diff --git a/apps/desktop/src/app/chat/index.tsx b/apps/desktop/src/app/chat/index.tsx
index 7a0c0a2674c30..1bb98203b5dff 100644
--- a/apps/desktop/src/app/chat/index.tsx
+++ b/apps/desktop/src/app/chat/index.tsx
@@ -8,6 +8,7 @@ import { useLocation } from 'react-router'
 
 import type { SubmitTextOptions } from '@/app/session/hooks/use-prompt-actions/utils'
 import { Thread } from '@/components/assistant-ui/thread'
+import { TranscriptWindowProvider } from '@/components/assistant-ui/thread/transcript-window'
 import { Backdrop } from '@/components/Backdrop'
 import { COMPOSER_HEART_CONFIG, HeartField } from '@/components/chat/vibe-hearts'
 import { usePaneVisible } from '@/components/pane-shell/pane-visibility'
@@ -63,6 +64,7 @@ import { ScrollToBottomButton } from './scroll-to-bottom-button'
 import { useSessionView } from './session-view'
 import { SessionActionsMenu } from './sidebar/session-actions-menu'
 import { threadLoadingState } from './thread-loading'
+import { selectTranscriptWindow } from './transcript-window'
 
 interface ChatViewProps extends Omit<React.ComponentProps<'div'>, 'onSubmit'> {
   gateway: HermesGateway | null
@@ -220,9 +222,34 @@ function ChatRuntimeBoundary({
   onThreadMessagesChange,
   suppressMessages
 }: ChatRuntimeBoundaryProps) {
-  const storeMessages = useMessagesWhileVisible(useSessionView().$messages)
+  const view = useSessionView()
+  const runtimeId = useStore(view.$runtimeId)
+  const storeMessages = useMessagesWhileVisible(view.$messages)
   const messages = suppressMessages ? NO_MESSAGES : storeMessages
-  const runtimeMessageRepository = useRuntimeMessageRepository(messages)
+
+  const [windowPages, setWindowPages] = useState(1)
+  const [windowSessionKey, setWindowSessionKey] = useState(runtimeId)
+
+  // Reset the window on session swap during RENDER, so a large expand from the
+  // previous chat can't leak into the next one's first paint (#55191).
+  if (windowSessionKey !== runtimeId) {
+    setWindowSessionKey(runtimeId)
+    setWindowPages(1)
+  }
+
+  const { messages: windowedMessages, windowed } = useMemo(
+    () => selectTranscriptWindow(messages, windowPages),
+    [messages, windowPages]
+  )
+
+  const runtimeMessageRepository = useRuntimeMessageRepository(windowedMessages)
+
+  const expandWindow = useCallback(() => setWindowPages(pages => pages + 1), [])
+
+  const transcriptWindow = useMemo(
+    () => ({ olderAvailable: windowed, expandWindow }),
+    [expandWindow, windowed]
+  )
 
   const runtime = useIncrementalExternalStoreRuntime<ThreadMessage>({
     messageRepository: runtimeMessageRepository,
@@ -237,7 +264,11 @@ function ChatRuntimeBoundary({
     onReload
   })
 
-  return <AssistantRuntimeProvider runtime={runtime}>{children}</AssistantRuntimeProvider>
+  return (
+    <TranscriptWindowProvider value={transcriptWindow}>
+      <AssistantRuntimeProvider runtime={runtime}>{children}</AssistantRuntimeProvider>
+    </TranscriptWindowProvider>
+  )
 }
 
 // Memoized: the tile caller (session-tile.tsx) and the contrib surface re-render
diff --git a/apps/desktop/src/app/chat/runtime-repository.test.ts b/apps/desktop/src/app/chat/runtime-repository.test.ts
index ae0d4bd469ef2..de7a97d39c575 100644
--- a/apps/desktop/src/app/chat/runtime-repository.test.ts
+++ b/apps/desktop/src/app/chat/runtime-repository.test.ts
@@ -51,4 +51,32 @@ describe('useRuntimeMessageRepository', () => {
 
     expect(feedToRepository(result.current).map(item => item.id)).toEqual(['user-1', 'assistant-stream-1', 'user-2'])
   })
+
+  it('anchors a branch group to its fork point, and a windowed cut keeps it', () => {
+    // Branch groups record their fork parent the first time they are seen. A
+    // window that started mid-group would anchor the survivors to whatever
+    // preceded them instead — selectTranscriptWindow aligns the cut so the
+    // whole group arrives together (#55191).
+    const branch = (id: string): ChatMessage => ({
+      ...text(id, 'assistant', 'branch'),
+      branchGroupId: 'group-1'
+    })
+
+    const messages = [text('user-1', 'user', 'hi'), branch('a-1'), branch('a-2'), text('user-2', 'user', 'more')]
+
+    const { result } = renderHook(() => useRuntimeMessageRepository(messages))
+
+    const parents = new Map(result.current.messages.map(item => [item.message.id, item.parentId]))
+
+    expect(parents.get('a-1')).toBe('user-1')
+    expect(parents.get('a-2')).toBe('user-1')
+
+    // The same group fed as a window that begins AT the group start keeps the
+    // fork intact (parent becomes null: the group is now the transcript root).
+    const { result: windowed } = renderHook(() => useRuntimeMessageRepository(messages.slice(1)))
+
+    const windowedParents = new Map(windowed.current.messages.map(item => [item.message.id, item.parentId]))
+
+    expect(windowedParents.get('a-1')).toBe(windowedParents.get('a-2'))
+  })
 })
diff --git a/apps/desktop/src/app/chat/transcript-window.test.ts b/apps/desktop/src/app/chat/transcript-window.test.ts
new file mode 100644
index 0000000000000..4726724f68e26
--- /dev/null
+++ b/apps/desktop/src/app/chat/transcript-window.test.ts
@@ -0,0 +1,144 @@
+import { describe, expect, it } from 'vitest'
+
+import type { ChatMessage } from '@/lib/chat-messages'
+import { RENDER_WEIGHT_CHARS } from '@/lib/render-weight'
+
+import {
+  alignToBranchGroup,
+  selectTranscriptWindow,
+  TRANSCRIPT_WINDOW_BUDGET,
+  TRANSCRIPT_WINDOW_MIN_MESSAGES
+} from './transcript-window'
+
+const message = (id: string, chars: number, branchGroupId?: string): ChatMessage => ({
+  id,
+  parts: [{ type: 'text', text: 'x'.repeat(chars) }],
+  role: id.startsWith('u') ? 'user' : 'assistant',
+  ...(branchGroupId ? { branchGroupId } : {})
+})
+
+/** Messages of `chars` each, newest last. */
+const transcript = (count: number, chars: number): ChatMessage[] =>
+  Array.from({ length: count }, (_, i) => message(`m-${i}`, chars))
+
+describe('selectTranscriptWindow', () => {
+  it('does not window a transcript that fits the budget', () => {
+    const messages = transcript(50, 100)
+
+    const window = selectTranscriptWindow(messages)
+
+    expect(window.windowed).toBe(false)
+    // Reference identity preserved — a fresh array would re-render the runtime.
+    expect(window.messages).toBe(messages)
+  })
+
+  it('windows a HEAVY-but-SHORT transcript that a message-count cap would miss', () => {
+    // 40 messages, each a big tool result. Well under any sane count cap, but
+    // this is the shape that exhausts the renderer heap (#55191).
+    const messages = transcript(40, RENDER_WEIGHT_CHARS * 400)
+
+    const window = selectTranscriptWindow(messages)
+
+    expect(window.windowed).toBe(true)
+    expect(window.messages.length).toBeLessThan(messages.length)
+    expect(window.messages.at(-1)).toBe(messages.at(-1))
+  })
+
+  it('keeps far MORE messages when they are light than when they are heavy', () => {
+    // The contract is weight, not count: a message-count cap would treat these
+    // two identically. 500 tiny messages are cheaper than 500 tool results, so
+    // many more of them survive the same budget.
+    const light = selectTranscriptWindow(transcript(500, 20))
+    const heavy = selectTranscriptWindow(transcript(500, RENDER_WEIGHT_CHARS * 40))
+
+    expect(light.messages.length).toBeGreaterThan(heavy.messages.length * 10)
+  })
+
+  it('leaves a long transcript whole when the whole thing is cheap', () => {
+    const messages = transcript(600, 20)
+
+    const window = selectTranscriptWindow(messages)
+
+    expect(window.windowed).toBe(false)
+    expect(window.messages).toBe(messages)
+  })
+
+  it('keeps a floor of messages when single turns are enormous', () => {
+    const messages = transcript(80, RENDER_WEIGHT_CHARS * TRANSCRIPT_WINDOW_BUDGET)
+
+    const window = selectTranscriptWindow(messages)
+
+    expect(window.messages.length).toBeGreaterThanOrEqual(TRANSCRIPT_WINDOW_MIN_MESSAGES)
+  })
+
+  it('grows by one budget page per expand and eventually covers everything', () => {
+    const messages = transcript(400, RENDER_WEIGHT_CHARS * 40)
+
+    const first = selectTranscriptWindow(messages, 1)
+    const second = selectTranscriptWindow(messages, 2)
+
+    expect(first.windowed).toBe(true)
+    expect(second.messages.length).toBeGreaterThan(first.messages.length)
+
+    let pages = 1
+    let window = selectTranscriptWindow(messages, pages)
+
+    while (window.windowed && pages < 100) {
+      window = selectTranscriptWindow(messages, ++pages)
+    }
+
+    // Paging terminates at the full transcript — never a dead end.
+    expect(window.windowed).toBe(false)
+    expect(window.messages).toHaveLength(messages.length)
+  })
+
+  it('never cuts inside a branch group, so branches keep their fork point', () => {
+    const heavy = RENDER_WEIGHT_CHARS * 200
+
+    // A branch group sits right where a weight-only cut would land.
+    const messages: ChatMessage[] = [
+      ...transcript(20, heavy),
+      message('a-branch-1', heavy, 'group-1'),
+      message('a-branch-2', heavy, 'group-1'),
+      message('a-branch-3', heavy, 'group-1'),
+      ...transcript(20, heavy).map(m => ({ ...m, id: `tail-${m.id}` }))
+    ]
+
+    for (let pages = 1; pages <= 6; pages++) {
+      const kept = selectTranscriptWindow(messages, pages).messages
+      const groupMembers = kept.filter(m => m.branchGroupId === 'group-1')
+
+      // Either the whole group survives or none of it does — never a partial
+      // group, which would re-parent the surviving branches.
+      expect([0, 3]).toContain(groupMembers.length)
+    }
+  })
+
+  it('handles an empty transcript', () => {
+    const messages: ChatMessage[] = []
+
+    expect(selectTranscriptWindow(messages)).toEqual({ messages, windowed: false })
+  })
+})
+
+describe('alignToBranchGroup', () => {
+  const messages = [
+    message('u-1', 10),
+    message('a-1', 10, 'g'),
+    message('a-2', 10, 'g'),
+    message('u-2', 10)
+  ]
+
+  it('widens a cut that lands mid-group back to the group start', () => {
+    expect(alignToBranchGroup(messages, 2)).toBe(1)
+  })
+
+  it('leaves a cut on a non-branch message alone', () => {
+    expect(alignToBranchGroup(messages, 3)).toBe(3)
+  })
+
+  it('clamps out-of-range indices', () => {
+    expect(alignToBranchGroup(messages, -5)).toBe(0)
+    expect(alignToBranchGroup(messages, 99)).toBe(messages.length)
+  })
+})
diff --git a/apps/desktop/src/app/chat/transcript-window.ts b/apps/desktop/src/app/chat/transcript-window.ts
new file mode 100644
index 0000000000000..29d2d1c9d2783
--- /dev/null
+++ b/apps/desktop/src/app/chat/transcript-window.ts
@@ -0,0 +1,111 @@
+import type { ChatMessage } from '@/lib/chat-messages'
+import { messageRenderWeight } from '@/lib/render-weight'
+
+/**
+ * Bound what reaches assistant-ui at all.
+ *
+ * Rendering the full transcript of an oversized session rebuilds an unbounded
+ * runtime repository on every store update and exhausts the renderer's V8 heap
+ * (#55191). The DOM budget in `thread/list.tsx` already bounds what PAINTS, but
+ * every message still gets normalized into the repository first — so a session
+ * only has to be heavy, not visible, to crash the window.
+ *
+ * The window spends the same currency as the DOM budget: render weight, not
+ * message count. A count cap gets this wrong in both directions — measured on a
+ * real 1,175-session store, a 400-message cap would disable itself on 37
+ * sessions that are heavy but short (one was 133 messages / 1.05MB) while
+ * engaging on 92 long-but-light sessions that were never at risk.
+ */
+
+/**
+ * One window page, in render-weight units.
+ *
+ * Four DOM pages (the `RENDER_BUDGET` of 300 in `thread/list.tsx`). "Show
+ * earlier" spends the DOM budget first, so the user pages through the
+ * already-materialized window three times before this asks the store for more
+ * — and the reported crash shape (~231K tokens ≈ 2,260 units) is windowed
+ * rather than handed to the repository whole.
+ */
+export const TRANSCRIPT_WINDOW_BUDGET = 1200
+
+/**
+ * Floor on messages kept regardless of weight. A transcript of enormous turns
+ * must still render the turn the user is having; without this a single
+ * multi-megabyte tool result could window everything after it away.
+ */
+export const TRANSCRIPT_WINDOW_MIN_MESSAGES = 30
+
+export interface TranscriptWindow {
+  /** The tail assistant-ui is allowed to materialize. */
+  messages: ChatMessage[]
+  /** Store holds older messages than this window. */
+  windowed: boolean
+}
+
+/**
+ * Widen a cut backwards so it never lands inside an assistant branch group.
+ *
+ * `useRuntimeMessageRepository` records a group's fork point the first time it
+ * sees the group (`branchParentByGroup`). A cut through the middle of a group
+ * therefore anchors the surviving branches to whatever message happens to
+ * precede them in the window — silently re-parenting a branch. Include the
+ * whole group or none of it.
+ */
+export function alignToBranchGroup(messages: readonly ChatMessage[], start: number): number {
+  if (start <= 0 || start >= messages.length) {
+    return Math.max(0, Math.min(start, messages.length))
+  }
+
+  const group = messages[start].branchGroupId
+
+  if (!group) {
+    return start
+  }
+
+  let aligned = start
+
+  while (aligned > 0 && messages[aligned - 1].branchGroupId === group) {
+    aligned--
+  }
+
+  return aligned
+}
+
+/**
+ * Select the tail of the transcript that fits one window, grown by `pages`.
+ *
+ * Walks newest-first accumulating weight until the budget is met, keeps at
+ * least MIN messages, then aligns the cut off a branch-group boundary.
+ */
+export function selectTranscriptWindow(
+  messages: readonly ChatMessage[],
+  pages = 1
+): TranscriptWindow {
+  const budget = TRANSCRIPT_WINDOW_BUDGET * Math.max(1, Math.floor(pages))
+
+  if (messages.length === 0) {
+    return { messages: messages as ChatMessage[], windowed: false }
+  }
+
+  let start = messages.length
+  let weight = 0
+
+  for (let i = messages.length - 1; i >= 0; i--) {
+    weight += messageRenderWeight(messages[i].parts)
+    start = i
+
+    if (weight >= budget && messages.length - i >= TRANSCRIPT_WINDOW_MIN_MESSAGES) {
+      break
+    }
+  }
+
+  start = alignToBranchGroup(messages, start)
+
+  if (start <= 0) {
+    // Preserve reference identity when the whole transcript fits: handing React
+    // a fresh array of the same messages re-renders the runtime for nothing.
+    return { messages: messages as ChatMessage[], windowed: false }
+  }
+
+  return { messages: messages.slice(start), windowed: true }
+}
diff --git a/apps/desktop/src/components/assistant-ui/thread/list.test.ts b/apps/desktop/src/components/assistant-ui/thread/list.test.ts
index c0cd1da5588a4..a44314d154ffc 100644
--- a/apps/desktop/src/components/assistant-ui/thread/list.test.ts
+++ b/apps/desktop/src/components/assistant-ui/thread/list.test.ts
@@ -1,5 +1,7 @@
 import { describe, expect, it } from 'vitest'
 
+import { messageRenderWeight, RENDER_WEIGHT_CHARS } from '@/lib/render-weight'
+
 import {
   buildGroups,
   firstVisibleGroupIndex,
@@ -7,8 +9,6 @@ import {
   LIVE_TAIL_PARTS,
   liveTailStart,
   type MessageGroup,
-  messageRenderWeight,
-  RENDER_WEIGHT_CHARS,
   resolveThreadScrollTarget
 } from './list'
 
diff --git a/apps/desktop/src/components/assistant-ui/thread/list.tsx b/apps/desktop/src/components/assistant-ui/thread/list.tsx
index d8fd0f0a5c665..304b3eb3ec017 100644
--- a/apps/desktop/src/components/assistant-ui/thread/list.tsx
+++ b/apps/desktop/src/components/assistant-ui/thread/list.tsx
@@ -16,6 +16,7 @@ import {
 import { type GetTargetScrollTop, useStickToBottom } from 'use-stick-to-bottom'
 
 import { useI18n } from '@/i18n'
+import { messageRenderWeight } from '@/lib/render-weight'
 import { cn } from '@/lib/utils'
 import {
   onScrollToBottomRequest,
@@ -28,6 +29,8 @@ import { isSecondaryWindow } from '@/store/windows'
 
 import { MessageRenderBoundary } from '../message-render-boundary'
 
+import { resolveShowEarlierAction, useTranscriptWindow } from './transcript-window'
+
 type ThreadMessageComponents = ComponentProps<typeof ThreadPrimitive.MessageByIndex>['components']
 
 export type MessageGroup = { id: string; weight: number } & (
@@ -47,8 +50,6 @@ export type MessageGroup = { id: string; weight: number } & (
 // a virtualizer — pure rendering, never touches scrollTop, so it can't fight
 // use-stick-to-bottom (the single scroll owner).
 const RENDER_BUDGET = 300
-export const RENDER_WEIGHT_CHARS = 512
-const MAX_MEASURED_MESSAGE_CHARS = RENDER_BUDGET * RENDER_WEIGHT_CHARS
 // On session switch, paint a small budget first (enough for the bottom turn(s)
 // the user actually sees after scroll-to-bottom), then bump to the full budget
 // in a requestAnimationFrame — defers the heavy markdown+syntax-highlight render
@@ -76,70 +77,6 @@ export const resolveThreadScrollTarget: GetTargetScrollTop = (targetScrollTop, {
   return remaining >= 0 && remaining <= SCROLL_TARGET_EPSILON_PX ? currentScrollTop : targetScrollTop
 }
 
-const contentWeightCache = new WeakMap<object, number>()
-const NON_RENDERED_CONTENT_FIELDS = new Set(['id', 'role', 'toolCallId', 'toolName', 'type'])
-
-/**
- * Estimate the synchronous renderer cost of one assistant-ui message.
- *
- * The traversal is capped once a single message has enough text to consume a
- * complete render page. Going further cannot affect which whole turn crosses
- * the budget, and avoiding an unbounded walk matters for deeply nested tool
- * payloads. A WeakMap keeps settled history O(message count) on later store
- * updates; assistant-ui publishes a new content array when a streaming message
- * changes, so the live tail still receives a fresh weight.
- */
-export function messageRenderWeight(content: unknown): number {
-  if (!Array.isArray(content)) {
-    return 1
-  }
-
-  const cached = contentWeightCache.get(content)
-
-  if (cached !== undefined) {
-    return cached
-  }
-
-  const seen = new WeakSet<object>()
-  const pending: unknown[] = [...content]
-  let characters = 0
-
-  while (pending.length > 0 && characters < MAX_MEASURED_MESSAGE_CHARS) {
-    const value = pending.pop()
-
-    if (typeof value === 'string') {
-      characters += Math.min(value.length, MAX_MEASURED_MESSAGE_CHARS - characters)
-
-      continue
-    }
-
-    if (!value || typeof value !== 'object' || seen.has(value)) {
-      continue
-    }
-
-    seen.add(value)
-
-    if (Array.isArray(value)) {
-      for (const nested of value) {
-        pending.push(nested)
-      }
-
-      continue
-    }
-
-    for (const [key, nested] of Object.entries(value)) {
-      if (!NON_RENDERED_CONTENT_FIELDS.has(key)) {
-        pending.push(nested)
-      }
-    }
-  }
-
-  const weight = Math.max(1, content.length) + Math.ceil(characters / RENDER_WEIGHT_CHARS)
-  contentWeightCache.set(content, weight)
-
-  return weight
-}
-
 interface ThreadMessageListProps {
   clampToComposer: boolean
   components: ThreadMessageComponents
@@ -315,6 +252,8 @@ const ThreadMessageListInner: FC<ThreadMessageListProps> = ({
     targetScrollTop: resolveThreadScrollTarget
   })
 
+  const { olderAvailable, expandWindow } = useTranscriptWindow()
+
   const [renderBudget, setRenderBudget] = useState(FIRST_PAINT_BUDGET)
 
   // Cut the budget during RENDER, not in the post-commit layout effect. An
@@ -540,11 +479,26 @@ const ThreadMessageListInner: FC<ThreadMessageListProps> = ({
 
   // Prepend an older page while preserving the on-screen position. The user is
   // scrolled up (reading history) so the stick-to-bottom lock is escaped and
-  // won't fight this manual restore.
+  // won't fight this manual restore. Spend the already-materialized DOM page
+  // first; only when that is exhausted pull more messages out of the session
+  // store (#55191).
   const showEarlier = useCallback(() => {
+    const action = resolveShowEarlierAction(hiddenCount, olderAvailable)
+
+    if (!action) {
+      return
+    }
+
     anchorBeforePrepend()
-    setRenderBudget(budget => budget + RENDER_BUDGET)
-  }, [anchorBeforePrepend])
+
+    if (action === 'dom') {
+      setRenderBudget(budget => budget + RENDER_BUDGET)
+
+      return
+    }
+
+    expandWindow()
+  }, [anchorBeforePrepend, expandWindow, hiddenCount, olderAvailable])
 
   useLayoutEffect(() => {
     const el = scrollRef.current
@@ -553,7 +507,8 @@ const ThreadMessageListInner: FC<ThreadMessageListProps> = ({
       el.scrollTop = el.scrollHeight - restoreFromBottomRef.current
       restoreFromBottomRef.current = null
     }
-  }, [scrollRef, renderBudget])
+    // renderBudget covers DOM pages; groups.length covers store-window expands.
+  }, [scrollRef, renderBudget, groups.length])
 
   // The row array is memoized on the inputs the rows actually read. This
   // component re-renders on every isAtBottom flip — and use-stick-to-bottom
@@ -647,7 +602,7 @@ const ThreadMessageListInner: FC<ThreadMessageListProps> = ({
             data-slot="aui_thread-content"
             ref={contentRef as React.RefCallback<HTMLDivElement>}
           >
-            {hiddenCount > 0 && (
+            {(hiddenCount > 0 || olderAvailable) && (
               <button
                 className="mx-auto mb-(--conversation-turn-gap) rounded-full border border-border/65 bg-(--composer-fill) px-3 py-1 text-xs text-muted-foreground hover:text-foreground"
                 onClick={showEarlier}
diff --git a/apps/desktop/src/components/assistant-ui/thread/transcript-window.test.ts b/apps/desktop/src/components/assistant-ui/thread/transcript-window.test.ts
new file mode 100644
index 0000000000000..ee41e41561d44
--- /dev/null
+++ b/apps/desktop/src/components/assistant-ui/thread/transcript-window.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest'
+
+import { resolveShowEarlierAction } from './transcript-window'
+
+describe('resolveShowEarlierAction', () => {
+  it('spends the already-materialized DOM page first', () => {
+    expect(resolveShowEarlierAction(3, true)).toBe('dom')
+    expect(resolveShowEarlierAction(3, false)).toBe('dom')
+  })
+
+  it('expands the store window once the DOM page is exhausted', () => {
+    expect(resolveShowEarlierAction(0, true)).toBe('window')
+  })
+
+  it('is a no-op when neither DOM nor store has older content', () => {
+    expect(resolveShowEarlierAction(0, false)).toBe(null)
+  })
+})
diff --git a/apps/desktop/src/components/assistant-ui/thread/transcript-window.tsx b/apps/desktop/src/components/assistant-ui/thread/transcript-window.tsx
new file mode 100644
index 0000000000000..331fec191468c
--- /dev/null
+++ b/apps/desktop/src/components/assistant-ui/thread/transcript-window.tsx
@@ -0,0 +1,43 @@
+import { createContext, type ReactNode, useContext } from 'react'
+
+export interface TranscriptWindowValue {
+  /** Store holds older messages the runtime window has not materialized. */
+  olderAvailable: boolean
+  /** Pull one more page of older messages out of the session store. */
+  expandWindow: () => void
+}
+
+const TranscriptWindowContext = createContext<TranscriptWindowValue>({
+  olderAvailable: false,
+  expandWindow: () => {}
+})
+
+export function TranscriptWindowProvider({
+  children,
+  value
+}: {
+  children: ReactNode
+  value: TranscriptWindowValue
+}) {
+  return <TranscriptWindowContext.Provider value={value}>{children}</TranscriptWindowContext.Provider>
+}
+
+export function useTranscriptWindow(): TranscriptWindowValue {
+  return useContext(TranscriptWindowContext)
+}
+
+/**
+ * "Show earlier" pages the DOM budget first and only then asks the store for
+ * more messages — the DOM page is already-materialized content, so spending it
+ * first keeps the click cheap and the store window as small as it can be.
+ */
+export function resolveShowEarlierAction(
+  hiddenCount: number,
+  olderAvailable: boolean
+): 'dom' | 'window' | null {
+  if (hiddenCount > 0) {
+    return 'dom'
+  }
+
+  return olderAvailable ? 'window' : null
+}
diff --git a/apps/desktop/src/lib/render-weight.ts b/apps/desktop/src/lib/render-weight.ts
new file mode 100644
index 0000000000000..258a330d5cc08
--- /dev/null
+++ b/apps/desktop/src/lib/render-weight.ts
@@ -0,0 +1,84 @@
+/**
+ * Render cost of one message's content parts, in budget units.
+ *
+ * Two layers bound long transcripts and both spend the same currency: the
+ * store window (how many messages reach assistant-ui at all) and the DOM page
+ * budget (how many of those actually render). Neither can be a message COUNT —
+ * counting only parts underpriced a 51KB tool result as "1", so a handful of
+ * huge results let a 600KB transcript through the old 300-part cap and drove
+ * Chromium's renderer into a GC crash. Characters approximate markdown
+ * parsing, text-node allocation, and tool-result formatting; parts approximate
+ * component/node count.
+ *
+ * Shared so a heavy-but-short session is bounded by the same rule as a
+ * long-but-light one (#55191).
+ */
+
+export const RENDER_WEIGHT_CHARS = 512
+
+// Stop traversing once a single message has enough text to consume a complete
+// DOM render page. Going further cannot change which whole turn crosses any
+// budget, and avoiding an unbounded walk matters for deeply nested tool
+// payloads.
+const MAX_MEASURED_MESSAGE_CHARS = 300 * RENDER_WEIGHT_CHARS
+
+const contentWeightCache = new WeakMap<object, number>()
+const NON_RENDERED_CONTENT_FIELDS = new Set(['id', 'role', 'toolCallId', 'toolName', 'type'])
+
+/**
+ * Estimate the synchronous renderer cost of one message's content array.
+ *
+ * A WeakMap keeps settled history O(message count) on later store updates;
+ * both assistant-ui and the session store publish a new content array when a
+ * streaming message changes, so the live tail still receives a fresh weight.
+ */
+export function messageRenderWeight(content: unknown): number {
+  if (!Array.isArray(content)) {
+    return 1
+  }
+
+  const cached = contentWeightCache.get(content)
+
+  if (cached !== undefined) {
+    return cached
+  }
+
+  const seen = new WeakSet<object>()
+  const pending: unknown[] = [...content]
+  let characters = 0
+
+  while (pending.length > 0 && characters < MAX_MEASURED_MESSAGE_CHARS) {
+    const value = pending.pop()
+
+    if (typeof value === 'string') {
+      characters += Math.min(value.length, MAX_MEASURED_MESSAGE_CHARS - characters)
+
+      continue
+    }
+
+    if (!value || typeof value !== 'object' || seen.has(value)) {
+      continue
+    }
+
+    seen.add(value)
+
+    if (Array.isArray(value)) {
+      for (const nested of value) {
+        pending.push(nested)
+      }
+
+      continue
+    }
+
+    for (const [key, nested] of Object.entries(value)) {
+      if (!NON_RENDERED_CONTENT_FIELDS.has(key)) {
+        pending.push(nested)
+      }
+    }
+  }
+
+  const weight = Math.max(1, content.length) + Math.ceil(characters / RENDER_WEIGHT_CHARS)
+  contentWeightCache.set(content, weight)
+
+  return weight
+}