fix(web): keep ineligible message forks disabled

.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.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/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md
-2026-08-02-message-fork-actions-require-completed-turn-tail.md: 38a81d9e6966b459ed8d7dd19ba0cbdd790b85b6
-2026-08-02-message-fork-actions-require-completed-turn-tail.zh.md: 09d2ecc2b110e66348b7cd7c5eb253b4c8cdbb9a
+2026-08-02-message-fork-actions-require-completed-turn-tail.md: f2e7fd67b65a6ce4a86ba3f4405f78842be8f234
+2026-08-02-message-fork-actions-require-completed-turn-tail.zh.md: 2c3feeaa3ef01dbde67faa73257520918996f9c8

.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md

@@ -10,7 +10,7 @@ The Web conversation attached branch to the last assistant node with nonempty te
 
 ## Decision
 
-`ConversationSnapshot.turnEnds` retains the completed turn boundaries present in the raw event window. The conversation view walks transcript nodes through each boundary and exposes branch only when the boundary's last node is a user message, a durable steering message, or a content-bearing assistant message. Open turns have no eligible message, and a later tool result, reasoning-only interruption, turn error, or other transcript node suppresses branch on earlier messages. Copy and clock remain available under their existing message chrome, and the Host's completed-turn fork semantics remain unchanged.
+`ConversationSnapshot.turnEnds` retains the completed turn boundaries present in the raw event window. The conversation view walks transcript nodes through each boundary and enables branch only when the boundary's last node is a user message, a durable steering message, or a content-bearing assistant message. Open turns have no eligible message, and a later tool result, reasoning-only interruption, turn error, or other transcript node leaves branch unavailable on earlier messages. The unavailable control stays visible, focusable, and hoverable; `aria-disabled`, a tooltip, and `aria-describedby` explain the completed-tail requirement without sending a Host request. Copy and clock remain available under their existing message chrome, and the Host's completed-turn fork semantics remain unchanged.
 
 This narrows the message eligibility established by the earlier [Web session fork action decision](../feature/2026-07-27-web-session-fork-actions.md). Session-row forking still selects the latest completed turn, and eligible message actions still pass their event seq through the shared client runtime operation.
 
@@ -22,6 +22,8 @@ This narrows the message eligibility established by the earlier [Web session for
 
 **Hide branch from every interrupted turn.** Rejected because an aborted turn is durably closed and its final interrupted text can be the true transcript tail. Eligibility depends on the completed boundary and node order, not the outcome kind.
 
+**Hide ineligible message controls.** Rejected because a disappearing control does not explain the boundary requirement and shifts otherwise stable message chrome. A focusable unavailable control preserves the affordance while preventing the request.
+
 ## Consequences
 
-A branch icon now denotes the same completed-turn boundary that the Host will copy. In the reported response → tool → interrupted Think shape, the response keeps copy and clock but no longer advertises branch. This change deliberately does not provide same-turn transcript editing or a retry-before-turn operation; the Session-row action remains available when a reader wants to copy the latest completed turn in full. Runtime tests pin boundary projection and reference stability, while conversation tests cover assistant, user-only, and durable-steering tails plus suppression by later tool and interrupted reasoning rows.
+An enabled branch icon denotes the same completed-turn boundary that the Host will copy. In the reported response → tool → interrupted Think shape, the response keeps copy, clock, and a disabled branch control that explains why it cannot act. This change deliberately does not provide same-turn transcript editing or a retry-before-turn operation; the Session-row action remains available when a reader wants to copy the latest completed turn in full. Runtime tests pin boundary projection and reference stability, while conversation tests cover assistant, user-only, and durable-steering tails plus unavailable controls caused by later tool and interrupted reasoning rows.

.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.zh.md

@@ -10,7 +10,7 @@ Web 会话把分支操作挂到每个轮次中最后一个文本非空的 assist
 
 ## 决策
 
-`ConversationSnapshot.turnEnds` 保留原始事件窗口中的已完成轮次边界。会话视图按各边界遍历 transcript（文本记录）节点，仅当边界的最后一个节点是用户消息、持久 steering（中途引导）消息或含内容的 assistant 消息时才暴露分支操作。开放轮次没有符合条件的消息；如果后面还有工具结果、只有推理内容的中断、轮次错误或其他 transcript 节点，较早消息上的分支操作就会被抑制。复制和时钟仍可在既有消息 chrome 下使用，Host 按已完成轮次 fork 的语义保持不变。
+`ConversationSnapshot.turnEnds` 保留原始事件窗口中的已完成轮次边界。会话视图按各边界遍历 transcript（文本记录）节点，仅当边界的最后一个节点是用户消息、持久 steering（中途引导）消息或含内容的 assistant 消息时才启用分支操作。开放轮次没有符合条件的消息；如果后面还有工具结果、只有推理内容的中断、轮次错误或其他 transcript 节点，较早消息上的分支操作会保持不可用。不可用的控件仍然可见、可聚焦、可悬停；`aria-disabled`、tooltip 与 `aria-describedby` 会说明已完成尾部这一要求，且不会发送 Host 请求。复制和时钟仍可在既有消息 chrome 下使用，Host 按已完成轮次 fork 的语义保持不变。
 
 本决策收紧了较早的 [Web 会话 fork 操作决策](../feature/2026-07-27-web-session-fork-actions.md)所定义的消息资格。Session 行 fork 仍选择最新的已完成轮次；符合条件的消息操作仍通过共享 client 运行时操作传递其事件 seq。
 
@@ -22,6 +22,8 @@ Web 会话把分支操作挂到每个轮次中最后一个文本非空的 assist
 
 **对每个被中断轮次隐藏分支。** 不予采纳：已中止的轮次会持久关闭，其最终的中断文本可能正是真正的 transcript 尾部。资格取决于已完成边界与节点顺序，而非结果类别。
 
+**隐藏不符合条件的消息控件。** 不予采纳：消失的控件无法说明边界要求，还会让本应稳定的消息 chrome 发生位移。保留可聚焦但不可用的控件，既能维持操作提示，也能阻止请求。
+
 ## 后果
 
-分支图标现在表示的已完成轮次边界与 Host 实际复制的边界一致。在所报告的「响应 → 工具 → 被中断的 Think」形态中，响应仍保留复制和时钟，但不再显示分支。本变更刻意不提供同轮次 transcript 编辑，也不提供轮次前重试操作；当读者希望完整复制最新的已完成轮次时，仍可使用 Session 行操作。运行时测试固定边界投影和引用稳定性，会话测试则覆盖 assistant 尾部、纯用户消息尾部、持久 steering 尾部，以及后续工具行和被中断推理行对分支操作的抑制。
+启用的分支图标现在表示的已完成轮次边界与 Host 实际复制的边界一致。在所报告的「响应 → 工具 → 被中断的 Think」形态中，响应仍保留复制、时钟，以及一个说明无法操作原因的禁用分支控件。本变更刻意不提供同轮次 transcript 编辑，也不提供轮次前重试操作；当读者希望完整复制最新的已完成轮次时，仍可使用 Session 行操作。运行时测试固定边界投影和引用稳定性，会话测试则覆盖 assistant 尾部、纯用户消息尾部、持久 steering 尾部，以及后续工具行和被中断推理行导致的不可用控件。

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

@@ -95,7 +95,7 @@ describe('web e2e: message IconActions and clocks on settled history', () => {
     await scaffold?.close()
   })
 
-  it.skipIf(MODE === 'record')('shows branch only on the completed transcript tail', async () => {
+  it.skipIf(MODE === 'record')('enables branch only on the completed transcript tail', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-message-actions'))
     const groupRow = page.locator('[role="treeitem"]').first()
     await groupRow.waitFor({ timeout: 15_000 })
@@ -107,13 +107,20 @@ describe('web e2e: message IconActions and clocks on settled history', () => {
     await expect.poll(() => page.getByText('DONE', { exact: true }).count(), { timeout: 15_000 }).toBe(1)
 
     // Focus-reveal the footers (hover:hover keeps them opacity-hidden until
-    // hover/focus-within). All message rows keep copy, but only the final
-    // assistant at a completed transcript tail has branch.
+    // hover/focus-within). Every durable message footer keeps branch visible,
+    // but only the final assistant at a completed transcript tail enables it.
     const copyButtons = page.getByRole('button', { name: 'Copy' })
     await expect.poll(() => copyButtons.count(), { timeout: 10_000 }).toBeGreaterThanOrEqual(4)
     await copyButtons.first().focus()
-    await expect.poll(() => page.getByRole('button', { name: 'Branch into a new conversation' }).count(), { timeout: 5_000 })
-      .toBe(1)
+    const branchButtons = page.getByRole('button', { name: 'Branch into a new conversation' })
+    await expect.poll(() => branchButtons.count(), { timeout: 5_000 }).toBe(4)
+    await expect.poll(
+      () => branchButtons.evaluateAll(buttons => buttons.map(button => button.getAttribute('aria-disabled'))),
+      { timeout: 5_000 },
+    ).toEqual(['true', 'true', 'true', null])
+    await branchButtons.first().focus()
+    await expect.poll(() => page.getByRole('tooltip').textContent(), { timeout: 5_000 })
+      .toBe('Available only on the last message of a completed turn')
     await expect.poll(() => page.getByRole('button', { name: 'Edit' }).count(), { timeout: 5_000 }).toBe(0)
   }, 60_000)
 
@@ -132,7 +139,7 @@ describe('web e2e: message IconActions and clocks on settled history', () => {
 
   it.skipIf(MODE === 'record')('forks through the settled-message and session-row actions', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-message-fork'))
-    // The sole message action belongs to the completed second-turn assistant.
+    // The last message action belongs to the completed second-turn assistant.
     await page.getByRole('button', { name: 'Branch into a new conversation' }).last().click()
     await expect.poll(
       () => scaffold.ctx.agents.list().find(agent => agent.session.header.parentSession === SessionId(SEED_ID)),

apps/web/tests/snapshots/bash-abort-row/ui.expected.md

@@ -7,8 +7,9 @@
 - text: "Run two shell commands: wait for cancellation, then write skipped.txt. {{date}} {{clock}}"
 - button "Copy":
   - img
-- button "Branch into a new conversation":
+- button "Branch into a new conversation" [disabled]:
   - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/code-mode-round/ui.expected.md

@@ -7,6 +7,9 @@
 - text: "Using ONE run_code program: run bash `echo CODE_ROUND_OK`, then read the file missing.txt catching its error in the program. Return an object with both outcomes. Then reply DONE and stop. {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/cordis-tool-round/ui.expected.md

@@ -7,6 +7,9 @@
 - text: "Use only Cordis tools. First call cordis_inspect with what \"temporary\". Then call cordis_mount with this exact code: \"return { name: \\\"snapshot-noop\\\", apply(ctx) {} }\". Read its returned id and call cordis_unmount with that exact id. After all three calls succeed, reply exactly CORDIS_UI_DONE and stop. {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/fresh-round-trip/ui.expected.md

@@ -7,6 +7,9 @@
 - text: "Use the bash tool to run exactly: echo WEB_E2E_OK. Then reply with the single word DONE and stop. {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/lifecycle-chrome/reloaded.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with the single word LIGHTHOUSE and stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/live-interactions/cancel.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/live-interactions/error-auth.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/live-interactions/loading.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/live-interactions/retry.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

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

@@ -8,6 +8,9 @@
 - button "Copy":
   - img
 - tooltip "Copy"
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Think The user wants me to read a.txt and b.txt, then reply with \"DONE\". Let me do both reads in parallel.":
   - img
   - img
@@ -15,7 +18,9 @@
 - paragraph: I will read both files before answering.
 - button "Copy":
   - img
-- text: 7/25 {{clock}}
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn 7/25 {{clock}}
 - button "Read a.txt":
   - img
   - img
@@ -33,6 +38,9 @@
 - text: Stopped Now give the final answer. 7/25 {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - paragraph: DONE
 - button "Copy":
   - img

apps/web/tests/snapshots/plan-review/approved.expected.md

@@ -8,6 +8,9 @@
 - text: "plan Plan mode on. Use /plan off to leave. Plan a small change: add a --greeting flag to a CLI. Do not read or write any files. Call exit_plan_mode with a short plan of at most five bullet points. Once the plan is approved, reply with the single word DONE and stop. {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/question-composer/answered.expected.md

@@ -7,6 +7,9 @@
 - text: "Use the ask_user_question tool to ask me exactly one question with id \"color\", question \"Which color do you prefer?\", header \"Pick one\", and two options: label \"Blue\" with description \"A cool recessive hue that reads as calm and trustworthy in long reading sessions and dense dashboards.\", and label \"Green\" with description \"A restful mid-spectrum hue with the highest perceived brightness, easiest on the eye over long sessions.\" After I answer, reply with the single word DONE and stop. {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/queue-actions/collapsed.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/queue-actions/editing.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/queue-actions/preserved.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img
@@ -20,6 +23,9 @@
 - text: {{clock}} Edited queue item {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - paragraph: partial
 - status: Deep diving...
 - list:

apps/web/tests/snapshots/queue-actions/ui.expected.md

@@ -7,6 +7,9 @@
 - text: Reply with a one-sentence description of event sourcing, then stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/seeded-history/command-row.expected.md

@@ -7,6 +7,9 @@
 - text: "Use the read tool twice in one assistant message: read a.txt and b.txt. Then reply with the single word DONE and stop. 7/25 {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Think The user wants me to read a.txt and b.txt, then reply with \"DONE\". Let me do both reads in parallel.":
   - img
   - img

apps/web/tests/snapshots/seeded-history/ui.expected.md

@@ -7,6 +7,9 @@
 - text: "Use the read tool twice in one assistant message: read a.txt and b.txt. Then reply with the single word DONE and stop. 7/25 {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Think The user wants me to read a.txt and b.txt, then reply with \"DONE\". Let me do both reads in parallel.":
   - img
   - img

apps/web/tests/snapshots/steering/mid-steer.expected.md

@@ -7,6 +7,9 @@
 - text: Use the ask_user_question tool to ask me exactly one question with id "checkpoint", question "Ready to continue?", header "Checkpoint", and options labeled "Yes" and "No". After I answer, reply with one short sentence acknowledging my answer and stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

apps/web/tests/snapshots/steering/settled.expected.md

@@ -7,6 +7,9 @@
 - text: Use the ask_user_question tool to ask me exactly one question with id "checkpoint", question "Ready to continue?", header "Checkpoint", and options labeled "Yes" and "No". After I answer, reply with one short sentence acknowledging my answer and stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img
@@ -22,6 +25,9 @@
 - text: "Interjection: include the word BANANA in your final reply. {{clock}}"
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Think The user selected \"Yes\" and wants me to include the word \"BANANA\" in my final reply. Let me acknowledge their answer.":
   - img
   - img

apps/web/tests/snapshots/subagent-conversation/ui.expected.md

@@ -12,6 +12,9 @@
 - text: Explain event sourcing in one sentence. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img
@@ -28,6 +31,9 @@
 - text: {{clock}} Now give the same explanation to a human reader. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Think The user is asking for a one-sentence description of event sourcing. This is a straightforward knowledge question that doesn't require any skill loading or tool calls.":
   - img
   - img

apps/web/tests/snapshots/web-search-round/ui.expected.md

@@ -7,6 +7,9 @@
 - text: Use web_search to search exactly "DeepSeek Harness snapshot search". Then reply exactly SEARCH_DONE and stop. {{clock}}
 - button "Copy":
   - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
 - button "Context injection":
   - img
   - img

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: be4463343a04f7287cfba8c48bdd71fa4d2cade4
-README.zh.md: cec93ef091a0dec0a10c1de1b04cf86966b972bd
+README.md: 89e58f967f852bb0786a5b7d73fa8e924fa282e0
+README.zh.md: 960e2fceede1b500af9ee2063ec9283e2b7b271a

packages/client/runtime/README.md

@@ -26,7 +26,7 @@ SlotsService gives the renderer separate bare observables for `useSessions` and
 
 ## The human transcript
 
-`ConversationSnapshot.nodes` is the human transcript, not the model surface. `TranscriptAdapter` projects the raw window in log order — every append-origin surface event (`isAppendSurfaceEvent`) at its own log position, plus one `CompactionSummaryNode` marker per landed compaction checkpoint — and never consults surface order. `ConversationSnapshot.turnEnds` maps each completed turn in that window to its `turn/end` seq, retaining turn completion independently from the transcript so presentation can require a real boundary before exposing an action. A landed compaction therefore keeps the conversation it shadowed on the model side: the marker reports where the model stopped seeing that history instead of erasing it. Model-only replacement copies stay out: a pruned `tool/result` and a regenerated `assistant/message` rewrite one node for the model and mark no boundary. A checkpoint is a `user/message` carrying the compaction seam's plugin source that **replaced** a surface range; an appending plugin-sourced `user/message` is injected context, not a compaction. The adapter's plugin literal is pinned to the seam's own declaration by a type-only import of the cordis-free [`dsh-compact/checkpoint`](../../compact/compact/README.md) leaf, so renaming it there fails `tsc` here; a **value** import of the package would fail the client purity gate, and the package **root** is unreachable even as a type (it reaches `dsh-session`'s root, whose `Context` merge collides the host `sessions` with this program's). `tests/compact-checkpoint-pin.spec.ts` covers the same drift behaviorally.
+`ConversationSnapshot.nodes` is the human transcript, not the model surface. `TranscriptAdapter` projects the raw window in log order — every append-origin surface event (`isAppendSurfaceEvent`) at its own log position, plus one `CompactionSummaryNode` marker per landed compaction checkpoint — and never consults surface order. `ConversationSnapshot.turnEnds` maps each completed turn in that window to its `turn/end` seq, retaining turn completion independently from the transcript so presentation can require a real boundary before enabling an action. A landed compaction therefore keeps the conversation it shadowed on the model side: the marker reports where the model stopped seeing that history instead of erasing it. Model-only replacement copies stay out: a pruned `tool/result` and a regenerated `assistant/message` rewrite one node for the model and mark no boundary. A checkpoint is a `user/message` carrying the compaction seam's plugin source that **replaced** a surface range; an appending plugin-sourced `user/message` is injected context, not a compaction. The adapter's plugin literal is pinned to the seam's own declaration by a type-only import of the cordis-free [`dsh-compact/checkpoint`](../../compact/compact/README.md) leaf, so renaming it there fails `tsc` here; a **value** import of the package would fail the client purity gate, and the package **root** is unreachable even as a type (it reaches `dsh-session`'s root, whose `Context` merge collides the host `sessions` with this program's). `tests/compact-checkpoint-pin.spec.ts` covers the same drift behaviorally.
 
 Because the projection is log-ordered, the node array is seq-monotonic by construction: log-only `command/run` / `command/done` nodes splice in by seq, `Session` merges interrupted frozen nodes by their fractional seqs, and a window whose checkpoint cites a shadowed range outside it renders the marker with nothing logged. The marker's summary text comes from the checkpoint's `compact/summary` provenance; a window cut that left the provenance outside makes the row non-expandable rather than empty, and a later page that supplies it resolves the text. Performance contract: one append materializes at most one node and copies the projection only when it adds that node; an event that changes no node keeps the previous array reference (a chunk storm costs nothing), and unchanged nodes keep their object identity.
 

packages/client/runtime/README.zh.md

@@ -26,7 +26,7 @@ SlotsService 分别为 renderer 提供 `useSessions` 与 `useWorkspaces` 的裸
 
 ## 面向人的 transcript（文本记录）
 
-`ConversationSnapshot.nodes` 是面向人的 transcript，不是模型 surface。`TranscriptAdapter` 按日志顺序投影原始窗口——每个 append 来源的 surface 事件（`isAppendSurfaceEvent`）落在它自己的日志位置上，外加每次落地的压缩（compaction）检查点贡献一个 `CompactionSummaryNode` 标记——且从不查询 surface 顺序。`ConversationSnapshot.turnEnds` 把该窗口中的每个已完成轮次映射到其 `turn/end` seq；它独立于 transcript 保留轮次完成状态，使呈现层能够在暴露操作前要求存在真实边界。于是一次落地的压缩会保留它在模型侧遮蔽掉的对话：标记报告模型从哪里开始看不见那段历史，而不是把它抹掉。仅模型可见的 replacement 副本不进入记录：被裁剪的 `tool/result` 和重新生成的 `assistant/message` 只为模型重写一个节点，不标记任何边界。检查点是携带压缩 seam 插件来源、且**替换**了一段 surface 范围的 `user/message`；一条 append 的插件来源 `user/message` 是注入上下文，不是压缩。适配器的插件字面量通过对无 cordis 的 [`dsh-compact/checkpoint`](../../compact/compact/README.md) 叶子做仅类型导入，钉在压缩 seam 自己的声明上：在那里改名会让此处 `tsc` 失败；而对该包（package）做**值**导入会被客户端纯度门禁拒绝，包的**根**即便作为类型也无法到达（它会到达 `dsh-session` 的根，其 `Context` 合并会让 host 的 `sessions` 与本程序的冲突）。`tests/compact-checkpoint-pin.spec.ts` 从行为侧覆盖同一漂移。
+`ConversationSnapshot.nodes` 是面向人的 transcript，不是模型 surface。`TranscriptAdapter` 按日志顺序投影原始窗口——每个 append 来源的 surface 事件（`isAppendSurfaceEvent`）落在它自己的日志位置上，外加每次落地的压缩（compaction）检查点贡献一个 `CompactionSummaryNode` 标记——且从不查询 surface 顺序。`ConversationSnapshot.turnEnds` 把该窗口中的每个已完成轮次映射到其 `turn/end` seq；它独立于 transcript 保留轮次完成状态，使呈现层能够在启用操作前要求存在真实边界。于是一次落地的压缩会保留它在模型侧遮蔽掉的对话：标记报告模型从哪里开始看不见那段历史，而不是把它抹掉。仅模型可见的 replacement 副本不进入记录：被裁剪的 `tool/result` 和重新生成的 `assistant/message` 只为模型重写一个节点，不标记任何边界。检查点是携带压缩 seam 插件来源、且**替换**了一段 surface 范围的 `user/message`；一条 append 的插件来源 `user/message` 是注入上下文，不是压缩。适配器的插件字面量通过对无 cordis 的 [`dsh-compact/checkpoint`](../../compact/compact/README.md) 叶子做仅类型导入，钉在压缩 seam 自己的声明上：在那里改名会让此处 `tsc` 失败；而对该包（package）做**值**导入会被客户端纯度门禁拒绝，包的**根**即便作为类型也无法到达（它会到达 `dsh-session` 的根，其 `Context` 合并会让 host 的 `sessions` 与本程序的冲突）。`tests/compact-checkpoint-pin.spec.ts` 从行为侧覆盖同一漂移。
 
 由于投影按日志顺序，节点数组天然按 seq 单调：仅日志的 `command/run` / `command/done` 节点按 seq 插入，`Session` 按分数 seq 归并被打断的冻结节点，而检查点所引范围落在窗口之外的窗口会渲染出标记且不打印任何日志。标记的摘要文本来自检查点的 `compact/summary` 溯源；窗口切分把溯源留在窗口外时该行不可展开而非空白，后续补上溯源的分页会解析出文本。性能契约：一次追加最多物化一个节点，并且仅在加入该节点时复制投影；不改变任何节点的事件保持上一次的数组引用（分片风暴零成本），未变化的节点保持其对象标识。
 

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: 4f06b407a6c2288be2c58ad01655fd37e7e368ae
-README.zh.md: db0f31cc506f0d3d600296d5d00ae70e12df4fc8
+README.md: e1dbe7d4d5992b6b5b029fddfc9d9857ccae7443
+README.zh.md: fd82ad65f8e903a6f7106e8b8ff8eccbf1435957

packages/client/ui-conversation/README.md

@@ -38,7 +38,7 @@ The todo surfaces are two registrations over that shape, both plain registrant p
 
 `QueueDock` is the terminal input-dock entry at `order: 20`. It hides while empty, renders one pending row directly, and defaults two or more rows to a collapsed `"<n> 条排队消息"` header whose button expands or collapses the complete list. The header exposes `aria-expanded` and `aria-controls`; the expanded list scrolls within a 180px height bound. An active edit or mutation keeps its rows visible, and emptying the queue restores the collapsed default for the next queue. Each visible ordinary-session row remains a single-line preview with its exact-occurrence edit, delete, and strict-steer actions; addressed subagents retain the rows as a read-only projection because their continuation transport does not expose queue mutation. If strict steer loses to a closed window, the original occurrence remains queued for normal delivery; if the driver already claimed it, normal delivery is already underway. Neither converged race displays a failure, while transport and unknown failures do.
 
-The Host's placement-aware `session/queue` snapshot also carries pending steering. QueueDock filters it out, while ChatView projects it as a user-style bubble with Copy at the conversation tail; Fork stays absent because the message has not entered a durable turn. The Host delays steering retirement until the durable `steering/message` has entered the mux stream. On that accepted live event, the client runtime retires the first matching current steering occurrence before publishing the snapshot; historical events cannot hide later occurrences that reuse the same `MessageId`. The bubble therefore hands off without a gap or duplicate, immediately restores Copy from the durable node, exposes Fork only when that node is the completed turn's transcript tail, and survives reconnect from the same authority.
+The Host's placement-aware `session/queue` snapshot also carries pending steering. QueueDock filters it out, while ChatView projects it as a user-style bubble with Copy at the conversation tail; Fork stays absent because the message has not entered a durable turn. The Host delays steering retirement until the durable `steering/message` has entered the mux stream. On that accepted live event, the client runtime retires the first matching current steering occurrence before publishing the snapshot; historical events cannot hide later occurrences that reuse the same `MessageId`. The bubble therefore hands off without a gap or duplicate, immediately restores Copy and the branch control from the durable node, enables branch only when that node is the completed turn's transcript tail, and survives reconnect from the same authority.
 
 Keyboard message submission resolves delivery from the addressed session's running state and steering capability. While idle, Enter and Cmd/Ctrl+Enter both perform an ordinary Queue send. While a primary session is running, the browser-persisted General Settings preference assigns plain Enter to `Queue` (the default) or `Steer`, and Cmd/Ctrl+Enter performs the other behavior; Shift+Enter remains a newline. Addressed subagents keep both gestures on their Queue-only continuation transport even while running. The preference affects only the steer-capable busy-state gesture pair, and the send button and non-keyboard submit actions remain Queue. Composer Steer uses the existing best-effort `session.prompt(mode: 'steer')` contract: if the current next-step window closes before acceptance, AgentLoop admits the message as the next waking Queue turn without surfacing a failure or losing the draft transaction.
 
@@ -63,8 +63,8 @@ None; this package neither assembles nor sends a provider request.
 - **Compaction markers show no scale** — the row does not yet report how many messages or which range the checkpoint replaced.
 - **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 / clock, plus branch when eligible) ships under the last content-text assistant of each turn only; mid-turn narration and Think-only nodes stay chrome-free. Branch appears only when that message is also the last transcript node of a completed turn, then forks through that turn, increments the inherited title on the client, and opens the child; a fork or rename failure leaves the source selected ([decision](../../../.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md)).
-- **Sent user messages cannot be edited** — user bubbles retain clock and copy, while branch appears only for a completed turn whose transcript ends at that user message. Editing returns with the capability behind it: a client mutation over a settled user message, plus the host behavior for the turn that already consumed it ([decision](../../../.agents/notes/implemented/simplification/2026-07-31-drop-user-message-edit-stub.md)).
+- **Assistant per-message paging is a reserved slot** — drawn in the design, not implemented. The finalized content IconActions row (copy / clock / branch) ships under the last content-text assistant of each turn only; mid-turn narration and Think-only nodes stay chrome-free. Branch stays disabled unless that message is also the last transcript node of a completed turn; when enabled, it forks through that turn, increments the inherited title on the client, and opens the child. A fork or rename failure leaves the source selected ([decision](../../../.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md)).
+- **Sent user messages cannot be edited** — user bubbles retain clock, copy, and branch; branch stays disabled unless a completed turn's transcript ends at that user message. Editing returns with the capability behind it: a client mutation over a settled user message, plus the host behavior for the turn that already consumed it ([decision](../../../.agents/notes/implemented/simplification/2026-07-31-drop-user-message-edit-stub.md)).
 - **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

@@ -38,7 +38,7 @@ 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
 
 `QueueDock` 是 `order: 20` 的末端 input-dock 条目。队列为空时隐藏；只有一个待处理项时直接渲染该行；存在两个或更多待处理项时，默认收起为 `"<n> 条排队消息"` 表头，其按钮可展开或收起完整列表。表头暴露 `aria-expanded` 和 `aria-controls`；展开后的列表以 180px 为高度上限，并可滚动。存在进行中的编辑或变更时，列表行会保持可见；队列清空后，下一次出现队列时会恢复默认收起状态。普通会话中的每条可见行仍是单行预览，并提供针对精确单次入队项的编辑、删除和严格 steering（中途引导）操作；已寻址 subagent 则保留只读行，因为其继续执行传输不提供 Queue 变更。如果严格 steering 输给已关闭的窗口，原单次入队项会留在 Queue 中正常投递；如果驱动器已经认领该项，正常投递就已开始。这两种已收敛的竞态都不显示失败，传输和未知错误仍会显示。
 
-Host 带 placement 的 `session/queue` 快照也会携带待处理 steering。QueueDock 会将其过滤掉，ChatView 则把它投影为会话流末尾带复制操作的用户样式气泡；消息尚未进入持久轮次，因此不显示 fork。Host 会等持久 `steering/message` 进入 mux 流之后再退役 steering。客户端运行时接纳该实时事件时，会在发布快照前退役第一个匹配的当前 steering 单次入队项；历史事件无法隐藏后来复用同一 `MessageId` 的单次入队项。气泡交接时因而不会产生空档或重复，会立即从持久节点恢复复制操作，仅当该节点是已完成轮次的 transcript 尾部时才显示 fork，并能在重连后从同一权威恢复。
+Host 带 placement 的 `session/queue` 快照也会携带待处理 steering。QueueDock 会将其过滤掉，ChatView 则把它投影为会话流末尾带复制操作的用户样式气泡；消息尚未进入持久轮次，因此不显示 fork。Host 会等持久 `steering/message` 进入 mux 流之后再退役 steering。客户端运行时接纳该实时事件时，会在发布快照前退役第一个匹配的当前 steering 单次入队项；历史事件无法隐藏后来复用同一 `MessageId` 的单次入队项。气泡交接时因而不会产生空档或重复，会立即从持久节点恢复复制操作与分支控件，仅当该节点是已完成轮次的 transcript 尾部时才启用分支，并能在重连后从同一权威恢复。
 
 键盘消息提交会根据所寻址会话的运行状态和 steering 能力解析投递方式。空闲时，Enter 和 Cmd/Ctrl+Enter 都执行普通 Queue 发送。主会话运行期间，浏览器持久化的 General Settings 偏好会把普通 Enter 分配为 `Queue`（默认值）或 `Steer`，Cmd/Ctrl+Enter 则执行另一种行为；Shift+Enter 仍然换行。已寻址 subagent 即使正在运行，也会让这两个手势都使用其仅支持 Queue 的继续执行传输。该偏好只影响支持 steering 的繁忙态手势对，发送按钮与非键盘提交操作仍使用 Queue。Composer Steer 复用现有尽力而为的 `session.prompt(mode: 'steer')` 契约：如果当前 next-step 窗口在接纳前关闭，AgentLoop 会把消息接纳为下一条唤醒 Queue 轮次，不显示失败，也不会丢失草稿事务。
 
@@ -63,8 +63,8 @@ Host 带 placement 的 `session/queue` 快照也会携带待处理 steering。Qu
 - **压缩标记不显示规模**：该行尚不报告检查点替换了多少条消息或哪段范围。
 - **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。
 - **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／时钟，符合条件时再显示分支）只挂在每个轮次中最后一条带 text 内容的 assistant 下；轮次中间的叙述与纯 Think 节点不带 chrome。只有当该消息同时也是已完成轮次的最后一个 transcript 节点时才显示分支；随后 fork 到该轮次末尾，在 client 端递增继承标题并打开子会话；fork 或改名失败时源会话保持选中（[决策](../../../.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md)）。
-- **已发送的 user 消息无法编辑**：user 气泡保留时钟和复制；仅当已完成轮次的 transcript 结束于该 user 消息时才显示分支。编辑功能要与其背后的能力一起回归：既需要针对已定稿 user 消息的 client 变更，也需要 host 侧对已经消费过它的轮次给出行为（[决策](../../../.agents/notes/implemented/simplification/2026-07-31-drop-user-message-edit-stub.md)）。
+- **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／时钟／分支）只挂在每个轮次中最后一条带 text 内容的 assistant 下；轮次中间的叙述与纯 Think 节点不带 chrome。除非该消息同时也是已完成轮次的最后一个 transcript 节点，否则分支保持禁用；启用后，它会 fork 到该轮次末尾，在 client 端递增继承标题并打开子会话。fork 或改名失败时源会话保持选中（[决策](../../../.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md)）。
+- **已发送的 user 消息无法编辑**：user 气泡保留时钟、复制和分支；除非已完成轮次的 transcript 结束于该 user 消息，否则分支保持禁用。编辑功能要与其背后的能力一起回归：既需要针对已定稿 user 消息的 client 变更，也需要 host 侧对已经消费过它的轮次给出行为（[决策](../../../.agents/notes/implemented/simplification/2026-07-31-drop-user-message-edit-stub.md)）。
 - **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/chat/AssistantMarkdown.tsx

@@ -5,7 +5,7 @@
 // step-summary flow). Shared by finalized nodes and the streaming partial;
 // the turn-level loading dots live in the chat view's tail, not here.
 // Finalized content (text) nodes append IconActions once streaming ends
-// (`time` is omitted for mid-turn narration); their branch action is present
+// (`time` is omitted for mid-turn narration); their branch action is enabled
 // only when the node is also the completed turn's transcript tail. Think /
 // tool-head-only nodes stay chrome-free.
 
@@ -29,8 +29,10 @@ export interface AssistantMarkdownProps {
   time?: number | undefined
   /** Event sequence used as the fork boundary; omitted while streaming. */
   seq?: number | undefined
-  /** Fork the session through this finalized message's completed turn. */
+  /** Fork the session through this finalized message's completed turn when eligible. */
   onFork?: ((seq: number) => void) | undefined
+  /** The message is not the transcript tail of a completed turn. */
+  forkUnavailable?: boolean | undefined
   /** The owning view's locale seat, passed down as a plain prop. */
   t: ChatViewSlotProps['t']
 }
@@ -77,7 +79,7 @@ function ThinkRow({ text, running, t }: { text: string; running: boolean; t: Ass
 }
 
 export const AssistantMarkdown = memo(function AssistantMarkdown({
-  blocks, streaming, interrupted, time, seq, onFork, t,
+  blocks, streaming, interrupted, time, seq, onFork, forkUnavailable, t,
 }: AssistantMarkdownProps) {
   // Stable per locale revision (t identity changes on switch): a fresh object
   // per render would rebuild MarkdownText's component table every chunk.
@@ -121,6 +123,7 @@ export const AssistantMarkdown = memo(function AssistantMarkdown({
           time={time}
           clock="end"
           onBranch={onFork === undefined || seq === undefined ? undefined : () => { onFork(seq) }}
+          branchUnavailable={forkUnavailable}
           className={css.actions}
           t={t}
         />

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

@@ -414,7 +414,8 @@ export function ChatView({
           interrupted={node.interrupted}
           time={actionSeqs.has(node.seq) ? node.time : undefined}
           seq={node.seq}
-          onFork={branchSeqs.has(node.seq) ? forkAt : undefined}
+          onFork={forkAt}
+          forkUnavailable={!branchSeqs.has(node.seq)}
           t={t}
         />
       )
@@ -429,7 +430,8 @@ export function ChatView({
         key={item.key}
         node={node}
         retryActive={node.kind === 'model-retry' && node.seq === activeRetry}
-        {...branchSeqs.has(node.seq) ? { onFork: forkAt } : {}}
+        onFork={forkAt}
+        forkUnavailable={!branchSeqs.has(node.seq)}
         t={t}
       />
     )

packages/client/ui-conversation/src/client/chat/MessageIconActions.module.css

@@ -43,3 +43,23 @@
   background: var(--dsw-alias-interactive-bg-hover);
   color: var(--dsw-alias-label-secondary);
 }
+
+/* Unavailable stays focusable and hoverable so Tooltip can explain why. */
+.action[data-unavailable] {
+  cursor: default;
+  opacity: 0.4;
+}
+
+.action[data-unavailable]:hover {
+  background: transparent;
+  color: var(--dsw-alias-label-tertiary);
+}
+
+.visuallyHidden {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  overflow: hidden;
+  clip: rect(0 0 0 0);
+  white-space: nowrap;
+}

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

@@ -1,7 +1,7 @@
 // Shared IconActions chrome for user, steering, and assistant messages: copy
 // live, optional branch wiring, and an optional date-aware clock.
 
-import { useCallback } from 'react'
+import { useCallback, useId } from 'react'
 import {
   IconBranchOutline16, IconCopyOutline16, Tooltip,
 } from '@deepseek-ai/dsh-client-ui-primitives'
@@ -19,6 +19,8 @@ export interface MessageIconActionsProps {
   clock: 'start' | 'end'
   /** Fork the session at this message; omission hides the branch action. */
   onBranch?: (() => void) | undefined
+  /** The message is not a completed transcript tail, so branch stays visible but unavailable. */
+  branchUnavailable?: boolean | undefined
   /** Additional branch visibility gate for transient message chrome; defaults to true. */
   showBranch?: boolean | undefined
   /** Parent layout class composed onto the actions row. */
@@ -33,9 +35,10 @@ export interface MessageIconActionsProps {
  * @returns The actions row element.
  */
 export function MessageIconActions({
-  text, time, clock, onBranch, showBranch = true, className, t,
+  text, time, clock, onBranch, branchUnavailable = false, showBranch = true, className, t,
 }: MessageIconActionsProps) {
   const day = useCalendarDay()
+  const reasonId = useId()
   const onCopy = useCallback(() => {
     void writeClipboard(text)
   }, [text])
@@ -53,12 +56,24 @@ export function MessageIconActions({
         </button>
       </Tooltip>
       {showBranch && onBranch !== undefined && (
-        <Tooltip label={t('message.branch')} side="bottom">
-          <button type="button" className={css.action} aria-label={t('message.branch')} onClick={onBranch}>
+        <Tooltip label={branchUnavailable ? t('message.branchUnavailable') : t('message.branch')} side="bottom">
+          {/* Native disabled buttons do not deliver the hover/focus events Tooltip needs. */}
+          <button
+            type="button"
+            className={css.action}
+            aria-label={t('message.branch')}
+            aria-disabled={branchUnavailable || undefined}
+            aria-describedby={branchUnavailable ? reasonId : undefined}
+            data-unavailable={branchUnavailable || undefined}
+            onClick={branchUnavailable ? undefined : onBranch}
+          >
             <IconBranchOutline16 />
           </button>
         </Tooltip>
       )}
+      {showBranch && onBranch !== undefined && branchUnavailable && (
+        <span id={reasonId} className={css.visuallyHidden}>{t('message.branchUnavailable')}</span>
+      )}
       {clock === 'end' ? clockEl : null}
     </div>
   )

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

@@ -26,8 +26,10 @@ export interface MessageItemProps {
     | TurnErrorNode
     | UnknownSurfaceNode
   retryActive?: boolean
-  /** Fork through this message's completed turn when it is the transcript tail. */
+  /** Fork through this message's completed turn when eligible. */
   onFork?: (seq: number) => void
+  /** The message is not the transcript tail of a completed turn. */
+  forkUnavailable?: boolean
   /** The owning view's locale seat, passed down as a plain prop. */
   t: ChatViewSlotProps['t']
 }
@@ -220,7 +222,7 @@ export function PendingSteeringBubble({ content, t }: {
 }
 
 export const MessageItem = memo(function MessageItem({
-  node, retryActive = false, onFork, t,
+  node, retryActive = false, onFork, forkUnavailable = false, t,
 }: MessageItemProps) {
   const truncated = (total: number): string => t('json.truncated', { total })
   switch (node.kind) {
@@ -236,6 +238,7 @@ export const MessageItem = memo(function MessageItem({
               time={node.time}
               clock="start"
               onBranch={onFork === undefined ? undefined : () => { onFork(node.seq) }}
+              branchUnavailable={forkUnavailable}
               className={css.actions}
               t={t}
             />

packages/client/ui-conversation/src/client/chat/chat-flow.ts

@@ -50,8 +50,8 @@ export function assistantActionsSeqs(nodes: readonly ConversationNode[]): Readon
 /**
  * Seq set of message rows that may fork: the last transcript node of a
  * completed turn, when that node owns message chrome. A later tool, reasoning,
- * error, or other transcript node suppresses the earlier message's branch
- * action even though the Host would include the whole turn.
+ * error, or other transcript node leaves the earlier message's branch action
+ * unavailable because the Host would include the whole turn.
  * @param nodes - snapshot nodes in event order.
  * @param turnEnds - completed turn boundaries retained from the event window.
  * @returns Message seq values whose visible position matches the fork boundary.

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

@@ -59,6 +59,7 @@ export const zh = {
   'message.unknownBlock': '未知内容块',
   'message.stopped': '已停止',
   'message.branch': '在新对话中分支',
+  'message.branchUnavailable': '仅可从已完成轮次的最后一条消息分支',
   'message.retry.active': '正在重试模型请求',
   'message.retry.cancelled': '模型请求重试已取消',
   'message.retry.started': '已重试模型请求',
@@ -166,6 +167,7 @@ export const en = {
   'message.unknownBlock': 'Unknown content block',
   'message.stopped': 'Stopped',
   'message.branch': 'Branch into a new conversation',
+  'message.branchUnavailable': 'Available only on the last message of a completed turn',
   'message.retry.active': 'Retrying model request',
   'message.retry.cancelled': 'Model request retry cancelled',
   'message.retry.started': 'Retried model request',

packages/client/ui-conversation/tests/chat-branch-tails.spec.tsx

@@ -78,6 +78,30 @@ describe('MessageItem arms', () => {
     expect(exec).toHaveBeenCalledWith('copy')
   })
 
+  it('keeps an unavailable branch focusable and explains why without sending a fork', () => {
+    const onFork = vi.fn()
+    render(
+      <MessageItem t={t} node={{
+        kind: 'user', seq: 1, time: 1_000,
+        content: [{ type: 'text', text: 'open turn' }] as never,
+        source: null,
+      }}
+      onFork={onFork}
+      forkUnavailable
+      />,
+    )
+    const branch = screen.getByRole('button', { name: '在新对话中分支' }) as HTMLButtonElement
+    expect(branch.disabled).toBe(false)
+    expect(branch.getAttribute('aria-disabled')).toBe('true')
+    const reasonId = branch.getAttribute('aria-describedby')
+    expect(reasonId).not.toBeNull()
+    expect(document.getElementById(reasonId!)?.textContent).toBe('仅可从已完成轮次的最后一条消息分支')
+    fireEvent.click(branch)
+    expect(onFork).not.toHaveBeenCalled()
+    fireEvent.focus(branch)
+    expect(screen.getByRole('tooltip').textContent).toBe('仅可从已完成轮次的最后一条消息分支')
+  })
+
   it('user copy stays quiet when execCommand throws or is absent', () => {
     Object.defineProperty(navigator, 'clipboard', {
       configurable: true,

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

@@ -325,14 +325,19 @@ describe('ChatView', () => {
     expect(view.getAllByText('interrupt now')).toHaveLength(1)
     expect(view.container.querySelector('[data-pending-steering]')).toBeNull()
     expect(view.getAllByRole('button', { name: '复制' })).toHaveLength(2)
-    expect(view.queryByRole('button', { name: '在新对话中分支' })).toBeNull()
+    const durableBubble = view.getByText('interrupt now').closest('[class*="userRow"]') as HTMLElement
+    const unavailable = within(durableBubble).getByRole('button', { name: '在新对话中分支' })
+    expect(unavailable.getAttribute('aria-disabled')).toBe('true')
+    fireEvent.click(unavailable)
+    expect(h.forkAt).not.toHaveBeenCalled()
 
     act(() => {
       h.set({ running: false, turnEnds: new Map([[1, 3]]) })
     })
     const branchButtons = view.getAllByRole('button', { name: '在新对话中分支' })
-    expect(branchButtons).toHaveLength(1)
-    fireEvent.click(branchButtons[0]!)
+    expect(branchButtons).toHaveLength(2)
+    expect(branchButtons.map(button => button.getAttribute('aria-disabled'))).toEqual(['true', null])
+    fireEvent.click(branchButtons[1]!)
     expect(h.forkAt).toHaveBeenCalledWith(2)
   })
 
@@ -435,24 +440,28 @@ describe('ChatView', () => {
       turnEnds: new Map([[1, 4], [2, 6]]),
     })
     const view = render(<h.ChatView {...h.props} />)
-    // User rows keep copy/clock, while only the two completed assistant tails may branch.
+    // Every message footer keeps branch visible; only completed assistant tails enable it.
     expect(view.getAllByRole('button', { name: '复制' })).toHaveLength(4)
-    expect(view.getAllByRole('button', { name: '在新对话中分支' })).toHaveLength(2)
+    const branchButtons = view.getAllByRole('button', { name: '在新对话中分支' })
+    expect(branchButtons).toHaveLength(4)
+    expect(branchButtons.map(button => button.getAttribute('aria-disabled'))).toEqual(['true', null, 'true', null])
   })
 
-  it('forks only from a finalized assistant at the completed transcript tail', () => {
+  it('enables fork only on the finalized assistant at the completed transcript tail', () => {
     const h = makeHarness({
       nodes: [user(1, 'question'), assistant(2, 'answer')],
       turnEnds: new Map([[1, 3]]),
     })
     const view = render(<h.ChatView {...h.props} />)
     const buttons = view.getAllByRole('button', { name: '在新对话中分支' })
-    expect(buttons).toHaveLength(1)
+    expect(buttons).toHaveLength(2)
+    expect(buttons.map(button => button.getAttribute('aria-disabled'))).toEqual(['true', null])
     fireEvent.click(buttons[0]!)
+    fireEvent.click(buttons[1]!)
     expect(h.forkAt.mock.calls).toEqual([[2]])
   })
 
-  it('keeps copy chrome but hides branch when tool and interrupted Think follow the response', () => {
+  it('keeps branch visible but unavailable when tool and interrupted Think follow the response', () => {
     const interruptedThink: AssistantMessageNode = {
       kind: 'assistant', seq: 4.1, time: 4_100, turn: 1, step: 2,
       blocks: [{ kind: 'reasoning', text: 'bad path' }], interrupted: true,
@@ -463,7 +472,12 @@ describe('ChatView', () => {
     })
     const view = render(<h.ChatView {...h.props} />)
     expect(view.getAllByRole('button', { name: '复制' })).toHaveLength(2)
-    expect(view.queryByRole('button', { name: '在新对话中分支' })).toBeNull()
+    const buttons = view.getAllByRole('button', { name: '在新对话中分支' })
+    expect(buttons).toHaveLength(2)
+    expect(buttons.every(button => button.getAttribute('aria-disabled') === 'true')).toBe(true)
+    fireEvent.click(buttons[0]!)
+    fireEvent.click(buttons[1]!)
+    expect(h.forkAt).not.toHaveBeenCalled()
   })
 
   it('renders assistant Markdown across history, streaming, final, and interrupted states while user text stays literal', () => {