ sessionId: 'child', title: childTitle }])
+    await Promise.resolve()
+    expect(b.svc.list.getSnapshot().byId[sid('child')]).toMatchObject({
+      title: childTitle,
+      displayTitle: childTitle,
+      parentId: 'source',
+    })
+  })
+
+  it('does not rename without the title policy or a durable source title', async () => {
+    const b = bench()
+    await feedList(b, [{ id: 'source', cwd: '/work' }])
+    b.api.onFork = () => Promise.resolve(ok({ sessionId: sid('child') }))
+    await expect(b.svc.fork({ sessionId: sid('source'), increaseTitle: true })).resolves.toBe('child')
+    expect(b.api.callsOf('session.rename')).toEqual([])
+
+    b.api.onFork = () => Promise.resolve(ok({ sessionId: sid('child-2') }))
+    await expect(b.svc.fork({ sessionId: sid('source') })).resolves.toBe('child-2')
+    expect(b.api.callsOf('session.rename')).toEqual([])
+  })
+
+  it('rejects when child rename fails while keeping the published child addressable', async () => {
+    const b = bench()
+    b.svc.handleMuxEnvelope({
+      rpcId: 'source-title' as never,
+      payload: { type: 'session/projection', sessionId: sid('source'), key: 'title', value: 'Roadmap', seq: 2 } as never,
+    })
+    await feedList(b, [{ id: 'source' }])
+    b.api.onFork = () => Promise.resolve(ok({ sessionId: sid('child') }))
+    b.api.onRename = () => Promise.resolve(err({
+      code: 'title-invalid', message: 'rejected', details: { sessionId: sid('child') },
+    }))
+
+    await expect(b.svc.fork({ sessionId: sid('source'), increaseTitle: true }))
+      .rejects.toThrow('fork child rename failed: title-invalid: rejected')
+    expect(b.svc.binding(sid('child'))).toBeDefined()
+  })
+})
+
 describe('scope lifecycle rides the list mirror (entity parity: no client-side pre-birth)', () => {
   it('a session-added frame births the row (blank) and makes the scope resolvable; removal prunes it', async () => {
     const b = bench()

packages/client/test-runtime/src/sessions.ts

@@ -169,7 +169,7 @@ export class TestSessions implements ISessions {
   private readonly channel: SessionProvideChannel
 
   /** Calls observed on the service-level face (open/clear), newest last. */
-  readonly calls: { method: 'open' | 'clear'; args: unknown[] }[] = []
+  readonly calls: { method: 'open' | 'clear' | 'fork'; args: unknown[] }[] = []
 
   /**
    * @param stabilize - the owning runtime's act wrapper.
@@ -392,6 +392,17 @@ export class TestSessions implements ISessions {
     this.list.update((draft) => { draft.current = undefined })
   }
 
+  /**
+   * Recorded fork stub: no child materializes (benches asserting the full
+   * fork flow drive the production service; this face only proves the call).
+   * @param opts - source session id, optional cut anchor, and client title policy.
+   * @returns the source id (no child record is created).
+   */
+  fork(opts: { sessionId: SessionId; atSeq?: number; increaseTitle?: boolean }): Promise<SessionId> {
+    this.calls.push({ method: 'fork', args: [opts] })
+    return Promise.resolve(opts.sessionId)
+  }
+
   /**
    * The session face of a fixture (typed view for assertions; fixture
    * behavior methods are grafted onto it).

packages/client/test-runtime/tests/runtime.spec.tsx

@@ -201,7 +201,7 @@ describe('sessions', () => {
     await runtime.dispose()
   })
 
-  it('records service-face calls; open() moves the selection and clear() empties it', async () => {
+  it('records service-face calls; open() moves selection, clear() empties it, and fork() echoes the source', async () => {
     const runtime = await runtimeWithFrame()
     await runtime.sessions.add({ id: 's1' })
     await runtime.sessions.add({ id: 's2' })
@@ -211,9 +211,13 @@ describe('sessions', () => {
     runtime.sessions.clear()
     await runtime.flush()
     expect(runtime.sessions.list.getSnapshot().current).toBeUndefined()
+    await expect(runtime.sessions.fork({
+      sessionId: 's1' as SessionId, atSeq: 7, increaseTitle: true,
+    })).resolves.toBe('s1')
     expect(runtime.sessions.calls).toEqual([
       { method: 'open', args: ['s1'] },
       { method: 'clear', args: [] },
+      { method: 'fork', args: [{ sessionId: 's1', atSeq: 7, increaseTitle: true }] },
     ])
     await runtime.dispose()
   })

packages/client/ui-conversation/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-conversation/README.md
-README.md: e3625f40a1f6b0d4097cbef51818c250c88cd0e8
-README.zh.md: a1c10c085d30dd5241823492d666f3c0b58d7942
+README.md: 60000c35c3e30883c4b29cc8410d30e58021318c
+README.zh.md: 8f9fe7278d2d4bf5251582867de44290d0727710

packages/client/ui-conversation/README.md

@@ -40,7 +40,7 @@ None; this package neither assembles nor sends a provider request.
 
 - **Stats-line durations cover the in-window flow only** — LLM and tool wall times fold the snapshot's assistant `timing` and tool call/result pairs, so nodes outside the loaded event window (older history) are not counted.
 - **Details panel is the minimal form and currently has no entry point** — selected call args/result raw display; the Input/Output/Metadata switch, Prev/Next stepping, and See-in-trajectory deep link are deferred. Tool rows stopped being details-panel click targets and nothing replaced that gesture, so `ChatViewInjected.openDetails` is implemented but uncalled and the panel (including its terminal card) is unreachable in the assembled application; its rendering stays covered by mounting it with a selection directly.
-- **Assistant per-message paging is a reserved slot** — drawn in the design, not implemented. The finalized content IconActions row (copy / branch / clock) ships under text output only; branch remains a chrome stub.
+- **Assistant per-message paging is a reserved slot** — drawn in the design, not implemented. The finalized content IconActions row (copy / branch / clock) ships under text output only; branch forks through the turn containing that message, increments the inherited title on the client, and then opens the child, while a fork or rename failure leaves the source selected.
 - **The sparkle icon for the others tool row is a hand-drawn approximation** — the design glyph's vector geometry is not exportable locally; promotion into ui-primitives waits on an exact export.
 - **The approval panel's "Always allow this type" is deferred** — durable grants need a grant-storage design; only allow-once/reject answer today.
 - **TodoPanel truncates long item text to one ellipsized line** — the figma strip has no wrap or expand affordance; full text is not readable inline.

packages/client/ui-conversation/README.zh.md

@@ -38,9 +38,9 @@ todo 两个面就是在该形状上的两个注册项，都是普通注册方插
 
 ## 已知限制与暂缓事项
 
-- **统计行的耗时只覆盖窗口内消息流**：LLM 与工具墙钟时间由快照的 assistant `timing` 与工具 call/result 配对折算，落在已加载事件窗口之外的节点（更早的历史）不计入。
+- **统计行的耗时只覆盖窗口内消息流**：LLM（大语言模型）与工具墙钟时间由快照的 assistant `timing` 与工具 call/result 配对折算，落在已加载事件窗口之外的节点（更早的历史）不计入。
 - **详情面板是最小形态，且当前没有入口**：以原始形式显示已选择调用的参数／结果；Input/Output/Metadata 切换、Prev/Next 步进与 See-in-trajectory 深链接暂缓实现。工具行已不再是详情面板的点击目标，且没有任何手势接替它，因此 `ChatViewInjected.openDetails` 虽已实现却无人调用，该面板（含其终端卡片）在组装后的应用中不可达；其渲染仍由直接以选中态挂载它来覆盖。
-- **assistant 逐消息分页是预留 slot**：设计中已有图稿，尚未实现。已定稿的内容 IconActions 行（复制／分支／时钟）只挂在 text 输出下；分支仍是 chrome stub。
+- **assistant 逐消息分页是预留 slot**：设计中已有图稿，尚未实现。已定稿的内容 IconActions 行（复制／分支／时钟）只挂在 text 输出下；分支会 fork 到包含该消息的轮次末尾，在 client 端递增继承标题后打开子会话，而 fork 或改名失败时源会话保持选中。
 - **others 工具行的闪光图标是手绘近似版本**：无法在本地导出设计字形的矢量几何；等到存在精确导出后再将其提升到 ui-primitives。
 - **审批面板的「始终允许此类」暂缓**：持久授权需要授权存储设计；今天只能回答允许一次／拒绝。
 - **TodoPanel 将过长条目截成单行省略号**：figma 条没有换行或展开入口，完整文本无法在行内读完。

packages/client/ui-conversation/src/client/apply.ts

@@ -262,6 +262,13 @@ export function apply(ctx: Context): void {
           })
         },
         loadOlder: () => { void scoped.loadOlder() },
+        forkAt: (seq) => {
+          sessions.fork({ sessionId, atSeq: seq, increaseTitle: true })
+            .then((childId) => { sessions.open(childId) })
+            .catch(() => {
+              // Fork or child-rename failure keeps the source view untouched.
+            })
+        },
       }
     },
   }, ChatView)

packages/client/ui-conversation/src/client/chat/AssistantMarkdown.tsx

@@ -23,6 +23,10 @@ export interface AssistantMarkdownProps {
   interrupted?: boolean | undefined
   /** Unix epoch ms for the finalized IconActions clock; omitted while streaming. */
   time?: number | undefined
+  /** Event sequence used as the fork boundary; omitted while streaming. */
+  seq?: number | undefined
+  /** Fork the session through the turn containing this finalized message. */
+  onFork?: ((seq: number) => void) | undefined
 }
 
 function firstLine(text: string): string {
@@ -60,7 +64,7 @@ function ThinkRow({ text, running }: { text: string; running: boolean }) {
 }
 
 export const AssistantMarkdown = memo(function AssistantMarkdown({
-  blocks, streaming, interrupted, time,
+  blocks, streaming, interrupted, time, seq, onFork,
 }: AssistantMarkdownProps) {
   const last = blocks.length - 1
   // Tool-call heads render as tool rows in the chat view's grouping pass, so
@@ -91,6 +95,7 @@ export const AssistantMarkdown = memo(function AssistantMarkdown({
           text={copyText(blocks)}
           time={time}
           clock="end"
+          onBranch={onFork === undefined || seq === undefined ? undefined : () => { onFork(seq) }}
           className={css.actions}
         />
       )}

packages/client/ui-conversation/src/client/chat/ChatView.tsx

@@ -230,7 +230,7 @@ function StreamingTail({ useSession, onGrow }: {
  * The chat view slot entry: pure component over the composed props (tool rows
  * render through the declared keyed hole's renderSlot share).
  */
-export function ChatView({ useSession, useSessions, useStore, renderSlot, sessionId, openFile, loadOlder }: ChatViewSlotProps) {
+export function ChatView({ useSession, useSessions, useStore, renderSlot, sessionId, openFile, loadOlder, forkAt }: ChatViewSlotProps) {
   const nodes = useSession(s => s.nodes)
   // Workspace root off the session list row: path summaries display relative to it.
   const cwd = useSessions(s => s.byId[sessionId]?.cwd)
@@ -377,6 +377,8 @@ export function ChatView({ useSession, useSessions, useStore, renderSlot, sessio
           streaming={false}
           interrupted={node.interrupted}
           time={node.time}
+          seq={node.seq}
+          onFork={forkAt}
         />
       )
     }
@@ -385,7 +387,7 @@ export function ChatView({ useSession, useSessions, useStore, renderSlot, sessio
     }
     /* v8 ignore next -- tool-result never reaches here: deriveChatFlow folds them into groups. */
     if (node.kind === 'tool-result') return null
-    return <MessageItem key={item.key} node={node} />
+    return <MessageItem key={item.key} node={node} onFork={forkAt} />
   }
 
   return (

packages/client/ui-conversation/src/client/chat/MessageIconActions.tsx

@@ -1,5 +1,6 @@
-// Shared IconActions chrome for user and assistant messages: copy / branch
-// live (branch still a stub), date-aware clock, optional edit stub.
+// Shared IconActions chrome for user and assistant messages: copy live,
+// branch wired through onBranch, date-aware clock,
+// optional edit stub.
 
 import { useCallback } from 'react'
 import {
@@ -18,17 +19,19 @@ export interface MessageIconActionsProps {
   clock: 'start' | 'end'
   /** When true, append the stub edit control (user bubble). */
   edit?: boolean | undefined
+  /** Fork the session at this message. */
+  onBranch?: (() => void) | undefined
   /** Parent layout class composed onto the actions row. */
   className?: string | undefined
 }
 
 /**
  * Copy / branch (/ clock) IconActions row shared by user and assistant chrome.
- * @param props - Copy text, event time, clock side, optional edit, className.
+ * @param props - Copy text, event time, clock side, optional edit, branch callback, className.
  * @returns The actions row element.
  */
 export function MessageIconActions({
-  text, time, clock, edit, className,
+  text, time, clock, edit, onBranch, className,
 }: MessageIconActionsProps) {
   const day = useCalendarDay()
   const onCopy = useCallback(() => {
@@ -48,7 +51,7 @@ export function MessageIconActions({
         </button>
       </Tooltip>
       <Tooltip label="在新对话中分支" side="bottom">
-        <button type="button" className={css.action} aria-label="在新对话中分支">
+        <button type="button" className={css.action} aria-label="在新对话中分支" onClick={onBranch}>
           <IconBranchOutline16 />
         </button>
       </Tooltip>

packages/client/ui-conversation/src/client/chat/MessageItem.tsx

@@ -16,6 +16,8 @@ import css from './MessageItem.module.css'
 
 export interface MessageItemProps {
   node: UserMessageNode | SteeringMessageNode | ContextMessageNode | UnknownSurfaceNode
+  /** Fork the session through the turn containing this message (user-bubble branch action). */
+  onFork?: (seq: number) => void
 }
 
 function contentText(content: readonly unknown[]): { text: string; rest: unknown[] } {
@@ -61,7 +63,7 @@ function projectUserText(text: string): ReactNode {
   return <>{parts}</>
 }
 
-export const MessageItem = memo(function MessageItem({ node }: MessageItemProps) {
+export const MessageItem = memo(function MessageItem({ node, onFork }: MessageItemProps) {
   switch (node.kind) {
     case 'user': {
       const { text, rest } = contentText(node.content)
@@ -76,6 +78,7 @@ export const MessageItem = memo(function MessageItem({ node }: MessageItemProps)
             time={node.time}
             clock="start"
             edit
+            onBranch={onFork === undefined ? undefined : () => { onFork(node.seq) }}
             className={css.actions}
           />
         </div>

packages/client/ui-conversation/src/client/contract/slots.ts

@@ -419,6 +419,8 @@ export interface ChatViewInjected {
    */
   openFile: (path: string) => void
   loadOlder: () => void
+  /** Fork the session through the turn containing the message at `seq`, then open the child. */
+  forkAt: (seq: number) => void
 }
 
 /** Full chat-view component props: runtime share & the declared toolview/commandview holes' render share & store share & injected share. */

packages/client/ui-conversation/tests/apply-inject.spec.tsx

@@ -122,6 +122,13 @@ describe('conversation slot inject surface', () => {
     const chatView = b.chatViewSurface(ROOT)
     chatView.injected.loadOlder()
     expect(b.sessionFake.loadOlder).toHaveBeenCalledTimes(1)
+    chatView.injected.forkAt(17)
+    await vi.waitFor(() => {
+      expect(b.runtime.sessions.calls).toContainEqual({ method: 'open', args: [ROOT] })
+    })
+    expect(b.runtime.sessions.calls).toContainEqual({
+      method: 'fork', args: [{ sessionId: ROOT, atSeq: 17, increaseTitle: true }],
+    })
     await b.runtime.dispose()
   })
 

packages/client/ui-conversation/tests/chat-view.spec.tsx

@@ -94,6 +94,7 @@ function makeHarness(init?: Partial<ConversationSnapshot>) {
   const openDetails = vi.fn<(t: SelectionTarget) => void>()
   const openFile = vi.fn<(path: string) => void>()
   const loadOlder = vi.fn()
+  const forkAt = vi.fn()
   // Selection rides the REAL chat store (same construction path as
   // production; the view reads it through the PropsStore useStore share).
   // renderSlot stub renders the render-site fallback (an empty keyed ledger:
@@ -120,9 +121,10 @@ function makeHarness(init?: Partial<ConversationSnapshot>) {
     openDetails,
     openFile,
     loadOlder,
+    forkAt,
   }
   const setSelection = (next: SelectionTarget | null): void => { chat.actions.select(next) }
-  return { set, ChatView, props, openDetails, openFile, loadOlder, setSelection }
+  return { set, ChatView, props, openDetails, openFile, loadOlder, forkAt, setSelection }
 }
 
 describe('chat-flow derivation', () => {
@@ -194,6 +196,16 @@ describe('ChatView', () => {
     expect(view.getByText('run a')).toBeTruthy()
   })
 
+  it('forks from both user and finalized assistant message actions at their event seq', () => {
+    const h = makeHarness({ nodes: [user(1, 'question'), assistant(2, 'answer')] })
+    const view = render(<h.ChatView {...h.props} />)
+    const buttons = view.getAllByRole('button', { name: '在新对话中分支' })
+    expect(buttons).toHaveLength(2)
+    fireEvent.click(buttons[0]!)
+    fireEvent.click(buttons[1]!)
+    expect(h.forkAt.mock.calls).toEqual([[1], [2]])
+  })
+
   it('renders assistant Markdown across history, streaming, final, and interrupted states while user text stays literal', () => {
     const markdown = '# Rendered\n\n- **one**\n- `two`'
     const h = makeHarness({ nodes: [user(1, markdown), assistant(2, markdown)] })

packages/client/ui-workspace/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-workspace/README.md
-README.md: a1b58f4abe0925be3b426d10344777e46caa9ba0
-README.zh.md: a472507bc45549c8feb55a75d294cbd7b3138cc5
+README.md: 860c24b8a25a1e9968261f586c16163579131a1c
+README.zh.md: 5a8e88051fc6f4fd46c5f2f6dcdc185eb4559ac6

packages/client/ui-workspace/README.zh.md

@@ -6,6 +6,8 @@
 
 该选择器通过全局 `useWorkspaces` hook 列出真实的 Host Workspace 实体。选择 Workspace 会调用 slot owner 的 `onPick` 回调，重新定位前端 Session 对象。每个注册各自声明一个**目录流子洞**（`single` kind：`conversation.hero.workspace.directoryFlow`／`sidebar.workspaces.directoryFlow`），由组合的选择器包 client half 填入其选取交互——今天是 [`-native`](../../host/directory-picker-native/README.md) 后端的无渲染 OS 选择器驱动，`-browse` 组合下则是应用内浏览对话框。平铺显示的 **打开本地文件夹…** 操作仅在本表层的洞被占用时渲染（每次菜单渲染读取占用状态；洞为空意味着该组合没有选目录能力——seam 文档化的无流程默认行为）。本包持有触发与接纳：占用者经洞的 owner 会话（`open`/`busy`/`onPicked`/`onCancel`/`onError`）每次打开上报一个所选路径，owner 通过对象层接纳它，并等待 Workspace 列表投影刷新后才选中已提交的 Workspace；取消操作不会显示提示，错误落入可重试的文件夹对话框，其 **重新选择** 会重新打开流程。**创建新工作区** 操作保留名称对话框，并禁用列表中已有的名称，而 Host 对并发或非 UI 调用方仍具有最终决定权。运行时 Session 与 Workspace 服务负责物化。Workspace 行内的 Delete 操作会打开确认框，说明保留边界、阻止重复提交，并在失败时保持打开；成功后，该分组会被移除，其 Session 则留在 Ungrouped 下。Session 行内的 Rename 操作打开同款浏览器持有的对话框，并以该行的显示标题预填：客户端不设名称冲突规则（host 负责规范化，可能以 `title-invalid` 拒绝，错误渲染在对话框告警区）；确认未修改的标题是有意允许的——这正是把当前自动标题钉住、不再被重新生成覆盖的手势。
 
+Session 行内的 Fork 操作在源会话最后一个已完成轮次处 fork，在 client 端递增继承的持久化标题后再打开子会话；尾部半角或全角括号编号会原样式递增，无编号标题追加 ` (1)`。源会话与子会话在 workspace 组内始终作为同级行展示，谱系只保留为 session 数据。Fork 或改名失败都不会改变当前选中项，改名失败时已创建的子会话仍会留在列表中。
+
 两个目标 slot 都由其他插件声明，因此 `apply` 通过声明感知的延迟机制完成注册，并在声明该 slot 的插件恢复后重新注册。
 
 ## 模型体验
@@ -18,5 +20,5 @@
 
 ## 已知限制与暂缓事项
 
-- **没有 Session 删除与 fork 控件**：Session 菜单的 Fork 与 Delete 行仍仅提供视觉效果（Rename 已接线）；删除 Workspace 注册记录不会删除 Session。
+- **没有 Session 删除控件**：Session 菜单的 Delete 行仍仅提供视觉效果；删除 Workspace 注册记录不会删除 Session。
 - **原生文件夹选择依赖本地 Host 载体**：在 `-native` 组合下，仅使用 fixture（测试前置数据）的部署或远程浏览器部署无法打开本地操作系统对话框；模态框会显示平台故障，并允许重试。可远程的选取是 `-browse` 组合的应用内流程。

packages/client/ui-workspace/src/client/WorkspaceBrowser.tsx

@@ -83,7 +83,7 @@ interface DragState {
 
 type SessionTreeProps = Pick<
   WorkspaceBrowserProps,
-  'useSessions' | 'startSession' | 'open' | 'insertSessionBefore'
+  'useSessions' | 'startSession' | 'open' | 'forkSession' | 'insertSessionBefore'
 > & {
   workspaces: readonly WorkspaceView[]
   /** Live search filter owned by the browser root (the query outlives the tree). */
@@ -98,13 +98,12 @@ type SessionTreeProps = Pick<
 
 /** The scrolling session tree; unmounting at collapse settle drops the sessions subscription and expansion state. */
 function SessionTree({
-  useSessions, startSession, open, workspaces, query,
+  useSessions, startSession, open, forkSession, workspaces, query,
   onRenameRequest, onDeleteRequest, onSessionRename, insertSessionBefore,
 }: SessionTreeProps) {
   const list = useSessions(s => s)
   const current = list.current
   const [expandedProjects, setExpandedProjects] = useState<string[]>([])
-  const [expandedSessions, setExpandedSessions] = useState<string[]>([])
   // Transient drag viewing state (never store-bound; order truth stays Host-side).
   const [drag, setDrag] = useState<DragState | null>(null)
   const currentGroup = current === undefined
@@ -116,8 +115,8 @@ function SessionTree({
     setExpandedProjects(l => (l.includes(currentGroup) ? l : [...l, currentGroup]))
   }, [current, currentGroup])
   const groups = useMemo(
-    () => deriveGroups(list, workspaces, { expandedProjects, expandedSessions, query }),
-    [list, workspaces, expandedProjects, expandedSessions, query],
+    () => deriveGroups(list, workspaces, { expandedProjects, query }),
+    [list, workspaces, expandedProjects, query],
   )
   const now = Date.now()
 
@@ -128,7 +127,7 @@ function SessionTree({
           <div className={css.empty}>{query === '' ? 'No sessions yet' : 'No matches'}</div>
         )}
         {groups.map(group => (
-          // Group section: header row + expanded session subtree. The
+          // Group section: header row + expanded top-level session rows. The
           // inter-group breathing room (former flat-list batch separator)
           // is the section's own margin (WorkspaceBrowser.module.css).
           <div key={group.key} className={css.groupSection}>
@@ -152,7 +151,7 @@ function SessionTree({
                 }}
             />
             {group.sessions.map((node, index) => {
-              // Draggable: real-workspace group roots outside search. The drag
+              // Draggable: real-workspace session rows outside search. The drag
               // never leaves its group — rows of other groups show no markers
               // and reject drops (visual movement confined to this section).
               const draggable = group.workspaceId !== undefined && query === ''
@@ -170,15 +169,15 @@ function SessionTree({
                 drop: (half: 'before' | 'after') => {
                   /* v8 ignore next -- narrowing guard: Rows gates drop on `active`, which is false while the drag state is null. */
                   if (drag === null) return
-                  const roots = group.sessions
+                  const sessions = group.sessions
                   // Anchor = the row the insert line points at ('after' means
                   // the next root; end-of-list omits the anchor → append).
-                  const anchor = half === 'before' ? node.id : roots[index + 1]?.id
+                  const anchor = half === 'before' ? node.id : sessions[index + 1]?.id
                   setDrag(null)
                   if (anchor === drag.sessionId) return
                   // No-op when the drop lands back on the source position.
-                  const sourceIndex = roots.findIndex(r => r.id === drag.sessionId)
-                  const anchorIndex = anchor === undefined ? roots.length : roots.findIndex(r => r.id === anchor)
+                  const sourceIndex = sessions.findIndex(r => r.id === drag.sessionId)
+                  const anchorIndex = anchor === undefined ? sessions.length : sessions.findIndex(r => r.id === anchor)
                   if (sourceIndex !== -1 && (anchorIndex === sourceIndex || anchorIndex === sourceIndex + 1)) return
                   insertSessionBefore(drag.workspaceId, drag.sessionId, anchor).catch((reason: unknown) => {
                     console.warn('session reorder rejected:', reason)
@@ -190,12 +189,11 @@ function SessionTree({
                 <SessionNodeItem
                   key={node.id}
                   node={node}
-                  depth={0}
                   currentId={current}
                   now={now}
                   onOpen={open}
                   onRename={onSessionRename}
-                  onToggle={(id) => { setExpandedSessions(l => toggled(l, id)) }}
+                  onFork={forkSession}
                   drag={dragProps}
                 />
               )
@@ -209,7 +207,7 @@ function SessionTree({
 }
 
 /** The flat "In one list" body: every session a top-level row, newest-first. */
-function FlatList({ useSessions, open, onSessionRename, query }: Pick<SessionTreeProps, 'useSessions' | 'open' | 'onSessionRename' | 'query'>) {
+function FlatList({ useSessions, open, forkSession, onSessionRename, query }: Pick<SessionTreeProps, 'useSessions' | 'open' | 'forkSession' | 'onSessionRename' | 'query'>) {
   const list = useSessions(s => s)
   const rows = useMemo(() => deriveFlat(list, { query }), [list, query])
   const now = Date.now()
@@ -223,14 +221,11 @@ function FlatList({ useSessions, open, onSessionRename, query }: Pick<SessionTre
           <SessionNodeItem
             key={node.id}
             node={node}
-            depth={0}
             currentId={list.current}
             now={now}
             onOpen={open}
             onRename={onSessionRename}
-            /* v8 ignore next -- required-prop filler: flat rows render no twist, so it never fires. */
-            onToggle={() => {}}
-            flat
+            onFork={forkSession}
           />
         ))}
       </div>
@@ -254,6 +249,7 @@ export function WorkspaceBrowser({
   startSession,
   open,
   renameSession,
+  forkSession,
   renameWorkspace,
   deleteWorkspace,
   insertSessionBefore,
@@ -462,11 +458,12 @@ export function WorkspaceBrowser({
           itself is wide-only. */}
       <div className={css.listArea}>
         {wide && (groupBy === 'flat'
-          ? <FlatList useSessions={useSessions} open={open} onSessionRename={onSessionRename} query={query} />
+          ? <FlatList useSessions={useSessions} open={open} forkSession={forkSession} onSessionRename={onSessionRename} query={query} />
           : (
             <SessionTree
               useSessions={useSessions}
               onSessionRename={onSessionRename}
+              forkSession={forkSession}
               workspaces={workspaces}
               startSession={startSession}
               open={open}

packages/client/ui-workspace/src/client/contract/slots.ts

@@ -95,6 +95,8 @@ export type WorkspaceBrowserInjected = DirectoryPickingInjected & {
   open: (sessionId: SessionId) => void
   /** Rename a Session (explicit user title; resolves on host acceptance). */
   renameSession: (sessionId: SessionId, title: string) => Promise<void>
+  /** Fork a Session at its last completed turn and open the child. */
+  forkSession: (sessionId: SessionId) => void
   /** Rename a Host Workspace (rejects on name conflict; resolves on durability). */
   renameWorkspace: (workspaceId: WorkspaceId, title: string) => Promise<void>
   /** Delete only a Host Workspace registration; directory and Session logs remain. */

packages/client/ui-workspace/src/client/index.ts

@@ -59,6 +59,13 @@ export function apply(ctx: ClientContext): void {
       const result = await session.rename(title)
       if (!result.ok) throw new Error(result.error.message)
     },
+    forkSession: (sessionId) => {
+      ctx.sessions.fork({ sessionId, increaseTitle: true })
+        .then((childId) => { ctx.sessions.open(childId) })
+        .catch(() => {
+          // Fork or child-rename failure keeps the current selection.
+        })
+    },
     renameWorkspace: async (workspaceId, title) => { await ctx.workspaces.rename(workspaceId, title) },
     deleteWorkspace: async (workspaceId) => { await ctx.workspaces.delete(workspaceId) },
     insertSessionBefore: async (workspaceId, sessionId, beforeSessionId) => {

packages/client/ui-workspace/src/client/rows/Rows.module.css

@@ -39,9 +39,7 @@
   height: 20px;
 }
 
-/* Session cell (figma): pad 8, adjacent 16px twist + status slots, then a 4px
-   gap to the title — the slots butt together, so the row gap is zeroed and
-   the title carries its own margins. */
+/* Session cell (figma): pad 8, a 16px status slot, then a 4px title gap. */
 .sessionRow {
   height: 34px;
   gap: 0;
@@ -168,7 +166,7 @@
   background: var(--dsw-alias-interactive-bg-hover);
 }
 
-/* Drag reorder insert line (workspace-group roots): 2px accent above or
+/* Drag reorder insert line (workspace-group session rows): 2px accent above or
    below the hovered row, drawn with box-shadow so no layout shift. */
 .sessionRow.dropBefore {
   box-shadow: 0 -2px 0 0 var(--dsw-alias-state-business-primary);
@@ -233,33 +231,9 @@
   color: var(--dsw-alias-label-primary);
 }
 
-/* Session expand twist occupies the leading 16px slot; keep a spacer when absent
-   so titles align across sibling rows. Duplicates the .iconButton reset instead
-   of `composes:` — the tsdown CSS-modules pipeline drops composes mappings, which
-   left the raw UA button box showing. */
-.twist {
-  flex: none;
-  display: inline-flex;
-  align-items: center;
-  justify-content: center;
-  width: 16px;
-  height: 20px;
-  border: none;
-  border-radius: 4px;
-  padding: 0;
-  background: transparent;
-  cursor: pointer;
-}
-
-.twist:hover {
-  color: var(--dsw-alias-label-primary);
-}
-
-/* Chevrons and tree twists ride the caption grey (#ADB2B8); the folder glyph
-   stays one step darker (tertiary, #81858C) per the cell spec. Declared last
-   to win over the composed .iconButton color. */
-.chevron,
-.twist {
+/* Chevrons ride the caption grey (#ADB2B8); the folder glyph stays one step
+   darker (tertiary, #81858C) per the cell spec. */
+.chevron {
   color: var(--dsw-alias-label-caption);
 }
 

packages/client/ui-workspace/src/client/rows/Rows.tsx

@@ -2,8 +2,8 @@
  * Workspace browser tree row components (figma Cell set 14:3080): pure presentational —
  * all data and callbacks arrive via props. Hover swaps (folder->chevron,
  * time->ellipsis, action buttons) are CSS-only. Row ... menus are visual-only
- * except workspace Rename/Delete and session Rename; the session and workspace
- * hover cards are suppressed while a menu is open.
+ * except workspace Rename/Delete and session Rename/Fork; the session and
+ * workspace hover cards are suppressed while a menu is open.
  */
 import { useState } from 'react'
 import clsx from 'clsx'
@@ -16,9 +16,6 @@ import type { GroupNode, SessionNode } from '../tree.ts'
 import { formatRelativeTime } from '../tree.ts'
 import css from './Rows.module.css'
 
-/** Indent step per tree level: one 16px slot (figma session cell). */
-const INDENT_STEP = 16
-
 const SESSION_MENU_ITEMS = [
   { id: 'rename', label: 'Rename', icon: <IconEditOutline16 /> },
   { id: 'fork', label: 'Fork session', icon: <IconBranchOutline16 /> },
@@ -135,16 +132,12 @@ export function ProjectRowItem({ group, onToggle, onCreate, actions }: {
 }
 
 /**
- * One session subtree: the node's own 34px row (indent by depth, expand
- * twist when it has children, running dot, relative time) plus its visible
- * children, recursively — the component tree mirrors the derived tree.
+ * One top-level 34px session row with running dot and relative time.
  * @param props.node - derived session node.
- * @param props.depth - 0 = directly under the group header.
  * @param props.currentId - selected session id (row highlight).
  * @param props.now - epoch ms for relative-time formatting.
  * @param props.onOpen - open a session by id.
- * @param props.onToggle - unfold/fold a subtree by id.
- * @returns the node's row followed by its children.
+ * @returns the session row.
  */
 /** Hover-card body: full title, relative time, and the status line (running/idle until wire status lands). */
 function SessionHoverContent({ node, now }: { node: SessionNode; now: number }) {
@@ -161,7 +154,7 @@ function SessionHoverContent({ node, now }: { node: SessionNode; now: number })
 }
 
 /**
- * Root-row drag wiring supplied by the group owner (workspace groups only).
+ * Session-row drag wiring supplied by the group owner (workspace groups only).
  * `drop` reports the half of the row the pointer released on: 'before'
  * inserts above this row, 'after' below it (the owner resolves the anchor).
  */
@@ -184,26 +177,22 @@ function rowHalf(e: { clientY: number; currentTarget: HTMLElement }): 'before' |
   return e.clientY < rect.top + rect.height / 2 ? 'before' : 'after'
 }
 
-export function SessionNodeItem({ node, depth, currentId, now, onOpen, onRename, onToggle, drag, flat = false }: {
+export function SessionNodeItem({ node, currentId, now, onOpen, onRename, onFork, drag }: {
   node: SessionNode
-  depth: number
   currentId: string | undefined
   now: number
   onOpen: (id: SessionNode['id']) => void
   /** Open the browser-owned session rename dialog (row menu action). */
   onRename: (id: SessionNode['id'], currentTitle: string) => void
-  onToggle: (id: SessionNode['id']) => void
-  /** Present only on draggable rows (workspace-group roots outside search). */
+  /** Fork a session at its last completed turn (row menu action). */
+  onFork: (id: SessionNode['id']) => void
+  /** Present only on draggable rows (workspace-group sessions outside search). */
   drag?: RowDragProps | undefined
-  /** Flat-list variant: no twist slot (figma flat cell) — titles align on the status slot. */
-  flat?: boolean
 }) {
   const row = node
   const selected = node.id === currentId
   const [menuOpen, setMenuOpen] = useState(false)
-  // Rail (figma session cell: pad 8, twist slot 16, status slot 16, gap 4 to
-  // the title): both slots are always reserved so titles align whether or not
-  // the twist/dot is lit. Extra depth rides the left padding.
+  // Figma session cell: pad 8, status slot 16, then a 4px title gap.
   const ownRow = (
     <div
       className={clsx(
@@ -212,8 +201,6 @@ export function SessionNodeItem({ node, depth, currentId, now, onOpen, onRename,
       )}
       role="treeitem"
       aria-selected={selected}
-      {...(row.hasChildren ? { 'aria-expanded': row.expanded } : {})}
-      style={{ paddingLeft: 8 + depth * INDENT_STEP }}
       onClick={() => { onOpen(node.id) }}
       draggable={drag !== undefined}
       onDragStart={drag === undefined
@@ -239,18 +226,6 @@ export function SessionNodeItem({ node, depth, currentId, now, onOpen, onRename,
           drag.drop(rowHalf(e))
         }}
     >
-      {row.hasChildren && !flat
-        ? (
-          <button
-            type="button"
-            className={css.twist}
-            aria-label={row.expanded ? 'Collapse' : 'Expand'}
-            onClick={(e) => { e.stopPropagation(); onToggle(node.id) }}
-          >
-            <IconTriangleRightFill14 className={clsx(css.arrow, row.expanded && css.arrowOpen)} />
-          </button>
-        )
-        : null}
       <span className={css.slot}>{row.running && <StateDot state="ongoing" />}</span>
       <span className={css.title}>{row.title}</span>
       <span className={css.time}>{formatRelativeTime(row.updatedAt, now)}</span>
@@ -261,7 +236,8 @@ export function SessionNodeItem({ node, depth, currentId, now, onOpen, onRename,
           items={SESSION_MENU_ITEMS}
           onSelect={(id) => {
             setMenuOpen(false)
-            if (id === 'rename') onRename(node.id, row.title) // fork/delete stay visual-only.
+            if (id === 'rename') onRename(node.id, row.title)
+            if (id === 'fork') onFork(node.id) // delete stays visual-only.
           }}
           portal
           closeOnPointerLeave
@@ -280,24 +256,10 @@ export function SessionNodeItem({ node, depth, currentId, now, onOpen, onRename,
     </div>
   )
   return (
-    <>
-      <HoverCard
-        anchor={ownRow}
-        content={<SessionHoverContent node={node} now={now} />}
-        disabled={menuOpen || drag?.active === true}
-      />
-      {node.children.map(child => (
-        <SessionNodeItem
-          key={child.id}
-          node={child}
-          depth={depth + 1}
-          currentId={currentId}
-          now={now}
-          onOpen={onOpen}
-          onRename={onRename}
-          onToggle={onToggle}
-        />
-      ))}
-    </>
+    <HoverCard
+      anchor={ownRow}
+      content={<SessionHoverContent node={node} now={now} />}
+      disabled={menuOpen || drag?.active === true}
+    />
   )
 }

packages/client/ui-workspace/src/client/tree.ts

@@ -11,20 +11,15 @@ export const UNGROUPED_KEY = ''
 /** Display label for the ungrouped bucket row. */
 export const UNGROUPED_LABEL = 'Ungrouped'
 
-/** One session node of a group's visible tree (34px row; children render indented one step). */
+/** One top-level session row in a group or the flat list. */
 export interface SessionNode {
   id: SessionId
   title: string
-  /** Visible children, already expansion/search-filtered (empty when folded). */
-  children: readonly SessionNode[]
-  /** The session HAS children in the data (the twist renders even while folded). */
-  hasChildren: boolean
-  expanded: boolean
   running: boolean
   updatedAt: number
 }
 
-/** One workspace group section: header row facts + the visible session tree. */
+/** One workspace group section: header row facts + visible top-level session rows. */
 export interface GroupNode {
   /** Group key: the workspace id or {@link UNGROUPED_KEY}. */
   key: string
@@ -39,14 +34,13 @@ export interface GroupNode {
   expanded: boolean
   /** The group contains the selected session (active folder tint; supplied here so the renderer never scans). */
   containsCurrent: boolean
-  /** Visible roots (empty while the group is folded). */
+  /** Visible session rows (empty while the group is folded). */
   sessions: readonly SessionNode[]
 }
 
-/** Viewing state consumed by the derivation — the component's local useState arrays, taken as-is. */
+/** Viewing state consumed by the derivation. */
 export interface TreeView {
   expandedProjects: readonly string[]
-  expandedSessions: readonly string[]
   query: string
 }
 
@@ -56,9 +50,7 @@ interface Group {
   cwd: string | undefined
   createdAt: number | undefined
   label: string
-  summaries: Map<SessionId, SessionSummary>
-  roots: SessionId[]
-  children: Map<SessionId, SessionId[]>
+  sessions: SessionSummary[]
 }
 
 /**
@@ -89,7 +81,7 @@ function sessionTitle(session: SessionSummary): string {
   return session.blank ? 'New Session' : session.displayTitle
 }
 
-/** Build one group's parent/child tree from an ordered member list. */
+/** Build one group without projecting session lineage into presentation. */
 function buildGroup(
   key: string,
   workspaceId: WorkspaceId | undefined,
@@ -99,54 +91,11 @@ function buildGroup(
   members: readonly SessionSummary[],
   order: 'account' | 'recency',
 ): Group {
-  const summaries = new Map(members.map(m => [m.id, m]))
-  const children = new Map<SessionId, SessionId[]>()
-  const roots: SessionSummary[] = []
-  for (const m of members) {
-    // A session is a tree child only when its parent lives in the same
-    // group; cross-group or unknown parents degrade to group roots.
-    if (m.parentId !== undefined && m.parentId !== m.id && summaries.has(m.parentId)) {
-      const kids = children.get(m.parentId)
-      if (kids === undefined) children.set(m.parentId, [m.id])
-      else kids.push(m.id)
-    } else {
-      roots.push(m)
-    }
-  }
-  // Workspace order is the member iteration order (workspace.sessionIds), so
-  // attached groups keep insertion order; Ungrouped sorts by recency.
-  if (order === 'recency') {
-    roots.sort(byRecency)
-    for (const kids of children.values()) {
-      kids.sort((a, b) => {
-        const sa = summaries.get(a)
-        const sb = summaries.get(b)
-        /* v8 ignore next -- unreachable: kid ids are inserted alongside their summaries. */
-        if (sa === undefined || sb === undefined) return 0
-        return byRecency(sa, sb)
-      })
-    }
-  }
-  const rootIds = roots.map(r => r.id)
-  // parentId cycles (host bug) leave members unreachable from any root;
-  // surface them as extra roots — the flatten walk's visited set stops
-  // loops. Each node sits in at most one kids list and roots have no
-  // in-group parent, so the scan pushes every reachable node exactly once.
-  const reachable = new Set<SessionId>(rootIds)
-  const stack = [...rootIds]
-  while (stack.length > 0) {
-    const top = stack.pop()
-    /* v8 ignore next -- unreachable: the loop condition guarantees a non-empty stack. */
-    if (top === undefined) break
-    for (const kid of children.get(top) ?? []) {
-      reachable.add(kid)
-      stack.push(kid)
-    }
-  }
-  for (const m of members) {
-    if (!reachable.has(m.id)) rootIds.push(m.id)
-  }
-  return { key, workspaceId, cwd, createdAt, label, summaries, roots: rootIds, children }
+  const sessions = [...members]
+  // Workspace order is workspace.sessionIds; only Ungrouped lacks an account
+  // order and therefore falls back to recency.
+  if (order === 'recency') sessions.sort(byRecency)
+  return { key, workspaceId, cwd, createdAt, label, sessions }
 }
 
 /**
@@ -181,72 +130,24 @@ function groupByWorkspace(list: SessionListState, workspaces: readonly Workspace
   return groups
 }
 
-function sessionNode(s: SessionSummary, children: readonly SessionNode[], hasChildren: boolean, expanded: boolean): SessionNode {
+function sessionNode(s: SessionSummary): SessionNode {
   return {
     id: s.id,
     title: sessionTitle(s),
-    children,
-    hasChildren,
-    expanded,
     running: s.running,
     updatedAt: s.updatedAt,
   }
 }
 
-function buildVisible(g: Group, expandedSessions: ReadonlySet<string>): SessionNode[] {
-  const visited = new Set<SessionId>()
-  const walk = (id: SessionId): SessionNode | null => {
-    if (visited.has(id)) return null
-    visited.add(id)
-    const s = g.summaries.get(id)
-    /* v8 ignore next -- unreachable: walked ids come from the grouped summaries. */
-    if (s === undefined) return null
-    const kids = g.children.get(id) ?? []
-    const expanded = expandedSessions.has(id)
-    const children = expanded ? kids.map(walk).filter((n): n is SessionNode => n !== null) : []
-    return sessionNode(s, children, kids.length > 0, expanded)
-  }
-  return g.roots.map(walk).filter((n): n is SessionNode => n !== null)
-}
-
-/** Matched sessions plus their ancestor chains (forced visible under search). */
-function searchVisible(g: Group, q: string): Set<SessionId> {
-  const visible = new Set<SessionId>()
-  for (const m of g.summaries.values()) {
-    if (!sessionTitle(m).toLowerCase().includes(q)) continue
-    let cur: SessionSummary | undefined = m
-    while (cur !== undefined && !visible.has(cur.id)) {
-      visible.add(cur.id)
-      cur = cur.parentId !== undefined && cur.parentId !== cur.id ? g.summaries.get(cur.parentId) : undefined
-    }
-  }
-  return visible
-}
-
-function buildSearch(g: Group, visible: ReadonlySet<SessionId>): SessionNode[] {
-  const visited = new Set<SessionId>()
-  const walk = (id: SessionId): SessionNode | null => {
-    if (visited.has(id) || !visible.has(id)) return null
-    visited.add(id)
-    const s = g.summaries.get(id)
-    /* v8 ignore next -- unreachable: walked ids come from the grouped summaries. */
-    if (s === undefined) return null
-    const kids = (g.children.get(id) ?? []).filter(kid => visible.has(kid))
-    const children = kids.map(walk).filter((n): n is SessionNode => n !== null)
-    return sessionNode(s, children, kids.length > 0, kids.length > 0)
-  }
-  return g.roots.map(walk).filter((n): n is SessionNode => n !== null)
-}
-
 /**
- * Derive the nested workspace browser group structure.
+ * Derive the workspace browser groups with every session as a top-level row.
  *
  * Normal mode: every group shows; sessions populate under expanded groups,
- * descending only into expanded sessions. Search mode (non-blank query,
+ * preserving Host account order. Search mode (non-blank query,
  * case-insensitive display-title substring): expansion state is ignored —
- * matched sessions and their ancestor chains are forced visible, groups
- * without a display-title or label hit are dropped, and a label-only hit
- * keeps the bare group header. Blank sessions are excluded everywhere.
+ * matching sessions are forced visible, groups without a display-title or
+ * label hit are dropped, and a label-only hit
+ * keeps the bare group header. Non-current blank sessions are excluded.
  * @param list - sessions list snapshot (`current` feeds containsCurrent).
  * @param workspaces - real workspaces in stable Host order.
  * @param view - local expansion arrays and search query.
@@ -259,7 +160,6 @@ export function deriveGroups(
 ): GroupNode[] {
   const q = view.query.trim().toLowerCase()
   const expandedProjects = new Set(view.expandedProjects)
-  const expandedSessions = new Set(view.expandedSessions)
   const currentGroup = list.current === undefined
     ? undefined
     : (workspaces.find(w => w.sessionIds.includes(list.current as SessionId))?.workspaceId as string | undefined)
@@ -274,24 +174,24 @@ export function deriveGroups(
         cwd: g.cwd,
         createdAt: g.createdAt,
         label: g.label,
-        sessionCount: g.summaries.size,
+        sessionCount: g.sessions.length,
         expanded,
         containsCurrent: g.key === currentGroup,
-        sessions: expanded ? buildVisible(g, expandedSessions) : [],
+        sessions: expanded ? g.sessions.map(sessionNode) : [],
       })
     } else {
-      const visible = searchVisible(g, q)
-      if (visible.size === 0 && !g.label.toLowerCase().includes(q)) continue
+      const matches = g.sessions.filter(session => sessionTitle(session).toLowerCase().includes(q))
+      if (matches.length === 0 && !g.label.toLowerCase().includes(q)) continue
       groups.push({
         key: g.key,
         workspaceId: g.workspaceId,
         cwd: g.cwd,
         createdAt: g.createdAt,
         label: g.label,
-        sessionCount: g.summaries.size,
-        expanded: visible.size > 0,
+        sessionCount: g.sessions.length,
+        expanded: matches.length > 0,
         containsCurrent: g.key === currentGroup,
-        sessions: buildSearch(g, visible),
+        sessions: matches.map(sessionNode),
       })
     }
   }
@@ -301,9 +201,8 @@ export function deriveGroups(
 /**
  * Derive the flat session list ("In one list" mode): every session — fork
  * children included — as a top-level row, strictly newest-first. No grouping,
- * no parent/child adjacency; rows reuse SessionNode with children always
- * empty so the renderer stays branch-free. Search mode filters by
- * case-insensitive display-title substring.
+ * no parent/child adjacency. Search mode filters by case-insensitive
+ * display-title substring.
  * @param list - sessions list snapshot.
  * @param view - the search query (expansion state does not apply).
  * @returns flat rows in render order.
@@ -318,7 +217,7 @@ export function deriveFlat(list: SessionListState, view: Pick<TreeView, 'query'>
     rows.push(s)
   }
   rows.sort(byRecency)
-  return rows.map(s => sessionNode(s, [], false, false))
+  return rows.map(sessionNode)
 }
 
 /**

packages/client/ui-workspace/tests/apply.spec.ts

@@ -19,11 +19,17 @@ async function bench() {
   const insertSessionBefore = vi.fn(async () => ({}))
   const open = vi.fn()
   const clear = vi.fn()
+  const renameSession = vi.fn(async (title: string) => ({ ok: true, value: { title, seq: 1 } }))
+  const binding = vi.fn(() => ({ session: { rename: renameSession } }))
+  const fork = vi.fn(async () => 'forked' as never)
   ctx.provide('workspaces', {
     create, startSession, rename, insertSessionBefore,
   } as never)
-  ctx.provide('sessions', { open, clear } as never)
-  return { ctx, slots: ctx.get('slots') as SlotsService, create, startSession, rename, insertSessionBefore, open, clear }
+  ctx.provide('sessions', { open, clear, binding, fork } as never)
+  return {
+    ctx, slots: ctx.get('slots') as SlotsService, create, startSession, rename,
+    insertSessionBefore, open, clear, renameSession, binding, fork,
+  }
 }
 
 type HoleName = 'sidebar.workspaces' | 'conversation.hero.workspace' | 'conversation.empty.workspace'
@@ -66,6 +72,14 @@ describe('ui-workspace apply', () => {
     expect(b.startSession).toHaveBeenLastCalledWith(undefined)
     browser.open('session' as never)
     expect(b.open).toHaveBeenCalledWith('session')
+    await browser.renameSession('session' as never, 'renamed session')
+    expect(b.binding).toHaveBeenCalledWith('session')
+    expect(b.renameSession).toHaveBeenCalledWith('renamed session')
+    browser.forkSession('session' as never)
+    await vi.waitFor(() => {
+      expect(b.open).toHaveBeenCalledWith('forked')
+    })
+    expect(b.fork).toHaveBeenCalledWith({ sessionId: 'session', increaseTitle: true })
     await browser.renameWorkspace('ws' as never, 'renamed')
     expect(b.rename).toHaveBeenCalledWith('ws', 'renamed')
     await browser.insertSessionBefore('ws' as never, 's1' as never, 's2' as never)

packages/client/ui-workspace/tests/rows.spec.tsx

@@ -56,46 +56,22 @@ describe('workspace browser rows', () => {
     expect(onToggle).toHaveBeenCalledOnce()
   })
 
-  it('renders and operates selected, running, recursive Session nodes', () => {
-    const child: SessionNode = {
-      id: sid('child'), title: 'Child', children: [], hasChildren: false,
-      expanded: false, running: false, updatedAt: 0,
-    }
-    const parent: SessionNode = {
-      id: sid('parent'), title: 'Parent', children: [child], hasChildren: true,
-      expanded: true, running: true, updatedAt: 0,
+  it('renders and opens a selected running Session row', () => {
+    const node: SessionNode = {
+      id: sid('session'), title: 'Session', running: true, updatedAt: 0,
     }
     const onOpen = vi.fn()
-    const onToggle = vi.fn()
-    const view = render(
-      <SessionNodeItem node={parent} depth={0} currentId={parent.id} now={0} onOpen={onOpen}
-        onRename={vi.fn()} onToggle={onToggle} />,
+    render(
+      <SessionNodeItem node={node} currentId={node.id} now={0} onOpen={onOpen}
+        onRename={vi.fn()} onFork={vi.fn()} />,
     )
 
-    const parentRow = screen.getByText('Parent').closest('[role="treeitem"]')!
-    const childRow = screen.getByText('Child').closest('[role="treeitem"]')!
-    expect(parentRow.getAttribute('aria-selected')).toBe('true')
-    expect(parentRow.getAttribute('aria-expanded')).toBe('true')
-    expect(childRow.getAttribute('aria-selected')).toBe('false')
-    expect(childRow.hasAttribute('aria-expanded')).toBe(false)
-
-    fireEvent.click(screen.getByRole('button', { name: 'Collapse' }))
-    expect(onToggle).toHaveBeenCalledWith(parent.id)
-    expect(onOpen).not.toHaveBeenCalled()
-    fireEvent.click(parentRow)
-    fireEvent.click(childRow)
-    expect(onOpen.mock.calls).toEqual([[parent.id], [child.id]])
-
-    view.rerender(
-      <SessionNodeItem
-        node={{ ...parent, children: [], expanded: false, running: false }}
-        depth={1} currentId={undefined} now={0} onOpen={onOpen}
-        onRename={vi.fn()} onToggle={onToggle}
-      />,
-    )
-    expect(screen.getByRole('button', { name: 'Expand' })).toBeTruthy()
-    expect(screen.getByRole('treeitem').getAttribute('aria-selected')).toBe('false')
-    expect(screen.getByRole('treeitem').style.paddingLeft).toBe('24px')
+    const row = screen.getByRole('treeitem')
+    expect(row.getAttribute('aria-selected')).toBe('true')
+    expect(row.hasAttribute('aria-expanded')).toBe(false)
+    expect(screen.queryByRole('button', { name: /Expand|Collapse/ })).toBeNull()
+    fireEvent.click(row)
+    expect(onOpen).toHaveBeenCalledWith(node.id)
   })
 
   it('workspace row menu opens on the ellipsis, renames, and shows the danger delete row', () => {
@@ -156,15 +132,15 @@ describe('workspace browser rows', () => {
     expect(screen.queryByRole('button', { name: /Workspace actions/ })).toBeNull()
   })
 
-  it('session row menu opens without opening the session and dispatches rename', () => {
+  it('session row menu opens without opening the session and dispatches rename and fork', () => {
     const onOpen = vi.fn()
     const onRename = vi.fn()
+    const onFork = vi.fn()
     const node: SessionNode = {
-      id: sid('s1'), title: 'One', children: [], hasChildren: false,
-      expanded: false, running: false, updatedAt: 0,
+      id: sid('s1'), title: 'One', running: false, updatedAt: 0,
     }
-    render(<SessionNodeItem node={node} depth={0} currentId={undefined} now={0} onOpen={onOpen}
-      onRename={onRename} onToggle={vi.fn()} />)
+    render(<SessionNodeItem node={node} currentId={undefined} now={0} onOpen={onOpen}
+      onRename={onRename} onFork={onFork} />)
     fireEvent.click(screen.getByRole('button', { name: 'Session actions for One' }))
     expect(onOpen).not.toHaveBeenCalled()
     expect(screen.getByRole('menuitem', { name: 'Delete session' }).className).toMatch(/danger/)
@@ -173,9 +149,10 @@ describe('workspace browser rows', () => {
     expect(screen.queryByRole('menu')).toBeNull()
     expect(onRename).toHaveBeenCalledWith(node.id, 'One')
     expect(onOpen).not.toHaveBeenCalled()
-    // Fork and Delete stay visual-only.
     fireEvent.click(screen.getByRole('button', { name: 'Session actions for One' }))
     fireEvent.click(screen.getByRole('menuitem', { name: 'Fork session' }))
+    expect(onFork).toHaveBeenCalledWith(node.id)
+    // Delete stays visual-only.
     fireEvent.click(screen.getByRole('button', { name: 'Session actions for One' }))
     fireEvent.click(screen.getByRole('menuitem', { name: 'Delete session' }))
     expect(onRename).toHaveBeenCalledOnce()
@@ -185,25 +162,14 @@ describe('workspace browser rows', () => {
     expect(screen.queryByRole('menu')).toBeNull()
   })
 
-  it('flat variant renders no twist even for a parent and ignores toggling', () => {
-    const node: SessionNode = {
-      id: sid('p'), title: 'Parent', children: [], hasChildren: true,
-      expanded: false, running: false, updatedAt: 0,
-    }
-    render(<SessionNodeItem node={node} depth={0} currentId={undefined} now={0} onOpen={vi.fn()}
-      onRename={vi.fn()} onToggle={vi.fn()} flat />)
-    expect(screen.queryByRole('button', { name: 'Expand' })).toBeNull()
-  })
-
   it('shows the hover card after the dwell and suppresses it while the row menu is open', () => {
     vi.useFakeTimers()
     try {
       const node: SessionNode = {
-        id: sid('s1'), title: 'Hovered', children: [], hasChildren: false,
-        expanded: false, running: true, updatedAt: 0,
+        id: sid('s1'), title: 'Hovered', running: true, updatedAt: 0,
       }
-      render(<SessionNodeItem node={node} depth={0} currentId={undefined} now={60_000} onOpen={vi.fn()}
-        onRename={vi.fn()} onToggle={vi.fn()} />)
+      render(<SessionNodeItem node={node} currentId={undefined} now={60_000} onOpen={vi.fn()}
+        onRename={vi.fn()} onFork={vi.fn()} />)
       const wrapper = screen.getByRole('treeitem').parentElement as HTMLElement
       fireEvent.pointerEnter(wrapper)
       act(() => { vi.advanceTimersByTime(500) })
@@ -226,11 +192,10 @@ describe('workspace browser rows', () => {
     vi.useFakeTimers()
     try {
       const node: SessionNode = {
-        id: sid('s1'), title: 'Quiet', children: [], hasChildren: false,
-        expanded: false, running: false, updatedAt: 0,
+        id: sid('s1'), title: 'Quiet', running: false, updatedAt: 0,
       }
-      render(<SessionNodeItem node={node} depth={0} currentId={undefined} now={0} onOpen={vi.fn()}
-        onRename={vi.fn()} onToggle={vi.fn()} />)
+      render(<SessionNodeItem node={node} currentId={undefined} now={0} onOpen={vi.fn()}
+        onRename={vi.fn()} onFork={vi.fn()} />)
       fireEvent.pointerEnter(screen.getByRole('treeitem').parentElement as HTMLElement)
       act(() => { vi.advanceTimersByTime(500) })
       expect(screen.getByText('Idle')).toBeTruthy()
@@ -242,13 +207,12 @@ describe('workspace browser rows', () => {
 
   it('draggable row wires start/end and gates hover/drop on an active same-group drag', () => {
     const node: SessionNode = {
-      id: sid('s1'), title: 'Drag me', children: [], hasChildren: false,
-      expanded: false, running: false, updatedAt: 0,
+      id: sid('s1'), title: 'Drag me', running: false, updatedAt: 0,
     }
     const inactive = dragProps()
     const { rerender } = render(
-      <SessionNodeItem node={node} depth={0} currentId={undefined} now={0} onOpen={vi.fn()}
-        onRename={vi.fn()} onToggle={vi.fn()} drag={inactive} />,
+      <SessionNodeItem node={node} currentId={undefined} now={0} onOpen={vi.fn()}
+        onRename={vi.fn()} onFork={vi.fn()} drag={inactive} />,
     )
     const row = screen.getByRole('treeitem')
     stubRect(row)
@@ -265,8 +229,8 @@ describe('workspace browser rows', () => {
 
     const active = dragProps({ active: true, marker: 'before' })
     rerender(
-      <SessionNodeItem node={node} depth={0} currentId={undefined} now={0} onOpen={vi.fn()}
-        onRename={vi.fn()} onToggle={vi.fn()} drag={active} />,
+      <SessionNodeItem node={node} currentId={undefined} now={0} onOpen={vi.fn()}
+        onRename={vi.fn()} onFork={vi.fn()} drag={active} />,
     )
     stubRect(screen.getByRole('treeitem'))
     // Top half hovers/drops 'before'; bottom half 'after' (row mid = 117).
@@ -279,8 +243,8 @@ describe('workspace browser rows', () => {
 
     const after = dragProps({ active: true, marker: 'after' })
     rerender(
-      <SessionNodeItem node={node} depth={0} currentId={undefined} now={0} onOpen={vi.fn()}
-        onRename={vi.fn()} onToggle={vi.fn()} drag={after} />,
+      <SessionNodeItem node={node} currentId={undefined} now={0} onOpen={vi.fn()}
+        onRename={vi.fn()} onFork={vi.fn()} drag={after} />,
     )
     expect(screen.getByRole('treeitem').className).toMatch(/dropAfter/)
   })

packages/client/ui-workspace/tests/tree.spec.ts

@@ -21,7 +21,7 @@ const workspace = (id: string, sessionIds: string[]): WorkspaceView => ({
   sessionIds: sessionIds.map(sid), createdAt: '2026-01-01T00:00:00.000Z', updatedAt: '2026-01-01T00:00:00.000Z',
 })
 const view = (expandedProjects: readonly string[] = [], query = '') => ({
-  expandedProjects, expandedSessions: [] as string[], query,
+  expandedProjects, query,
 })
 
 describe('deriveGroups', () => {
@@ -74,7 +74,7 @@ describe('deriveGroups', () => {
     expect(groups[0]!.sessionCount).toBe(1)
   })
 
-  it('builds, sorts, expands, and cycle-guards an ungrouped session tree', () => {
+  it('ignores fork lineage and sorts every ungrouped session as a top-level row', () => {
     const parent = summary('parent', 1)
     const oldChild = { ...summary('old-child', 10), parentId: parent.id }
     const newChild = { ...summary('new-child', 20), parentId: parent.id }
@@ -87,15 +87,13 @@ describe('deriveGroups', () => {
     const groups = deriveGroups(
       list(parent, oldChild, newChild, tieB, tieA, self, orphan, cycleA, cycleB),
       [],
-      { expandedProjects: [UNGROUPED_KEY], expandedSessions: [parent.id, cycleA.id, cycleB.id], query: '' },
+      { expandedProjects: [UNGROUPED_KEY], query: '' },
     )
 
     expect(groups).toHaveLength(1)
     expect(groups[0]!.sessions.map(node => node.id)).toEqual([
-      sid('orphan'), sid('self'), parent.id, sid('cycle-a'),
-    ])
-    expect(groups[0]!.sessions[2]!.children.map(node => node.id)).toEqual([
       newChild.id, tieA.id, tieB.id, oldChild.id,
+      cycleB.id, cycleA.id, orphan.id, self.id, parent.id,
     ])
 
     // Equal timestamps use ids as a deterministic tiebreak in either input order.
@@ -113,7 +111,7 @@ describe('deriveGroups', () => {
     expect(groups[0]!.sessions.map(node => node.id)).toEqual([sid('present')])
   })
 
-  it('searches descendants with ancestors and handles cycles, self parents, and label-only hits', () => {
+  it('searches rows independently of lineage and keeps label-only hits', () => {
     const root = { ...summary('root', 1), displayTitle: 'Ancestor' }
     const match = { ...summary('match', 2), displayTitle: 'Needle child', parentId: root.id }
     const sibling = { ...summary('sibling', 3), displayTitle: 'Other child', parentId: root.id }
@@ -124,8 +122,8 @@ describe('deriveGroups', () => {
     const sessions = list(root, match, sibling, self, orphan, cycleA, cycleB)
     const groups = deriveGroups(sessions, [workspace('project', sessions.ids)], view([], 'needle'))
 
-    expect(groups[0]!.sessions.flatMap(node => [node.id, ...node.children.map(child => child.id)])).toEqual([
-      root.id, match.id, self.id, orphan.id, cycleA.id, cycleB.id,
+    expect(groups[0]!.sessions.map(node => node.id)).toEqual([
+      match.id, self.id, orphan.id, cycleA.id, cycleB.id,
     ])
 
     const labelOnly = deriveGroups(
@@ -157,8 +155,6 @@ describe('deriveFlat', () => {
     const tieA = summary('tie-a', 20)
     const rows = deriveFlat(list(parent, child, tieB, tieA), { query: '' })
     expect(rows.map(row => row.id)).toEqual([sid('child'), sid('tie-a'), sid('tie-b'), sid('parent')])
-    // Rows are branch-free: no children, no expansion.
-    expect(rows.every(row => row.children.length === 0 && !row.hasChildren && !row.expanded)).toBe(true)
   })
 
   it('search filters by case-insensitive display-title substring', () => {

packages/client/ui-workspace/tests/workspace-browser.spec.tsx

@@ -56,6 +56,7 @@ function mount(overrides: Partial<WorkspaceBrowserProps> = {}) {
     startSession: vi.fn(),
     open: vi.fn(),
     renameSession: vi.fn(async () => {}),
+    forkSession: vi.fn(),
     renameWorkspace: vi.fn(async () => {}),
     deleteWorkspace: vi.fn(async () => {}),
     insertSessionBefore: vi.fn(async () => {}),
@@ -124,7 +125,7 @@ describe('WorkspaceBrowser', () => {
     expect(screen.queryByText('alpha-s')).toBeNull()
   })
 
-  it('unfolds a session subtree through the row twist', () => {
+  it('renders a fork child as a top-level row without a session twist', () => {
     const parent = summary('parent-s', 2)
     const child = { ...summary('child-s', 1), parentId: parent.id }
     mount({
@@ -132,11 +133,9 @@ describe('WorkspaceBrowser', () => {
       useWorkspaces: hook(workspaceState([workspace('alpha', ['parent-s', 'child-s'])])),
     })
     fireEvent.click(screen.getByText('alpha'))
-    expect(screen.queryByText('child-s')).toBeNull()
-    fireEvent.click(screen.getByRole('button', { name: 'Expand' }))
     expect(screen.getByText('child-s')).toBeTruthy()
-    fireEvent.click(screen.getByRole('button', { name: 'Collapse' }))
-    expect(screen.queryByText('child-s')).toBeNull()
+    expect(screen.queryByRole('button', { name: /Expand|Collapse/ })).toBeNull()
+    expect(screen.getByText('child-s').closest('[role="treeitem"]')?.getAttribute('draggable')).toBe('true')
   })
 
   it('auto-expands the selected session group and starts a session from the group ＋', () => {

packages/host/apiproxy/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/host/apiproxy/README.md
-README.md: 8f9deb6add7d30bf1609cc7febcb1febafe392c1
-README.zh.md: 399d45208b6d3f4152c27556523b6944432bec66
+README.md: 3ec21f90a495fe42e40c4407e0a81faa34a1e427
+README.zh.md: 5bcd310c23c35b851216176d69b13965dd2e1c3e

packages/host/apiproxy/README.md

@@ -16,6 +16,8 @@ The layering/protocol decisions are recorded in the [GUI layering and RPC protoc
 
 Session titles ride the generic projection pair like every other domain — the history-tail `projections` block plus `session/projection` frames under the `title` key (the bespoke `session/title` frame is retired). Titles do not join `session.list`; cold sessions remain metadata-only there until opening or resuming attaches their logs. `session.rename` accepts an explicit user title (resuming a cold session first), delegating to `ctx.sessionTitle.rename` — the accepted `session/title` event pins the title against automatic regeneration — and returns the normalized title plus its event seq so a client settles its `title` projection cell ahead of the push frame; a title that normalizes to empty returns `title-invalid`.
 
+`session.fork` maps an optional event anchor to the first `turn/end` at or after it, letting a message action include that message's whole turn. An omitted or past-end anchor selects the last completed turn; an in-log anchor whose turn remains open returns `fork-unavailable` rather than clipping backward. The published child inherits the source's seeded history, cwd, latest logged provider/model/reasoning target, and lineage before joining the source Workspace. If Workspace attachment fails, `workspace-attach-failed` carries the already-published child id so clients can reconcile it. The [SessionStore fork decision](../../../.agents/notes/implemented/feature/2026-06-30-session-store-fork-api.md) owns the boundary rationale.
+
 Session model routing is a session-domain contract. `session.models` returns the selected provider/model/reasoning target with provider-grouped advisory models, exact-route reasoning metadata, and provider-local lookup failures. `session.selectModel` validates the optional adapter-owned reasoning effort and replaces the complete target selected for the next prompt-assembly boundary. Catalog membership is not validation: an adapter may resolve an unlisted model, while an unavailable route or unsupported effort returns `model-unavailable`.
 
 Pending queued input is a live control-plane contract, not session history. The gateway mirrors queued `InboxItem` occurrences from `agent/inbox/*` and broadcasts authoritative `session/queue` snapshots on every queued change and reconnect; pending steering stays outside this Web projection. `session.updateQueue` addresses one `InboxItemId`: edit replaces pending content and remove discards it. A driver claim wins races by retiring the address before admission; a later operation returns `queue-item-not-found`. The operation queries only an attached Agent and never resumes a cold session because process-local inbox identities do not survive restart or disposal. The client never infers retirement from turn or status events.
@@ -43,7 +45,7 @@ None; this package neither assembles nor sends a provider request.
 ## Known Limitations and Deferred Work
 
 - **`respond` routing is shipped, but pending-interaction state is host-side work** — the wire shape (POST `/api/respond`, `RpcReceipt`) is final; the pending table that makes late/duplicate answers meaningful lives in `src/api-proxy.ts` and is still minimal (questions only, no approvals).
-- **Reserved seams stay out of `RpcMethodMap`** — `session.fork`, `prompt.mode: 'inject'`, `task.list`, `host.listModels`, and a describe `hostInstanceId` are documented reservations; an unknown method fails loud at envelope parse rather than getting a not-implemented code.
+- **Reserved seams stay out of `RpcMethodMap`** — `prompt.mode: 'inject'`, `task.list`, `host.listModels`, and a describe `hostInstanceId` are documented reservations; an unknown method fails loud at envelope parse rather than getting a not-implemented code.
 - **No protocol version field** — client and host ship together; `host.describe` gains a version negotiation field only when an independently released client exists.
 - **Linux native picker requires desktop tooling** — under the `native` capability, `host.pickDirectory` reports an actionable error when neither Zenity nor KDialog is installed; the browse backend is the composition-level fallback (see the [native backend README](../directory-picker-native/README.md)).
 - **A cold session's `updatedAt` counts a mere pickup as a write (per-file backends only)** — the attached projection excludes the `session/end-seed` boundary, because picking a session up is not activity, but a cold session's `updatedAt` is its log file's mtime and every durable write refreshes that, the boundary included. `agentFor()` resumes a cold session on first touch, so merely opening one in a client writes it. This applies only where `locate()` resolves a per-session artifact, i.e. JSONL; SQLite returns `undefined`, so its cold sessions fall back to `createdAt` and are skewed the other way — too old rather than too new — independently of this boundary. A session touched without being worked in therefore sorts newer than its last real activity until it attaches. Separating the two needs a log read, which is exactly what the mtime path exists to avoid; a stored last-activity field in the index would fix it at the source, scoped in the [last-activity-index Agent Note](../../../.agents/notes/proposed/architecture/2026-07-29-durable-last-activity-index.md).

packages/host/apiproxy/README.zh.md

@@ -16,7 +16,9 @@
 
 会话标题与其他所有领域一样搭乘这对通用投影机制——历史尾页的 `projections` 块外加 `title` 键下的 `session/projection` 帧（专设的 `session/title` 帧已下线）。标题不会加入 `session.list`；冷会话在其中仍只有元数据，直到打开或恢复操作附加其日志。`session.rename` 接受用户显式标题（冷会话先恢复），委托给 `ctx.sessionTitle.rename`——被接受的 `session/title` 事件将标题钉住、不再被自动生成覆盖——并返回规范化后的标题及其事件 seq，让 client 在推送帧到达前就结算自己的 `title` 投影格；规范化后为空的标题返回 `title-invalid`。
 
-会话模型路由属于会话领域契约。`session.models` 返回选中的提供方／模型／推理（reasoning）目标，以及按提供方分组的建议性模型、精确路由推理元数据和逐提供方查询失败记录。`session.selectModel` 校验由适配器持有的可选推理强度，并替换将在下一提示词组装边界使用的完整目标。目录成员关系不构成校验：适配器可以解析未列出的模型，而不可用路由或不受支持的推理强度会返回 `model-unavailable`。
+`session.fork` 将可选事件锚点映射到该锚点处或其后的首个 `turn/end`，使消息操作可包含该消息所在的完整轮次。锚点省略或超过末尾时，选择最后一个已完成轮次；若锚点已在日志中，而其所在轮次仍开放，则返回 `fork-unavailable`，不会向较早位置裁剪。发布后的子会话会先继承源会话的种子历史、cwd、日志中最新的提供方／模型／推理（reasoning）目标及谱系，再加入源 Workspace。如果附加到 Workspace 失败，`workspace-attach-failed` 会携带已发布的子会话 id，供客户端对账。[SessionStore fork 决策](../../../.agents/notes/implemented/feature/2026-06-30-session-store-fork-api.md)给出边界设计的理由。
+
+会话模型路由属于会话领域契约。`session.models` 返回选中的提供方／模型／推理目标，以及按提供方分组的建议性模型、精确路由推理元数据和逐提供方查询失败记录。`session.selectModel` 校验由适配器持有的可选推理强度，并替换将在下一提示词组装边界使用的完整目标。目录成员关系不构成校验：适配器可以解析未列出的模型，而不可用路由或不受支持的推理强度会返回 `model-unavailable`。
 
 待处理的 queued 输入属于实时控制平面契约，而非会话历史。网关镜像来自 `agent/inbox/*` 的 queued `InboxItem` 入队项，并在每次 queued 变更和重连时广播权威的 `session/queue` 快照；待处理 steering（中途引导）不进入此 Web 投影。`session.updateQueue` 通过 `InboxItemId` 寻址单个项：编辑会替换待处理内容，移除会将其丢弃。驱动器在接纳前退役寻址标识，因此认领会赢得竞态；之后的操作返回 `queue-item-not-found`。该操作只查询当前已挂载的 Agent，绝不恢复冷会话，因为进程本地 inbox 标识无法在重启或资源释放后存活。客户端绝不根据轮次或状态事件推断项已退役。
 
@@ -43,7 +45,7 @@ Workspace 列表与 Session 列表是相互独立的重连基线。`workspace.cr
 ## 已知限制与延期工作
 
 - **`respond` 路由已经发布，但待处理交互状态仍属宿主侧工作**：协议形状（POST `/api/respond`、`RpcReceipt`）已经定型；使延迟或重复回答具有明确语义的待处理表位于 `src/api-proxy.ts`，目前仍很精简（只支持问题，不支持审批）。
-- **预留 seam 不进入 `RpcMethodMap`**：`session.fork`、`prompt.mode: 'inject'`、`task.list`、`host.listModels` 和描述字段 `hostInstanceId` 都是已记录的预留项；未知方法会在信封解析时直接失败，而不会返回「尚未实现」错误码。
+- **预留 seam 不进入 `RpcMethodMap`**：`prompt.mode: 'inject'`、`task.list`、`host.listModels` 和描述字段 `hostInstanceId` 都是已记录的预留项；未知方法会在信封解析时直接失败，而不会返回「尚未实现」错误码。
 - **没有协议版本字段**：客户端与宿主一同发布；只有出现独立发布的客户端后，`host.describe` 才会增加版本协商字段。
 - **Linux 原生选择器依赖桌面工具**：在 `native` 能力下，Zenity 和 KDialog 均未安装时，`host.pickDirectory` 会给出包含解决建议的错误提示；组合层面的回退是 browse 后端（见 [native 后端 README](../directory-picker-native/README.md)）。
-- **冷会话的 `updatedAt` 会把一次单纯的拾起算作写入（仅逐文件后端）**：已附加投影排除了 `session/end-seed` 边界，因为接手一个会话不算活动；但冷会话的 `updatedAt` 取自其日志文件的 mtime，而每一次持久写入都会刷新它，包括这条边界。`agentFor()` 会在首次触碰时恢复一个冷会话，因此在客户端里仅仅打开一个会话就会写入它。这只适用于 `locate()` 能解析出逐会话产物的场景，即 JSONL；SQLite 返回 `undefined`，因此它的冷会话回退到 `createdAt`，偏差方向相反——偏旧而不是偏新——且与这条边界无关。于是一个被触碰过却没有在里面工作过的会话，在重新附加之前会排在它最后一次真实活动之后。要把两者区分开需要读取日志，而这恰恰是 mtime 路径存在的目的；在索引中存储一个最后活动字段可以从源头修好它，范围见[最后活动索引 Agent Note](../../../.agents/notes/proposed/architecture/2026-07-29-durable-last-activity-index.md)。
+- **冷会话的 `updatedAt` 会把一次单纯的拾起算作写入（仅逐文件后端）**：已附加投影排除了 `session/end-seed` 边界，因为接手一个会话不算活动；但冷会话的 `updatedAt` 取自其日志文件的 mtime，而每一次持久写入都会刷新它，包括这条边界。`agentFor()` 会在首次触碰时恢复一个冷会话，因此在客户端里仅仅打开一个会话就会写入它。这只适用于 `locate()` 能解析出逐会话产物的场景，即 JSONL；SQLite 返回 `undefined`，因此它的冷会话回退到 `createdAt`，偏差方向相反——偏旧而不是偏新——且与这条边界无关。于是一个被触碰过却没有在里面工作过的会话，在重新附加之前会按晚于其最后一次真实活动的时间排序。要把两者区分开需要读取日志，而这恰恰是 mtime 路径存在的目的；在索引中存储一个最后活动字段可以从源头修好它，范围见[最后活动索引 Agent Note（agent 决策记录）](../../../.agents/notes/proposed/architecture/2026-07-29-durable-last-activity-index.md)。

packages/host/apiproxy/src/api-proxy.ts

@@ -1148,6 +1148,75 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
         }
       },
 
+      async fork(request) {
+        const { sessionId, atSeq } = request.payload
+        const found = await agentFor(sessionId)
+        if ('error' in found) return err(request, found.error)
+        const source = found.agent.session
+        const events = source.events
+        // An in-log anchor belongs to the turn containing it and must never
+        // clip backward to an earlier completed turn. Omitted and past-end
+        // anchors retain the last-completed-turn shortcut.
+        const lastSeq = events.at(-1)?.seq ?? -1
+        const anchoredBoundary = atSeq === undefined
+          ? undefined
+          : events.find(e => e.type === 'turn/end' && e.seq >= atSeq)
+        const boundary = anchoredBoundary
+          ?? (atSeq === undefined || atSeq > lastSeq
+            ? events.findLast(e => e.type === 'turn/end')
+            : undefined)
+        if (boundary === undefined) {
+          return err(request, {
+            code: 'fork-unavailable',
+            message: atSeq !== undefined && atSeq <= lastSeq
+              ? `session "${sessionId}" has not completed the turn containing event ${String(atSeq)}`
+              : `session "${sessionId}" has no completed turn to fork from`,
+            details: { sessionId },
+          })
+        }
+        // Extend the cut through trailing out-of-band appends (session/title,
+        // injections) up to the next turn/start: they are standalone events, so
+        // the seed stays balanced, and the child inherits a title generated
+        // right after the boundary turn.
+        let cut = boundary.seq + 1
+        while (cut < events.length && events[cut]?.type !== 'turn/start') cut++
+        const childId = `session-${randomUUID()}` as SessionId
+        try {
+          await ctx.agents.create({
+            sessionId: childId,
+            seed: events.slice(0, cut),
+            meta: {
+              ...source.header.cwd === undefined ? {} : { cwd: source.header.cwd },
+              parentSession: source.id,
+              seedLength: cut,
+            },
+            agentOptions,
+            setup: installTarget,
+          })
+        } catch (error: unknown) {
+          return err(request, {
+            code: 'internal',
+            message: `failed to fork session "${sessionId}": ${String(error)}`,
+            details: {},
+          })
+        }
+        // Keep the child in the source's Workspace so the list nests it under
+        // its parent; the child is already published if the attach fails.
+        const workspace = ctx.workspace.list().find(w => w.sessionIds.includes(source.id))
+        if (workspace !== undefined) {
+          try {
+            await workspace.attachSession(childId)
+          } catch (error: unknown) {
+            return err(request, {
+              code: 'workspace-attach-failed',
+              message: `session "${childId}" was forked but could not attach to workspace "${workspace.id}": ${String(error)}`,
+              details: { sessionId: childId, workspaceId: workspace.id },
+            })
+          }
+        }
+        return ok(request, { sessionId: childId })
+      },
+
       async prompt(request) {
         const { sessionId, mode, content } = request.payload
         const found = await agentFor(sessionId)

packages/host/apiproxy/src/api/rpc-map.ts

@@ -24,6 +24,7 @@ export interface RpcMethodMap {
   'session.models': SessionsApi['models']
   'session.selectModel': SessionsApi['selectModel']
   'session.rename': SessionsApi['rename']
+  'session.fork': SessionsApi['fork']
   'session.prompt': SessionsApi['prompt']
   'session.updateQueue': SessionsApi['updateQueue']
   'session.cancel': SessionsApi['cancel']

packages/host/apiproxy/src/api/rpc.schema.ts

@@ -51,6 +51,7 @@ export const rpcErrorSchema: z.ZodType<RpcError> = z.discriminatedUnion('code',
   z.object({ code: z.literal('command-error'), message: z.string(), details: z.object({}) }),
   z.object({ code: z.literal('unknown-command'), message: z.string(), details: z.object({}) }),
   z.object({ code: z.literal('title-invalid'), message: z.string(), details: z.object({ sessionId: z.string() }) }),
+  z.object({ code: z.literal('fork-unavailable'), message: z.string(), details: z.object({ sessionId: z.string() }) }),
   z.object({ code: z.literal('internal'), message: z.string(), details: z.object({}) }),
 ]) as unknown as z.ZodType<RpcError>
 

packages/host/apiproxy/src/api/rpc.ts

@@ -51,6 +51,7 @@ export interface RpcErrorDetailsMap {
   /** A leading-/ prompt named no registered command; the message names the token. */
   'unknown-command': {}
   'title-invalid': { sessionId: SessionId }
+  'fork-unavailable': { sessionId: SessionId }
   'internal': {}
 }
 

packages/host/apiproxy/src/api/sessions.schema.ts

@@ -89,6 +89,17 @@ export const sessionRenameValueSchema = z.object({
   seq: z.number().int().nonnegative(),
 }) satisfies z.ZodType<Wire<ResponseValue<'session.rename'>>>
 
+/** session.fork request payload (atSeq anchors the completed-turn cut). */
+export const sessionForkRequestSchema = z.object({
+  sessionId: sessionIdSchema,
+  atSeq: z.number().int().nonnegative().optional(),
+}) satisfies z.ZodType<Wire<RequestPayload<'session.fork'>>>
+
+/** session.fork response value (the child session id). */
+export const sessionForkValueSchema = z.object({
+  sessionId: sessionIdSchema,
+}) satisfies z.ZodType<Wire<ResponseValue<'session.fork'>>>
+
 /** session.history request payload (beforeSeq/maxMessages page backwards from the window tail). */
 export const sessionHistoryRequestSchema = z.object({
   sessionId: sessionIdSchema,

packages/host/apiproxy/src/api/sessions.ts

@@ -238,6 +238,21 @@ export interface SessionsApi {
    * one — carried for future rendering; the state change is the feedback). A usage/state error is an
    * RPC error with code command-error; an unrecognized name is an RPC error with code unknown-command.
    */
+  /**
+   * Forks a new session from a completed-turn prefix of the source. `atSeq`
+   * anchors the cut: the boundary is the first `turn/end` at or after it
+   * (a message's fork button passes the message seq, so the fork includes
+   * that whole turn); a boundary past the log end, or an omitted `atSeq`,
+   * falls back to the source's last completed turn. An in-log anchor whose
+   * turn is still open fails with `fork-unavailable` instead of clipping to
+   * an earlier turn. The child inherits the source cwd, latest logged model
+   * target, workspace attachment, and `parentSessionId` lineage; the seed
+   * prefix carries the source title.
+   */
+  fork(request: RpcRequest<{ sessionId: SessionId; atSeq?: number }>):
+  Promise<RpcResponse<{ sessionId: SessionId }>>
+
+  /** Sends a message. content is core's ContentBlock[] verbatim; mode maps 1:1 — queue→send, steer→steer. */
   prompt(request: RpcRequest<{ sessionId: SessionId; mode: 'queue' | 'steer'; content: ContentBlock[] }>):
   Promise<RpcResponse<{ accepted: true; command?: { kind: 'success'; text?: string } }>>
 

packages/host/apiproxy/src/fetch/client.ts

@@ -20,6 +20,7 @@ import {
 import {
   sessionCancelValueSchema,
   sessionCreateValueSchema,
+  sessionForkValueSchema,
   sessionHistoryValueSchema,
   sessionListValueSchema,
   sessionModelsValueSchema,
@@ -69,6 +70,7 @@ export interface IApiClient {
     models(payload: RequestPayload<'session.models'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'session.models'>>>
     selectModel(payload: RequestPayload<'session.selectModel'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'session.selectModel'>>>
     rename(payload: RequestPayload<'session.rename'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'session.rename'>>>
+    fork(payload: RequestPayload<'session.fork'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'session.fork'>>>
     prompt(payload: RequestPayload<'session.prompt'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'session.prompt'>>>
     updateQueue(payload: RequestPayload<'session.updateQueue'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'session.updateQueue'>>>
     cancel(payload: RequestPayload<'session.cancel'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'session.cancel'>>>
@@ -121,6 +123,7 @@ const UNARY_VALUE_SCHEMAS: { [K in keyof RpcMethodMap]: z.ZodType<Wire<ResponseV
   'session.models': sessionModelsValueSchema,
   'session.selectModel': sessionSelectModelValueSchema,
   'session.rename': sessionRenameValueSchema,
+  'session.fork': sessionForkValueSchema,
   'session.prompt': sessionPromptValueSchema,
   'session.updateQueue': sessionUpdateQueueValueSchema,
   'session.cancel': sessionCancelValueSchema,
@@ -334,6 +337,7 @@ export abstract class AbstractApiClient implements IApiClient {
     models: (payload, signal) => this.callUnary('session.models', payload, signal),
     selectModel: (payload, signal) => this.callUnary('session.selectModel', payload, signal),
     rename: (payload, signal) => this.callUnary('session.rename', payload, signal),
+    fork: (payload, signal) => this.callUnary('session.fork', payload, signal),
     prompt: (payload, signal) => this.callUnary('session.prompt', payload, signal),
     updateQueue: (payload, signal) => this.callUnary('session.updateQueue', payload, signal),
     cancel: (payload, signal) => this.callUnary('session.cancel', payload, signal),

packages/host/apiproxy/src/fetch/handler.ts

@@ -17,6 +17,7 @@ import { clientRequestSchema, clientResponseSchema } from '../api/rpc.schema.ts'
 import {
   sessionCancelRequestSchema,
   sessionCreateRequestSchema,
+  sessionForkRequestSchema,
   sessionHistoryRequestSchema,
   sessionListRequestSchema,
   sessionModelsRequestSchema,
@@ -71,6 +72,7 @@ const UNARY_ROUTES: UnaryRoutes = {
   'session.models': { schema: sessionModelsRequestSchema, invoke: (api, r) => api.sessions.models(r) },
   'session.selectModel': { schema: sessionSelectModelRequestSchema, invoke: (api, r) => api.sessions.selectModel(r) },
   'session.rename': { schema: sessionRenameRequestSchema, invoke: (api, r) => api.sessions.rename(r) },
+  'session.fork': { schema: sessionForkRequestSchema, invoke: (api, r) => api.sessions.fork(r) },
   'session.prompt': { schema: sessionPromptRequestSchema, invoke: (api, r) => api.sessions.prompt(r) },
   'session.updateQueue': { schema: sessionUpdateQueueRequestSchema, invoke: (api, r) => api.sessions.updateQueue(r) },
   'session.cancel': { schema: sessionCancelRequestSchema, invoke: (api, r) => api.sessions.cancel(r) },

packages/host/apiproxy/tests/api-proxy-fork.spec.ts

@@ -0,0 +1,163 @@
+/** Session-fork boundaries, lineage, and inherited model routing. */
+
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import AgentRegistry, { agentEvents } from '@deepseek-ai/dsh-agent'
+import type { Agent, AgentHandle, CreateAgentOptions } from '@deepseek-ai/dsh-agent'
+import { createUserMessage, ReasoningEffortId } from '@deepseek-ai/dsh-llm'
+import type { LlmCallConfig } from '@deepseek-ai/dsh-llm'
+import SessionStore from '@deepseek-ai/dsh-session'
+import type { Session, SessionId } from '@deepseek-ai/dsh-session'
+import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
+import UserInteractionService from '@deepseek-ai/dsh-user-interaction'
+import type { RpcRequest } from '@deepseek-ai/dsh-host-apiproxy/api/rpc'
+import { RpcId } from '@deepseek-ai/dsh-host-apiproxy/api/rpc'
+import { createApiProxy } from '@deepseek-ai/dsh-host-apiproxy'
+
+const sid = (id: string): SessionId => id as SessionId
+
+let nextRpc = 1
+function request<P>(payload: P): RpcRequest<P> {
+  return { rpcId: RpcId(`fork-${String(nextRpc++)}`), payload }
+}
+
+async function composed(): Promise<Context> {
+  const ctx = new Context()
+  await ctx.plugin(SessionStore)
+  await ctx.plugin(SystemPrompt, { persona: '' })
+  await ctx.plugin(AgentRegistry)
+  await ctx.plugin(UserInteractionService)
+  ctx.provide('workspace', { list: () => [] } as never)
+  ctx.agents.setFactory({
+    createAgent: async (ownerCtx: Context, options: CreateAgentOptions): Promise<AgentHandle> => {
+      const session = ctx.sessions.create(options.sessionId, {
+        ...options.seed === undefined ? {} : { seed: [...options.seed] },
+        ...options.meta === undefined ? {} : { meta: options.meta },
+      })
+      const agent = {} as Agent
+      const agentCtx = ownerCtx.extend({ agent })
+      Object.assign(agent, { id: session.id, session, status: 'idle', ctx: agentCtx })
+      await options.setup?.(agentCtx)
+      ctx.agents.register(agent)
+      return { agent, dispose: () => Promise.resolve() }
+    },
+    resume: () => Promise.reject(new Error('fork test sources are live')),
+  })
+  return ctx
+}
+
+function liveAgent(ctx: Context, id: string, turns: number, openTail = false): Session {
+  const session = ctx.sessions.create(sid(id), { meta: { cwd: '/proj' } })
+  for (let turn = 1; turn <= turns; turn++) {
+    session.append('turn/start', { turn, trigger: { kind: 'message', source: { kind: 'user' } } })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: `prompt ${String(turn)}` }],
+      source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    session.append('turn/end', { turn, reason: { kind: 'completed' } })
+  }
+  if (openTail) {
+    session.append('turn/start', { turn: turns + 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'open prompt' }],
+      source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+  }
+  ctx.agents.register({ id: session.id, session, status: 'idle', ctx } as Agent)
+  return session
+}
+
+const api = (ctx: Context) => createApiProxy(ctx, {
+  provider: 'default-provider',
+  model: 'default-model',
+  cwd: '/tmp',
+  workspaceRoot: '/tmp',
+})
+
+describe('sessions.fork', () => {
+  it('cuts at the anchored completed turn and records lineage and cwd', async () => {
+    const ctx = await composed()
+    const source = liveAgent(ctx, 'session-source', 2)
+    const response = await api(ctx).sessions.fork(request({ sessionId: source.id, atSeq: 1 }))
+    expect(response.result.ok).toBe(true)
+    if (!response.result.ok) return
+    const child = ctx.sessions.get(response.result.value.sessionId)
+    expect(child?.events.map(event => event.type)).toEqual([
+      'turn/start', 'user/message', 'turn/end', 'session/end-seed',
+    ])
+    expect(child?.header.parentSession).toBe(source.id)
+    expect(child?.header.cwd).toBe('/proj')
+    await ctx.fiber.dispose()
+  })
+
+  it('uses the last completed turn only for omitted and past-end anchors', async () => {
+    const ctx = await composed()
+    const source = liveAgent(ctx, 'session-tail', 2, true)
+    const proxy = api(ctx)
+    const expectedTypes = [
+      'turn/start', 'user/message', 'turn/end',
+      'turn/start', 'user/message', 'turn/end',
+      'session/end-seed',
+    ]
+    const omitted = await proxy.sessions.fork(request({ sessionId: source.id }))
+    expect(omitted.result.ok).toBe(true)
+    if (omitted.result.ok) {
+      expect(ctx.sessions.get(omitted.result.value.sessionId)?.events.map(event => event.type))
+        .toEqual(expectedTypes)
+    }
+    const pastEnd = await proxy.sessions.fork(request({ sessionId: source.id, atSeq: 999 }))
+    expect(pastEnd.result.ok).toBe(true)
+    if (pastEnd.result.ok) {
+      expect(ctx.sessions.get(pastEnd.result.value.sessionId)?.events.map(event => event.type))
+        .toEqual(expectedTypes)
+    }
+    await ctx.fiber.dispose()
+  })
+
+  it('rejects an in-log anchor whose turn is still open', async () => {
+    const ctx = await composed()
+    const source = liveAgent(ctx, 'session-open', 1, true)
+    const anchor = source.events.at(-1)?.seq ?? 0
+    const response = await api(ctx).sessions.fork(request({ sessionId: source.id, atSeq: anchor }))
+    expect(response.result).toMatchObject({
+      ok: false,
+      error: { code: 'fork-unavailable', details: { sessionId: source.id } },
+    })
+    if (!response.result.ok) expect(response.result.error.message).toMatch(/has not completed/)
+    await ctx.fiber.dispose()
+  })
+
+  it('installs the latest logged model target before the child can run', async () => {
+    const ctx = await composed()
+    const source = liveAgent(ctx, 'session-routed', 1)
+    source.append('request/header', {
+      header: {
+        config: {
+          provider: 'inherited-provider',
+          model: 'inherited-model',
+          reasoningEffort: ReasoningEffortId('high'),
+        },
+      },
+      reason: 'initial',
+    })
+    const response = await api(ctx).sessions.fork(request({ sessionId: source.id }))
+    expect(response.result.ok).toBe(true)
+    if (!response.result.ok) return
+    const child = ctx.agents.get(response.result.value.sessionId)
+    if (child === undefined) throw new Error('fork did not publish the child agent')
+    const assembly = await child.ctx.systemPrompt.assemble()
+    expect(assembly.variables).toMatchObject({
+      provider: 'inherited-provider',
+      model: 'inherited-model',
+    })
+    const fallback: LlmCallConfig = { provider: 'default-provider', model: 'default-model' }
+    await expect(agentEvents(child.ctx, child).waterfall(
+      'agent/request', 1, 0, new AbortController().signal, () => Promise.resolve(fallback),
+    )).resolves.toMatchObject({
+      provider: 'inherited-provider',
+      model: 'inherited-model',
+      reasoningEffort: 'high',
+    })
+    await ctx.fiber.dispose()
+  })
+})

packages/host/apiproxy/tests/client-handler.spec.ts

@@ -47,6 +47,7 @@ function scriptedApi(overrides: {
         selected: { provider: r.payload.provider, model: r.payload.model },
       }),
       rename: r => ok(r, { title: 'renamed', seq: 0 }),
+      fork: r => ok(r, { sessionId: sid('s-fork') }),
       prompt: r => ok(r, { accepted: true as const }),
       updateQueue: r => ok(r, { accepted: true as const }),
       cancel: r => ok(r, { accepted: true as const }),
@@ -110,6 +111,21 @@ describe('unary round trip', () => {
     expect(response.result).toEqual({ ok: true, value: { items: [{ sessionId: 's1', updatedAt: 7, running: false, blank: false }] } })
   })
 
+  it('routes session fork with its optional cut anchor through the wire', async () => {
+    let seen: RpcRequest<{ sessionId: SessionId; atSeq?: number }> | undefined
+    const api = scriptedApi({
+      sessions: {
+        fork: (request) => {
+          seen = request
+          return ok(request, { sessionId: sid('s-child') })
+        },
+      },
+    })
+    const response = await client(api).sessions.fork({ sessionId: sid('s-parent'), atSeq: 7 })
+    expect(seen?.payload).toEqual({ sessionId: 's-parent', atSeq: 7 })
+    expect(response.result).toEqual({ ok: true, value: { sessionId: 's-child' } })
+  })
+
   it('routes workspace rename, delete, and insertSessionBefore through the wire', async () => {
     const api = scriptedApi()
     const c = client(api)

packages/host/apiproxy/tests/fetch-carrier.spec.ts

@@ -70,6 +70,9 @@ function fakeApi(overrides: Partial<{ muxFrames: MuxFrame[]; hostFrames: HostFra
       async rename(request) {
         return { rpcId: request.rpcId, result: { ok: true, value: { title: request.payload.title, seq: 0 } } }
       },
+      async fork(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: { sessionId: 's-fork' as never } } }
+      },
       async prompt(request) {
         return { rpcId: request.rpcId, result: { ok: true, value: { accepted: true as const } } }
       },