fix(web): restrict message forks to completed turn tails

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

@@ -4,9 +4,10 @@
 // view groups them into tool rows through its keyed toolview slot (figma
 // 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 turn-tail content (text) nodes append IconActions once streaming
-// ends (`time` is omitted for mid-turn narration); Think / tool-head-only
-// nodes stay chrome-free.
+// Finalized content (text) nodes append IconActions once streaming ends
+// (`time` is omitted for mid-turn narration); their branch action is present
+// only when the node is also the completed turn's transcript tail. Think /
+// tool-head-only nodes stay chrome-free.
 
 import { memo, useMemo } from 'react'
 import type { AssistantBlock } from '@deepseek-ai/dsh-client-runtime/client'
@@ -28,7 +29,7 @@ export interface AssistantMarkdownProps {
   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. */
+  /** Fork the session through this finalized message's completed turn. */
   onFork?: ((seq: number) => void) | undefined
   /** The owning view's locale seat, passed down as a plain prop. */
   t: ChatViewSlotProps['t']

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

@@ -30,7 +30,7 @@ import type {
 import type { SnapshotSelectorHook } from '@deepseek-ai/dsh-client-ui-slots'
 import { IconChevronDownOutline14 } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { ChatViewSlotProps } from '../contract/slots.ts'
-import { assistantActionsSeqs, deriveChatFlow, type ChatFlowItem } from './chat-flow.ts'
+import { assistantActionsSeqs, deriveChatFlow, messageBranchSeqs, type ChatFlowItem } from './chat-flow.ts'
 import { AssistantMarkdown } from './AssistantMarkdown.tsx'
 import { GenericCommandCard } from './GenericCommandCard.tsx'
 import { GenericToolCard } from './GenericToolCard.tsx'
@@ -236,6 +236,7 @@ export function ChatView({
   useSession, useSessions, useStore, renderSlot, sessionId, openFile, loadOlder, inspectCall, chatScroll, forkAt, t,
 }: ChatViewSlotProps) {
   const nodes = useSession(s => s.nodes)
+  const turnEnds = useSession(s => s.turnEnds)
   // Workspace root off the session list row: path summaries display relative to it.
   const cwd = useSessions(s => s.byId[sessionId]?.cwd)
   const running = useSession(s => s.running)
@@ -252,6 +253,7 @@ export function ChatView({
   // Only the last content assistant of each turn owns IconActions; mid-turn
   // text (before tools) omits `time` so AssistantMarkdown stays chrome-free.
   const actionSeqs = useMemo(() => assistantActionsSeqs(nodes), [nodes])
+  const branchSeqs = useMemo(() => messageBranchSeqs(nodes, turnEnds), [nodes, turnEnds])
 
   const listRef = useRef<HTMLDivElement | null>(null)
   const atBottomRef = useRef(true)
@@ -401,7 +403,7 @@ export function ChatView({
           interrupted={node.interrupted}
           time={actionSeqs.has(node.seq) ? node.time : undefined}
           seq={node.seq}
-          onFork={forkAt}
+          onFork={branchSeqs.has(node.seq) ? forkAt : undefined}
           t={t}
         />
       )
@@ -416,7 +418,7 @@ export function ChatView({
         key={item.key}
         node={node}
         retryActive={node.kind === 'model-retry' && node.seq === activeRetry}
-        onFork={forkAt}
+        {...branchSeqs.has(node.seq) ? { onFork: forkAt } : {}}
         t={t}
       />
     )

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

@@ -17,7 +17,7 @@ export interface MessageIconActionsProps {
   time: number
   /** Clock before icons (user) or after (assistant). */
   clock: 'start' | 'end'
-  /** Fork the session at this message. */
+  /** Fork the session at this message; omission hides the branch action. */
   onBranch?: (() => void) | undefined
   /** Parent layout class composed onto the actions row. */
   className?: string | undefined
@@ -50,11 +50,13 @@ export function MessageIconActions({
           <IconCopyOutline16 />
         </button>
       </Tooltip>
-      <Tooltip label={t('message.branch')} side="bottom">
-        <button type="button" className={css.action} aria-label={t('message.branch')} onClick={onBranch}>
-          <IconBranchOutline16 />
-        </button>
-      </Tooltip>
+      {onBranch !== undefined && (
+        <Tooltip label={t('message.branch')} side="bottom">
+          <button type="button" className={css.action} aria-label={t('message.branch')} onClick={onBranch}>
+            <IconBranchOutline16 />
+          </button>
+        </Tooltip>
+      )}
       {clock === 'end' ? clockEl : null}
     </div>
   )

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

@@ -26,7 +26,7 @@ export interface MessageItemProps {
     | TurnErrorNode
     | UnknownSurfaceNode
   retryActive?: boolean
-  /** Fork the session through the turn containing this message (user-bubble branch action). */
+  /** Fork through this message's completed turn when it is the transcript tail. */
   onFork?: (seq: number) => void
   /** The owning view's locale seat, passed down as a plain prop. */
   t: ChatViewSlotProps['t']

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

@@ -5,8 +5,8 @@
  * reuse the first notice's row while projecting the latest retry turn.
  * Item identity keys are stable across snapshots so the list parent can
  * subscribe to keys only while rows subscribe to content. IconActions ownership
- * (last content assistant per turn) is derived here too so ChatView and the
- * flow share one gate.
+ * and completed-turn branch points are derived here too so ChatView and the
+ * flow share their gates.
  */
 import type {
   AssistantBlock, ConversationNode, ToolResultNode,
@@ -47,6 +47,38 @@ export function assistantActionsSeqs(nodes: readonly ConversationNode[]): Readon
   return new Set(lastByTurn.values())
 }
 
+/**
+ * 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.
+ * @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.
+ */
+export function messageBranchSeqs(
+  nodes: readonly ConversationNode[],
+  turnEnds: ReadonlyMap<number, number>,
+): ReadonlySet<number> {
+  const result = new Set<number>()
+  const boundaries = [...turnEnds].sort((a, b) => a[1] - b[1])
+  let nodeIndex = 0
+  for (const [turn, endSeq] of boundaries) {
+    let tail: ConversationNode | undefined
+    while (nodeIndex < nodes.length) {
+      const candidate = nodes[nodeIndex]
+      if (candidate === undefined || candidate.seq > endSeq) break
+      tail = candidate
+      nodeIndex++
+    }
+    if (tail?.kind === 'user'
+      || (tail?.kind === 'assistant' && tail.turn === turn && hasContentText(tail.blocks))) {
+      result.add(tail.seq)
+    }
+  }
+  return result
+}
+
 /**
  * Group finalized nodes into the step-summary flow.
  * @param nodes - snapshot nodes in human-transcript and durable-notice order.

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

@@ -454,7 +454,7 @@ export interface ChatViewInjected {
     /** Last recorded offset, or null when pinned or never recorded. */
     read: () => number | null
   }
-  /** Fork the session through the turn containing the message at `seq`, then open the child. */
+  /** Fork through the completed turn ending at the eligible message `seq`, then open the child. */
   forkAt: (seq: number) => void
 }