diff --git a/src/main/ai-vault/session-transcript-message-content.test.ts b/src/main/ai-vault/session-transcript-message-content.test.ts
index c540335e415..d09d66ef411 100644
--- a/src/main/ai-vault/session-transcript-message-content.test.ts
+++ b/src/main/ai-vault/session-transcript-message-content.test.ts
@@ -1,7 +1,9 @@
-import { expect, it } from 'vitest'
-import { transcriptMessagesFromContent } from './session-transcript-message-content'
+import { expect, it, vi } from 'vitest'
+import { boundedText, transcriptMessagesFromContent } from './session-transcript-message-content'
+import { asRecord } from './session-scanner-record-value'
 
 const AT = '2026-05-01T10:00:00.000Z'
+const MESSAGE_LIMIT = 256 * 1024
 
 it('keeps a plain string turn under the record role', () => {
   expect(transcriptMessagesFromContent('user', 'just words', AT)).toEqual([
@@ -65,3 +67,174 @@ it('does not apply the list preview cap', () => {
   const [message] = transcriptMessagesFromContent('user', [{ type: 'text', text: long }], AT)
   expect(message.text).toHaveLength(5000)
 })
+
+it('bounds joins for a large multi-block turn and keeps its later tool messages', () => {
+  const blockText = 'x'.repeat(1024 * 1024)
+  const content = [
+    ...Array.from({ length: 72 }, () => ({ type: 'text', text: blockText })),
+    { type: 'tool_use', name: 'Read', input: { path: 'after-large-text' } },
+    {
+      type: 'tool_result',
+      content: [
+        { type: 'text', text: 'header' },
+        { type: 'text', text: blockText.repeat(50) },
+        { type: 'text', text: 'ignored after the existing result cap' }
+      ]
+    },
+    { type: 'tool_use', name: 'Bash', input: { command: 'pwd' } },
+    { type: 'tool_result', content: 'finished' }
+  ]
+  const join = Array.prototype.join
+  let largestJoinedLength = 0
+  const spy = vi.spyOn(Array.prototype, 'join').mockImplementation(function (
+    this: unknown[],
+    separator
+  ) {
+    let length = Math.max(0, this.length - 1) * (separator ?? ',').length
+    for (const value of this) {
+      length += typeof value === 'string' ? value.length : 0
+    }
+    largestJoinedLength = Math.max(largestJoinedLength, length)
+    return join.call(this, separator)
+  })
+  let messages
+  try {
+    messages = transcriptMessagesFromContent('assistant', content, AT)
+  } finally {
+    spy.mockRestore()
+  }
+  expect(messages).toEqual([
+    { role: 'assistant', text: 'x'.repeat(MESSAGE_LIMIT), timestamp: AT },
+    { role: 'tool', text: 'Read: after-large-text', timestamp: AT },
+    { role: 'tool', text: `header\n${'x'.repeat(MESSAGE_LIMIT - 7)}`, timestamp: AT },
+    { role: 'tool', text: 'Bash: pwd', timestamp: AT },
+    { role: 'tool', text: 'finished', timestamp: AT }
+  ])
+  expect(largestJoinedLength).toBeLessThanOrEqual(MESSAGE_LIMIT + 1)
+})
+
+function previousToolResultText(content: unknown): string | null {
+  if (typeof content === 'string') {
+    return boundedText(content)
+  }
+  if (!Array.isArray(content)) {
+    return null
+  }
+  const parts: string[] = []
+  let length = 0
+  for (const item of content) {
+    const text = typeof item === 'string' ? item : asRecord(item)?.text
+    if (typeof text === 'string' && text) {
+      parts.push(text)
+      length += text.length
+      if (length >= MESSAGE_LIMIT) {
+        break
+      }
+    }
+  }
+  return boundedText(parts.join('\n'))
+}
+
+it('matches the previous tool-result collector at newline and surrogate boundaries', () => {
+  const cases: unknown[][] = [
+    ['x'.repeat(MESSAGE_LIMIT - 2), '\ud800'],
+    ['x'.repeat(MESSAGE_LIMIT - 1), '\ud800'],
+    [`${'x'.repeat(MESSAGE_LIMIT - 1)}\ud800`, 'later'],
+    ['x'.repeat(MESSAGE_LIMIT - 2), '\ud800\udc00', 'later'],
+    ['x'.repeat(MESSAGE_LIMIT), '', 'later'],
+    ['', '', '\ud800', '\udfff', '\u0000'],
+    [' '.repeat(MESSAGE_LIMIT), 'visible beyond the cap'],
+    [null, 10, { type: 'image', text: 'still collected' }, { content: 'ignored' }]
+  ]
+  let seed = 0x6a09e667
+  const random = (): number => {
+    seed ^= seed << 13
+    seed ^= seed >>> 17
+    seed ^= seed << 5
+    return seed >>> 0
+  }
+  const lengths = [0, 1, 12, 4096, MESSAGE_LIMIT - 1, MESSAGE_LIMIT, MESSAGE_LIMIT + 1]
+  const atoms = ['a', ' ', '\n', '\ud800', '\udfff', '\ud83d\ude80', '\u0000', '漢']
+  for (let sample = 0; sample < 120; sample++) {
+    cases.push(
+      Array.from({ length: 1 + (random() % 6) }, (_, index) => {
+        const length = lengths[random() % lengths.length]
+        const atom = atoms[random() % atoms.length]
+        const text = atom.repeat(Math.ceil(length / atom.length)).slice(0, length)
+        return index % 2 === 0 ? text : { type: 'text', text }
+      })
+    )
+  }
+  for (const content of cases) {
+    const text = previousToolResultText(content)
+    expect(
+      transcriptMessagesFromContent(
+        'user',
+        [
+          { type: 'tool_result', content },
+          { type: 'tool_use', name: 'Read', input: { path: 'after' } },
+          { type: 'tool_result', content: 'finished' }
+        ],
+        AT
+      )
+    ).toEqual([
+      ...(text ? [{ role: 'tool', text, timestamp: AT }] : []),
+      { role: 'tool', text: 'Read: after', timestamp: AT },
+      { role: 'tool', text: 'finished', timestamp: AT }
+    ])
+  }
+})
+
+it('matches join-then-bound at newline and raw surrogate boundaries', () => {
+  const partsCases = [
+    ['x'.repeat(MESSAGE_LIMIT - 2), '\ud800'],
+    ['x'.repeat(MESSAGE_LIMIT - 1), '\ud800'],
+    [`${'x'.repeat(MESSAGE_LIMIT - 1)}\ud800`, 'later'],
+    ['x'.repeat(MESSAGE_LIMIT - 2), '\ud800\udc00', 'later'],
+    ['x'.repeat(MESSAGE_LIMIT - 3), '\ud800\udc00', 'later'],
+    ['x'.repeat(MESSAGE_LIMIT), '', 'later'],
+    ['', '', '\ud800', '\udfff', '\u0000'],
+    [' '.repeat(MESSAGE_LIMIT), 'visible beyond the cap']
+  ]
+  for (const parts of partsCases) {
+    const text = boundedText(parts.join('\n'))
+    expect(transcriptMessagesFromContent('user', parts, AT)).toEqual(
+      text ? [{ role: 'user', text, timestamp: AT }] : []
+    )
+  }
+})
+
+it('matches the previous join-then-bound output across generated block mixtures', () => {
+  let seed = 0x6a09e667
+  const random = (): number => {
+    seed ^= seed << 13
+    seed ^= seed >>> 17
+    seed ^= seed << 5
+    return seed >>> 0
+  }
+  const lengths = [0, 1, 12, 4096, MESSAGE_LIMIT - 1, MESSAGE_LIMIT, MESSAGE_LIMIT + 1]
+  const atoms = ['a', ' ', '\n', '\ud800', '\udfff', '\ud83d\ude80', '\u0000', '漢']
+  const roles = ['user', 'assistant', 'tool', 'system', 'unknown'] as const
+  for (let sample = 0; sample < 120; sample++) {
+    const parts = Array.from({ length: 2 + (random() % 7) }, () => {
+      const length = lengths[random() % lengths.length]
+      const atom = atoms[random() % atoms.length]
+      return atom.repeat(Math.ceil(length / atom.length)).slice(0, length)
+    })
+    const role = roles[random() % roles.length]
+    const content = parts.map((text, index) => (index % 2 === 0 ? text : { type: 'Text', text }))
+    const text = boundedText(parts.filter((part, index) => index % 2 === 0 || part).join('\n'))
+    expect(
+      transcriptMessagesFromContent(
+        role,
+        [...content, { type: 'tool_use', name: 'Bash', input: { command: 'pwd' } }],
+        AT
+      )
+    ).toEqual([
+      ...(text && (role === 'user' || role === 'assistant' || role === 'tool')
+        ? [{ role, text, timestamp: AT }]
+        : []),
+      { role: 'tool', text: 'Bash: pwd', timestamp: AT }
+    ])
+  }
+})
diff --git a/src/main/ai-vault/session-transcript-message-content.ts b/src/main/ai-vault/session-transcript-message-content.ts
index 299b494777f..48258ffef6d 100644
--- a/src/main/ai-vault/session-transcript-message-content.ts
+++ b/src/main/ai-vault/session-transcript-message-content.ts
@@ -61,6 +61,9 @@ function toolResultText(content: unknown): string | null {
       }
     }
   }
+  if (length + parts.length - 1 > TRANSCRIPT_MESSAGE_TEXT_LIMIT) {
+    return boundedJoinedText(parts)
+  }
   const joined = parts.join('\n')
   return joined.trim() ? joined : null
 }
@@ -113,7 +116,7 @@ export function transcriptMessagesFromContent(
   }
   if (textRole && textParts.length > 0) {
     // The record's own words lead; its tool blocks follow in transcript order.
-    const text = boundedText(textParts.join('\n'))
+    const text = boundedJoinedText(textParts)
     if (text) {
       messages.unshift({ role: textRole, text, timestamp })
     }
@@ -121,6 +124,27 @@ export function transcriptMessagesFromContent(
   return messages
 }
 
+function boundedJoinedText(parts: readonly string[]): string | null {
+  if (parts.length === 1) {
+    return boundedText(parts[0])
+  }
+  const prefixes: string[] = []
+  // One extra code unit preserves truncation at a trailing high surrogate.
+  let remaining = TRANSCRIPT_MESSAGE_TEXT_LIMIT + 1
+  for (const part of parts) {
+    if (prefixes.length > 0) {
+      remaining--
+    }
+    const prefix = part.slice(0, remaining)
+    prefixes.push(prefix)
+    remaining -= prefix.length
+    if (remaining === 0) {
+      break
+    }
+  }
+  return boundedText(prefixes.join('\n'))
+}
+
 function pushMessage(
   messages: TranscriptMessage[],
   role: TranscriptMessageRole,