diff --git a/src/main/native-chat/agent-session-journal/journal-payload-bounds-retention.test.ts b/src/main/native-chat/agent-session-journal/journal-payload-bounds-retention.test.ts
index 62bfe6e9439..34a99f038a4 100644
--- a/src/main/native-chat/agent-session-journal/journal-payload-bounds-retention.test.ts
+++ b/src/main/native-chat/agent-session-journal/journal-payload-bounds-retention.test.ts
@@ -72,7 +72,7 @@ describe('journal payload allocation bounds', () => {
       body,
       sequence: 1,
       observedAt: 1
-    })
+    }).item
     const encodedStringBytes = from.mock.calls.reduce(
       (largest, [value]) =>
         typeof value === 'string' ? Math.max(largest, Buffer.byteLength(value, 'utf8')) : largest,
diff --git a/src/main/native-chat/agent-session-wire/agent-session-bounded-frame-item.ts b/src/main/native-chat/agent-session-wire/agent-session-bounded-frame-item.ts
index 668f815de2b..1b8e0ebcfcf 100644
--- a/src/main/native-chat/agent-session-wire/agent-session-bounded-frame-item.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-bounded-frame-item.ts
@@ -5,6 +5,8 @@ import type { AgentJournalRenderItem } from '../../../shared/agent-session-journ
 import { boundJournalKeyComponent } from '../../../shared/agent-session-journal-item-key'
 import {
   BOUNDED_FRAME_CONTENT_BUDGET_BYTES,
+  measureHistoryItem,
+  type HistoryItemForm,
   oversizedHistoryItem
 } from './agent-session-history-page-bounds'
 import { shortenedAgentJournalItem } from './agent-session-item-wire-form'
@@ -36,17 +38,16 @@ function oversizedFrameItem(item: AgentJournalRenderItem): AgentJournalRenderIte
   }
 }
 
-function boundFrameItem(item: AgentJournalRenderItem): AgentJournalRenderItem {
-  return shortenedAgentJournalItem(item, BOUNDED_FRAME_ROW_BYTES) ?? oversizedFrameItem(item)
-}
-
 // Items are immutable per revision, so a catch-up or a second subscriber reuses the bound form.
-const bounded = new WeakMap<AgentJournalRenderItem, AgentJournalRenderItem>()
+const bounded = new WeakMap<AgentJournalRenderItem, HistoryItemForm>()
 
-export function boundedFrameItem(item: AgentJournalRenderItem): AgentJournalRenderItem {
+export function boundedFrameItem(formed: HistoryItemForm): HistoryItemForm {
+  const { item, bytes } = formed
   let result = bounded.get(item)
   if (result === undefined) {
-    result = boundFrameItem(item)
+    const fitted =
+      shortenedAgentJournalItem(item, BOUNDED_FRAME_ROW_BYTES, bytes) ?? oversizedFrameItem(item)
+    result = fitted === item ? formed : measureHistoryItem(fitted)
     bounded.set(item, result)
   }
   return result
diff --git a/src/main/native-chat/agent-session-wire/agent-session-bounded-frames.test.ts b/src/main/native-chat/agent-session-wire/agent-session-bounded-frames.test.ts
index 5a9b9b03818..186cefd9c74 100644
--- a/src/main/native-chat/agent-session-wire/agent-session-bounded-frames.test.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-bounded-frames.test.ts
@@ -132,13 +132,15 @@ describe('a bounded row', () => {
       })
     ]
     for (const row of rows) {
-      expect(jsonBytes(boundedFrameItem(row))).toBeLessThanOrEqual(BOUNDED_FRAME_ROW_BYTES)
+      const formed = boundedFrameItem({ item: row, bytes: jsonBytes(row) })
+      expect(formed.bytes).toBe(jsonBytes(formed.item))
+      expect(formed.bytes).toBeLessThanOrEqual(BOUNDED_FRAME_ROW_BYTES)
     }
   })
 
   it('is the stored row when it already fits', () => {
     const row = rendered({ body: message('hello') })
-    expect(boundedFrameItem(row)).toBe(row)
+    expect(boundedFrameItem({ item: row, bytes: jsonBytes(row) }).item).toBe(row)
   })
 })
 
diff --git a/src/main/native-chat/agent-session-wire/agent-session-history-page-bounds.ts b/src/main/native-chat/agent-session-wire/agent-session-history-page-bounds.ts
index 588ca592dfe..6c65e2bd3c6 100644
--- a/src/main/native-chat/agent-session-wire/agent-session-history-page-bounds.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-history-page-bounds.ts
@@ -65,10 +65,19 @@ export function oversizedHistoryItem(item: AgentJournalRenderItem): AgentJournal
   }
 }
 
+export type HistoryItemForm = {
+  item: AgentJournalRenderItem
+  bytes: number
+}
+
+export function measureHistoryItem(item: AgentJournalRenderItem): HistoryItemForm {
+  return { item, bytes: Buffer.byteLength(JSON.stringify(item), 'utf8') }
+}
+
 /** How a reader receives each item, and the bytes its page may hold of them. A page is measured
  *  in that form, so it carries what it will actually send. */
 export type HistoryReader = {
-  form: (item: AgentJournalRenderItem) => AgentJournalRenderItem
+  form: (item: AgentJournalRenderItem) => HistoryItemForm
   budgetBytes: number
 }
 
@@ -78,14 +87,18 @@ export function boundHistoryItemsByBytes(
   keep: 'newest' | 'oldest',
   submissionBytes: ReadonlyMap<string, number>,
   maxBytes: number,
-  form: HistoryReader['form'] = (item) => item
+  form: HistoryReader['form'] = measureHistoryItem
 ): { items: AgentJournalRenderItem[]; dropped: number } {
   const ordered = groupItemsBySequence(items, keep)
   const kept: AgentJournalRenderItem[][] = []
   let total = 0
   for (const stored of ordered) {
-    const group = stored.map(form)
-    const bytes = group.reduce((sum, item) => sum + historyEntryBytes(item, submissionBytes), 0)
+    const formed = stored.map(form)
+    const group = formed.map(({ item }) => item)
+    const bytes = formed.reduce(
+      (sum, { item, bytes }) => sum + bytes + (submissionBytes.get(item.itemId) ?? 0),
+      0
+    )
     if (kept.length === 0 && bytes > maxBytes) {
       kept.push(group.map((item) => oversizedHistoryItem(item)))
       break
diff --git a/src/main/native-chat/agent-session-wire/agent-session-history-page.ts b/src/main/native-chat/agent-session-wire/agent-session-history-page.ts
index 1189ce1e0cf..07aa122c2c8 100644
--- a/src/main/native-chat/agent-session-wire/agent-session-history-page.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-history-page.ts
@@ -32,7 +32,8 @@ import { CLIENT_HISTORY_READER } from './agent-session-item-wire-form'
 import {
   boundHistoryItemsByBytes,
   type HistoryReader,
-  historyEntryBytes,
+  type HistoryItemForm,
+  measureHistoryItem,
   newestWholeSequenceGroups,
   wholeSubagentGroupsStart,
   oversizedHistoryItem,
@@ -235,10 +236,13 @@ function readForward(
   // submissions AND removal ids — a legal pre-bounding tombstone id can dwarf
   // every item on the page.
   const pageContentBytes = (
-    items: readonly AgentJournalRenderItem[],
+    items: readonly HistoryItemForm[],
     removedItemIds: readonly string[]
   ): number =>
-    items.reduce((total, item) => total + historyEntryBytes(item, submissionBytes), 0) +
+    items.reduce(
+      (total, { item, bytes }) => total + bytes + (submissionBytes.get(item.itemId) ?? 0),
+      0
+    ) +
     removedItemIds.reduce(
       (total, itemId) => total + Buffer.byteLength(JSON.stringify(itemId), 'utf8') + 1,
       0
@@ -247,7 +251,7 @@ function readForward(
   // clipping projected items: dropping an item while advancing the cursor past
   // the rows that touched it would lose that revision for good.
   let rows = since.rows.slice(0, limit)
-  const project = (window: typeof rows): ReturnType<typeof projectJournalBatch> => {
+  const project = (window: typeof rows) => {
     const result = projectJournalBatch({
       rows: window,
       snapshot,
@@ -275,9 +279,11 @@ function readForward(
   // One row can still touch an over-budget item; degrade it visibly.
   let items = projected.batch.items
   if (contentBytes > reader.budgetBytes) {
-    items = items.map((item) => {
-      const bytes = historyEntryBytes(item, submissionBytes)
-      return bytes > reader.budgetBytes ? oversizedHistoryItem(item) : item
+    items = items.map((formed) => {
+      const bytes = formed.bytes + (submissionBytes.get(formed.item.itemId) ?? 0)
+      return bytes > reader.budgetBytes
+        ? measureHistoryItem(oversizedHistoryItem(formed.item))
+        : formed
     })
     contentBytes = pageContentBytes(items, projected.batch.removedItemIds)
   }
@@ -295,7 +301,7 @@ function readForward(
     page: buildPage({
       snapshot,
       direction: 'after',
-      items,
+      items: items.map(({ item }) => item),
       removedItemIds: projected.batch.removedItemIds,
       // Reading after a position means there is something before it.
       hasOlder: cursor.sequence > 0,
diff --git a/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.test.ts b/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.test.ts
index 801178f6d0b..068c28220c2 100644
--- a/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.test.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.test.ts
@@ -7,7 +7,9 @@ import { agentJournalItemForWire, shortenedAgentJournalItem } from './agent-sess
 it('preserves the full previously readable desktop message range', () => {
   const item: AgentJournalRenderItem = { itemId: 'message', revision: 1, sequence: 1, observedAt: 1,
     body: { kind: 'message', role: 'assistant', blocks: [{ type: 'text', text: `${'x'.repeat(1_200_000)}TAIL` }] } }
-  expect(agentJournalItemForWire(item)).toBe(item)
+  const formed = agentJournalItemForWire(item)
+  expect(formed.item).toBe(item)
+  expect(formed.bytes).toBe(Buffer.byteLength(JSON.stringify(item), 'utf8'))
 })
 
 it.each([73_728, HISTORY_PAGE_CONTENT_BUDGET_BYTES, 4 * 1024 * 1024])('keeps tool identity and output when cardinality exceeds %i bytes', (budget) => {
diff --git a/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.ts b/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.ts
index 8ba2b25d0f3..4d99b701c3d 100644
--- a/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-item-wire-form.ts
@@ -13,6 +13,7 @@ import type {
 import { boundToolInput, JOURNAL_RUNNING_PREVIEW_LIMITS, utf8Head } from '../agent-session-journal/journal-payload-bounds'
 import {
   HISTORY_PAGE_CONTENT_BUDGET_BYTES,
+  type HistoryItemForm,
   type HistoryReader
 } from './agent-session-history-page-bounds'
 import { jsonBytes, shortenJsonStringsToFit } from './json-string-shortening'
@@ -55,9 +56,10 @@ function wireBody(body: AgentJournalItemBody): AgentJournalItemBody {
  *  that cannot fit, which the page answers with its too-large marker. */
 export function shortenedAgentJournalItem(
   item: AgentJournalRenderItem,
-  maxBytes: number
+  maxBytes: number,
+  bytes = jsonBytes(item)
 ): AgentJournalRenderItem | null {
-  if (jsonBytes(item) <= maxBytes) {
+  if (bytes <= maxBytes) {
     return item
   }
   const budget = maxBytes - (jsonBytes({ ...item, body: null, shortened: true }) - jsonBytes(null))
@@ -71,17 +73,19 @@ export function shortenedAgentJournalItem(
   return body === null ? null : { ...item, body, shortened: true }
 }
 
-function wireItem(item: AgentJournalRenderItem): AgentJournalRenderItem {
+function wireItem(item: AgentJournalRenderItem): HistoryItemForm {
   const body = wireBody(item.body)
   const sent = body === item.body ? item : { ...item, body, shortened: true as const }
-  return shortenedAgentJournalItem(sent, CLIENT_ITEM_MAX_BYTES) ?? sent
+  const bytes = jsonBytes(sent)
+  const fitted = shortenedAgentJournalItem(sent, CLIENT_ITEM_MAX_BYTES, bytes) ?? sent
+  return { item: fitted, bytes: fitted === sent ? bytes : jsonBytes(fitted) }
 }
 
 // Items are immutable per revision, so a catch-up or a second subscriber reuses the sent form.
-const sentForms = new WeakMap<AgentJournalRenderItem, AgentJournalRenderItem>()
+const sentForms = new WeakMap<AgentJournalRenderItem, HistoryItemForm>()
 
 /** What a client is sent of `item`. */
-export function agentJournalItemForWire(item: AgentJournalRenderItem): AgentJournalRenderItem {
+export function agentJournalItemForWire(item: AgentJournalRenderItem): HistoryItemForm {
   let sent = sentForms.get(item)
   if (sent === undefined) {
     sent = wireItem(item)
diff --git a/src/main/native-chat/agent-session-wire/agent-session-stop-note-refresh.ts b/src/main/native-chat/agent-session-wire/agent-session-stop-note-refresh.ts
index 793969c7b1f..e8d228e941e 100644
--- a/src/main/native-chat/agent-session-wire/agent-session-stop-note-refresh.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-stop-note-refresh.ts
@@ -1,7 +1,7 @@
 import type { AgentJournalRenderItem } from '../../../shared/agent-session-journal-types'
 import { AGENT_SESSION_HISTORY_MAX_LIMIT } from '../../../shared/agent-session-wire'
 import type { AgentSessionJournal } from '../agent-session-journal/journal-store'
-import { oversizedHistoryItem } from './agent-session-history-page-bounds'
+import { measureHistoryItem, oversizedHistoryItem } from './agent-session-history-page-bounds'
 import { emptyAgentSessionBatch } from './agent-session-empty-batch'
 import { subscriberHistoryReader } from './agent-session-bounded-frames'
 import {
@@ -69,16 +69,16 @@ export function refreshDerivedStopNotes(
       continue
     }
     const formed = form(item)
-    const projectedBytes = Buffer.byteLength(JSON.stringify(formed), 'utf8')
-    const bounded = projectedBytes > budgetBytes ? oversizedHistoryItem(formed) : formed
-    const itemBytes = Buffer.byteLength(JSON.stringify(bounded), 'utf8') + 1
+    const bounded =
+      formed.bytes > budgetBytes ? measureHistoryItem(oversizedHistoryItem(formed.item)) : formed
+    const itemBytes = bounded.bytes + 1
     if (
       pending.length > 0 &&
       (pending.length === AGENT_SESSION_HISTORY_MAX_LIMIT || bytes + itemBytes > budgetBytes)
     ) {
       emit()
     }
-    pending.push(bounded)
+    pending.push(bounded.item)
     bytes += itemBytes
   }
   if (pending.length > 0 && port.isActive(subscriber)) {