fix(desktop): scope composer and transcript state to their own session

`$activeSessionId` only ever holds the primary chat's session, but surfaces
that render once per transcript were reading it as if it meant "the session
on screen." A preview produced inside a session tile was recorded under the
main chat's key and surfaced in the main chat's composer, which is what
prompted this.

The tool row now records under its own `SessionView`, and the same fix
applies to the other readers of that atom that render per surface:
attachment pills and inline preview links resolve relative paths against
their session's cwd, composer voice and auto-speak read and subscribe to
their own transcript, and the thread's compaction label, prompt-wait gate
and turn timer follow the session that mounted them. `ComposerScope` now
carries a `$messages` atom rather than a read closure so both the
imperative read and the subscription come from one place.

apps/desktop/src/components/assistant-ui/thread/status.tsx

@@ -1,7 +1,8 @@
 import { useAuiState } from '@assistant-ui/react'
 import { useStore } from '@nanostores/react'
-import { type FC, type ReactNode, useEffect, useState } from 'react'
+import { type FC, type ReactNode, useEffect, useMemo, useState } from 'react'
 
+import { useSessionView } from '@/app/chat/session-view'
 import { useElapsedSeconds } from '@/components/chat/activity-timer'
 import { ActivityTimerText } from '@/components/chat/activity-timer-text'
 import { Codicon } from '@/components/ui/codicon'
@@ -9,9 +10,9 @@ import { Loader } from '@/components/ui/loader'
 import { useI18n } from '@/i18n'
 import { cn } from '@/lib/utils'
 import { $backgroundResume } from '@/store/background-delegation'
-import { $compactionActive } from '@/store/compaction'
-import { $activeSessionAwaitingInput } from '@/store/prompts'
-import { $activeSessionId, $turnStartedAt } from '@/store/session'
+import { sessionCompacting } from '@/store/compaction'
+import { sessionAwaitingInput } from '@/store/prompts'
+import { $turnStartedAt } from '@/store/session'
 
 const StatusRow: FC<{ children: ReactNode; label: string } & React.ComponentPropsWithoutRef<'div'>> = ({
   children,
@@ -37,11 +38,23 @@ const CompactionHint: FC = () => (
   <span className="shimmer min-w-0 truncate text-muted-foreground/55">{COMPACTION_LABEL}</span>
 )
 
-function useActiveTurnTimerKey(): string | undefined {
-  const activeSessionId = useStore($activeSessionId)
+/** These indicators render inside whichever transcript mounted them, so every
+ *  session-scoped signal comes from that surface's view — a tile must never
+ *  show the primary chat's compaction, prompt-wait, or turn timer. */
+function useThreadSessionStatus() {
+  const sessionId = useStore(useSessionView().$runtimeId)
   const turnStartedAt = useStore($turnStartedAt)
+  const compacting = useStore(useMemo(() => sessionCompacting(sessionId), [sessionId]))
+  // A pending clarify / approval / sudo / secret means the turn is paused on the
+  // user, not working — so don't resurrect the "thinking" timer while they
+  // decide (matches the pet's awaitingInput pose taking priority over busy).
+  const awaitingInput = useStore(useMemo(() => sessionAwaitingInput(sessionId), [sessionId]))
 
-  return activeSessionId && turnStartedAt ? `turn:${activeSessionId}:${turnStartedAt}` : undefined
+  return {
+    awaitingInput,
+    compacting,
+    turnTimerKey: sessionId && turnStartedAt ? `turn:${sessionId}:${turnStartedAt}` : undefined
+  }
 }
 
 export const CenteredThreadSpinner: FC = () => {
@@ -67,9 +80,8 @@ export const CenteredThreadSpinner: FC = () => {
 
 export const ResponseLoadingIndicator: FC = () => {
   const { t } = useI18n()
-  const timerKey = useActiveTurnTimerKey()
-  const elapsed = useElapsedSeconds(true, timerKey)
-  const compacting = useStore($compactionActive)
+  const { compacting, turnTimerKey } = useThreadSessionStatus()
+  const elapsed = useElapsedSeconds(true, turnTimerKey)
 
   return (
     <StatusRow
@@ -147,12 +159,7 @@ export const StreamStallIndicator: FC = () => {
   // what lets the timer read "quiet for 12s" rather than the age of this
   // component, which is the whole turn so far.
   const [quietSince, setQuietSince] = useState<number | undefined>(undefined)
-  const compacting = useStore($compactionActive)
-  const turnTimerKey = useActiveTurnTimerKey()
-  // A pending clarify / approval / sudo / secret means the turn is paused on the
-  // user, not working — so don't resurrect the "thinking" timer while they
-  // decide (matches the pet's awaitingInput pose taking priority over busy).
-  const awaitingInput = useStore($activeSessionAwaitingInput)
+  const { awaitingInput, compacting, turnTimerKey } = useThreadSessionStatus()
 
   useEffect(() => {
     setQuietSince(undefined)

apps/desktop/src/components/assistant-ui/tool/fallback-preview-scope.test.tsx

@@ -0,0 +1,76 @@
+import { cleanup, render } from '@testing-library/react'
+import { atom } from 'nanostores'
+import type { ComponentProps, ReactNode } from 'react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { type SessionView, SessionViewProvider } from '@/app/chat/session-view'
+import { $previewStatusBySession } from '@/store/preview-status'
+import { $activeSessionId, $currentCwd } from '@/store/session'
+
+vi.mock('@assistant-ui/react', async importOriginal => ({
+  ...(await importOriginal<Record<string, unknown>>()),
+  useAuiState: (select: (state: unknown) => unknown) =>
+    select({ message: { id: 'msg-1', status: { type: 'complete' } }, thread: { isRunning: false } })
+}))
+
+const { ToolFallback } = await import('./fallback')
+
+const PRIMARY_ID = 'primary-session'
+const TILE_ID = 'tile-session'
+
+/** Minimal tile view: only the fields the tool row reads. */
+function tileView(): SessionView {
+  return {
+    ...({} as SessionView),
+    $cwd: atom('/tile/work'),
+    $messages: atom([]),
+    $runtimeId: atom<null | string>(TILE_ID),
+    kind: 'tile'
+  }
+}
+
+function renderToolRow(wrap: (node: ReactNode) => ReactNode) {
+  const props = {
+    args: { path: '/tile/work/report.html' },
+    result: { path: '/tile/work/report.html' },
+    toolCallId: 'call-1',
+    toolName: 'write_file'
+  } as unknown as ComponentProps<typeof ToolFallback>
+
+  render(<>{wrap(<ToolFallback {...props} />)}</>)
+}
+
+afterEach(() => {
+  cleanup()
+  $previewStatusBySession.set({})
+  $activeSessionId.set(null)
+  $currentCwd.set('')
+})
+
+describe('tool row preview recording', () => {
+  // The row used to record under the global (primary-only) $activeSessionId, so
+  // a preview produced inside a session TILE surfaced in the main chat's
+  // composer instead of the tile's own.
+  it('records into the session whose transcript the row is in, not the primary', () => {
+    $activeSessionId.set(PRIMARY_ID)
+    $currentCwd.set('/primary/work')
+
+    const view = tileView()
+
+    renderToolRow(node => <SessionViewProvider value={view}>{node}</SessionViewProvider>)
+
+    const recorded = $previewStatusBySession.get()
+
+    expect(Object.keys(recorded)).toEqual([TILE_ID])
+    expect(recorded[TILE_ID]?.[0]?.cwd).toBe('/tile/work')
+  })
+
+  it('still records into the primary session for the main chat', () => {
+    $activeSessionId.set(PRIMARY_ID)
+    $currentCwd.set('/primary/work')
+
+    renderToolRow(node => node)
+
+    expect(Object.keys($previewStatusBySession.get())).toEqual([PRIMARY_ID])
+  })
+})

apps/desktop/src/components/assistant-ui/tool/fallback.tsx

@@ -16,6 +16,7 @@ import {
   useState
 } from 'react'
 
+import { useSessionView } from '@/app/chat/session-view'
 import { AnsiText } from '@/components/assistant-ui/ansi-text'
 import { useElapsedSeconds } from '@/components/chat/activity-timer'
 import { ActivityTimerText } from '@/components/chat/activity-timer-text'
@@ -39,7 +40,6 @@ import { normalize } from '@/lib/text'
 import { useEnterAnimation } from '@/lib/use-enter-animation'
 import { cn } from '@/lib/utils'
 import { recordPreviewArtifact } from '@/store/preview-status'
-import { $activeSessionId, $currentCwd } from '@/store/session'
 import { $toolInlineDiff } from '@/store/tool-diffs'
 import { $toolRowDismissed, dismissToolRow } from '@/store/tool-dismiss'
 import { $toolDisclosureOpen, $toolViewMode, setToolDisclosureOpen } from '@/store/tool-view'
@@ -364,9 +364,12 @@ function ToolEntry({ part }: ToolEntryProps) {
 
   // Surface a previewable artifact (HTML file / localhost URL) as a compact link
   // in the composer status stack rather than a bulky inline card. Uses the same
-  // detected target the old inline card did, keyed to the active session the
-  // stack reads from. Idempotent + dedup'd, so re-renders don't churn.
+  // detected target the old inline card did. Idempotent + dedup'd, so re-renders
+  // don't churn.
   const previewTarget = view.previewTarget
+  // The session whose transcript this row is IN, which is not necessarily the
+  // primary one: a tool row inside a session tile must feed that tile's composer.
+  const { $cwd: $sessionCwd, $runtimeId: $sessionRuntimeId } = useSessionView()
 
   useEffect(() => {
     if (isPending || !previewTarget || !isPreviewableTarget(previewTarget)) {
@@ -376,12 +379,12 @@ function ToolEntry({ part }: ToolEntryProps) {
     // Read (don't subscribe) session/cwd: this only fires when a previewable
     // target appears, and subscribing re-rendered every tool row on any session
     // or cwd change.
-    const activeSessionId = $activeSessionId.get()
+    const sessionId = $sessionRuntimeId.get()
 
-    if (activeSessionId) {
-      recordPreviewArtifact(activeSessionId, previewTarget, $currentCwd.get() || '')
+    if (sessionId) {
+      recordPreviewArtifact(sessionId, previewTarget, $sessionCwd.get() || '')
     }
-  }, [isPending, previewTarget])
+  }, [$sessionCwd, $sessionRuntimeId, isPending, previewTarget])
 
   const detailSections = useMemo(() => {
     if (!view.detail) {

apps/desktop/src/components/chat/preview-attachment.tsx

@@ -1,33 +1,28 @@
 import { useStore } from '@nanostores/react'
 import { useEffect, useRef, useState } from 'react'
 
+import { useSessionView } from '@/app/chat/session-view'
 import { useI18n } from '@/i18n'
 import { MonitorPlay } from '@/lib/icons'
 import { normalizeOrLocalPreviewTarget } from '@/lib/local-preview'
 import { previewName } from '@/lib/preview-targets'
 import { notifyError } from '@/store/notifications'
-import {
-  $previewTarget,
-  dismissPreviewTarget,
-  type PreviewRecordSource,
-  setCurrentSessionPreviewTarget
-} from '@/store/preview'
-import { $currentCwd } from '@/store/session'
+import { $previewTabSources, closePreviewForSource, openPreview, type PreviewRecordSource } from '@/store/preview'
 
 export function PreviewAttachment({ source = 'manual', target }: { source?: PreviewRecordSource; target: string }) {
   const { t } = useI18n()
-  const cwd = useStore($currentCwd)
-  const activePreview = useStore($previewTarget)
+  // This link lives in one session's transcript; resolve it against THAT
+  // session's cwd, not the primary chat's.
+  const cwd = useStore(useSessionView().$cwd)
+  const openSources = useStore($previewTabSources)
   const [opening, setOpening] = useState(false)
-  const activePreviewRef = useRef(activePreview)
   const cwdRef = useRef(cwd)
   const mountedRef = useRef(false)
   const requestTokenRef = useRef(0)
   const targetRef = useRef(target)
   const name = previewName(target)
-  const isActive = activePreview?.source === target
+  const isActive = openSources.includes(target)
 
-  activePreviewRef.current = activePreview
   cwdRef.current = cwd
   targetRef.current = target
 
@@ -53,7 +48,7 @@ export function PreviewAttachment({ source = 'manual', target }: { source?: Prev
     }
 
     if (isActive) {
-      dismissPreviewTarget()
+      closePreviewForSource(target)
 
       return
     }
@@ -80,13 +75,7 @@ export function PreviewAttachment({ source = 'manual', target }: { source?: Prev
         throw new Error(`Could not open preview target: ${requestTarget}`)
       }
 
-      const currentPreview = activePreviewRef.current
-
-      if (currentPreview?.source === preview.source && currentPreview.url === preview.url) {
-        return
-      }
-
-      setCurrentSessionPreviewTarget(preview, source, requestTarget)
+      openPreview(preview, source)
     } catch (error) {
       if (
         !mountedRef.current ||