diff --git a/src/renderer/src/app-shell/shutdown-checkpoint-persist.test.ts b/src/renderer/src/app-shell/shutdown-checkpoint-persist.test.ts
new file mode 100644
index 00000000000..d7d4418c0cf
--- /dev/null
+++ b/src/renderer/src/app-shell/shutdown-checkpoint-persist.test.ts
@@ -0,0 +1,138 @@
+import { describe, expect, it, vi } from 'vitest'
+import {
+  runShutdownCheckpointPersist,
+  type ShutdownCheckpointPersistDeps
+} from './shutdown-checkpoint-persist'
+
+function makeDeps(
+  overrides: Partial<ShutdownCheckpointPersistDeps> = {}
+): ShutdownCheckpointPersistDeps {
+  return {
+    shouldCaptureSession: () => true,
+    captureTerminalBuffers: vi.fn(),
+    captureSleepingAgentSessions: vi.fn(),
+    buildSessionSnapshots: () => [{ state: { activeTabId: 't1' } }] as never,
+    buildUiPatch: () => ({ activeView: 'workspace' }) as never,
+    hasDirtyOpenFiles: () => false,
+    isIntentionalAppRestartInProgress: () => true,
+    stageBeforeUnloadSync: vi.fn(),
+    ...overrides
+  }
+}
+
+describe('runShutdownCheckpointPersist', () => {
+  it('does not fail the checkpoint when the sleeping-agent quit capture throws (STA-5505)', () => {
+    vi.spyOn(console, 'error').mockImplementation(() => {})
+    const deps = makeDeps({
+      captureSleepingAgentSessions: () => {
+        throw new Error('capture exploded')
+      }
+    })
+
+    expect(() => runShutdownCheckpointPersist(deps)).not.toThrow()
+    // The full snapshot must still be staged — losing at most one minute of
+    // resume records is strictly better than blocking the update.
+    expect(deps.stageBeforeUnloadSync).toHaveBeenCalledWith({
+      sessions: [{ state: { activeTabId: 't1' } }],
+      ui: { activeView: 'workspace' }
+    })
+    vi.restoreAllMocks()
+  })
+
+  it('degrades to durable-only staging when full staging throws during an intentional restart (STA-5505)', () => {
+    vi.spyOn(console, 'error').mockImplementation(() => {})
+    const stageBeforeUnloadSync = vi.fn((args: { sessions: unknown[] }) => {
+      if (args.sessions.length > 0) {
+        throw new Error('sync IPC staging failed')
+      }
+    })
+    const deps = makeDeps({ stageBeforeUnloadSync })
+
+    expect(() => runShutdownCheckpointPersist(deps)).not.toThrow()
+    expect(stageBeforeUnloadSync).toHaveBeenCalledTimes(2)
+    expect(stageBeforeUnloadSync).toHaveBeenLastCalledWith({
+      sessions: [],
+      ui: { activeView: 'workspace' }
+    })
+    vi.restoreAllMocks()
+  })
+
+  it('still fails the checkpoint when staging throws and dirty editor buffers exist', () => {
+    const deps = makeDeps({
+      hasDirtyOpenFiles: () => true,
+      stageBeforeUnloadSync: vi.fn(() => {
+        throw new Error('sync IPC staging failed')
+      })
+    })
+
+    expect(() => runShutdownCheckpointPersist(deps)).toThrow('sync IPC staging failed')
+  })
+
+  it('still fails the checkpoint when staging throws outside an intentional restart', () => {
+    const deps = makeDeps({
+      isIntentionalAppRestartInProgress: () => false,
+      stageBeforeUnloadSync: vi.fn(() => {
+        throw new Error('sync IPC staging failed')
+      })
+    })
+
+    expect(() => runShutdownCheckpointPersist(deps)).toThrow('sync IPC staging failed')
+  })
+
+  it('fails the checkpoint when even durable-only staging throws', () => {
+    vi.spyOn(console, 'error').mockImplementation(() => {})
+    const deps = makeDeps({
+      buildSessionSnapshots: () => {
+        throw new Error('snapshot build failed')
+      },
+      stageBeforeUnloadSync: vi.fn(() => {
+        throw new Error('durable staging failed')
+      })
+    })
+
+    expect(() => runShutdownCheckpointPersist(deps)).toThrow('durable staging failed')
+    vi.restoreAllMocks()
+  })
+
+  it('keeps the durable-session fallback for snapshot build failures', () => {
+    vi.spyOn(console, 'error').mockImplementation(() => {})
+    const deps = makeDeps({
+      buildSessionSnapshots: () => {
+        throw new Error('snapshot build failed')
+      }
+    })
+
+    expect(() => runShutdownCheckpointPersist(deps)).not.toThrow()
+    expect(deps.stageBeforeUnloadSync).toHaveBeenCalledTimes(1)
+    expect(deps.stageBeforeUnloadSync).toHaveBeenCalledWith({
+      sessions: [],
+      ui: { activeView: 'workspace' }
+    })
+    vi.restoreAllMocks()
+  })
+
+  it('rethrows snapshot build failures when dirty editor buffers exist', () => {
+    const deps = makeDeps({
+      hasDirtyOpenFiles: () => true,
+      buildSessionSnapshots: () => {
+        throw new Error('snapshot build failed')
+      }
+    })
+
+    expect(() => runShutdownCheckpointPersist(deps)).toThrow('snapshot build failed')
+    expect(deps.stageBeforeUnloadSync).not.toHaveBeenCalled()
+  })
+
+  it('skips capture and stages empty sessions before hydration completes', () => {
+    const deps = makeDeps({ shouldCaptureSession: () => false })
+
+    runShutdownCheckpointPersist(deps)
+
+    expect(deps.captureTerminalBuffers).not.toHaveBeenCalled()
+    expect(deps.captureSleepingAgentSessions).not.toHaveBeenCalled()
+    expect(deps.stageBeforeUnloadSync).toHaveBeenCalledWith({
+      sessions: [],
+      ui: { activeView: 'workspace' }
+    })
+  })
+})
diff --git a/src/renderer/src/app-shell/shutdown-checkpoint-persist.ts b/src/renderer/src/app-shell/shutdown-checkpoint-persist.ts
new file mode 100644
index 00000000000..90bbb3e39fb
--- /dev/null
+++ b/src/renderer/src/app-shell/shutdown-checkpoint-persist.ts
@@ -0,0 +1,64 @@
+import type { PersistedUIState } from '../../../shared/persisted-ui-state-types'
+import type { WorkspaceSessionHostSnapshot } from '../lib/workspace-session-host-persistence'
+
+export type ShutdownCheckpointStageArgs = {
+  sessions: WorkspaceSessionHostSnapshot[]
+  ui: Partial<PersistedUIState>
+}
+
+export type ShutdownCheckpointPersistDeps = {
+  shouldCaptureSession: () => boolean
+  /** Per-pane failures are already swallowed inside the capture loop. */
+  captureTerminalBuffers: () => void
+  captureSleepingAgentSessions: () => void
+  buildSessionSnapshots: () => WorkspaceSessionHostSnapshot[]
+  buildUiPatch: () => Partial<PersistedUIState>
+  hasDirtyOpenFiles: () => boolean
+  isIntentionalAppRestartInProgress: () => boolean
+  stageBeforeUnloadSync: (args: ShutdownCheckpointStageArgs) => void
+}
+
+/** The synchronous body of the shutdown checkpoint: capture renderer-owned state,
+ *  then stage everything durable through one main-process call. Throwing here fails
+ *  the checkpoint and blocks the restart, so only unstageable data may throw. */
+export function runShutdownCheckpointPersist(deps: ShutdownCheckpointPersistDeps): void {
+  const shouldCaptureSession = deps.shouldCaptureSession()
+  if (shouldCaptureSession) {
+    deps.captureTerminalBuffers()
+    try {
+      deps.captureSleepingAgentSessions()
+    } catch (error) {
+      // Why: the periodic capture refreshes resume records every minute, so a failed
+      // quit capture costs at most that window — never strand the restart on it.
+      console.error('[app] Sleeping-agent quit capture failed; continuing checkpoint', error)
+    }
+  }
+  // Why: dirty drafts exist only in the full session snapshot, so their loss is the
+  // one thing this checkpoint may never trade away for an update.
+  const canDegradeToDurableSession = (): boolean =>
+    deps.isIntentionalAppRestartInProgress() && !deps.hasDirtyOpenFiles()
+  let sessionSnapshots: WorkspaceSessionHostSnapshot[] = []
+  let degraded = false
+  try {
+    sessionSnapshots = shouldCaptureSession ? deps.buildSessionSnapshots() : []
+  } catch (error) {
+    if (!canDegradeToDurableSession()) {
+      throw error
+    }
+    console.error('[app] Full renderer session snapshot failed; using durable session', error)
+    degraded = true
+  }
+  try {
+    deps.stageBeforeUnloadSync({
+      sessions: degraded ? [] : sessionSnapshots,
+      ui: deps.buildUiPatch()
+    })
+  } catch (error) {
+    // A durable-only stage has nothing safer left to fall back to.
+    if (degraded || !canDegradeToDurableSession()) {
+      throw error
+    }
+    console.error('[app] Staging the full renderer session failed; using durable session', error)
+    deps.stageBeforeUnloadSync({ sessions: [], ui: deps.buildUiPatch() })
+  }
+}
diff --git a/src/renderer/src/app-shell/use-app-session-persistence.ts b/src/renderer/src/app-shell/use-app-session-persistence.ts
index 5047feac381..68b24d0588b 100644
--- a/src/renderer/src/app-shell/use-app-session-persistence.ts
+++ b/src/renderer/src/app-shell/use-app-session-persistence.ts
@@ -20,6 +20,7 @@ import {
   createShutdownCheckpointBeforeUnloadHandler,
   createShutdownCheckpointGuard
 } from '../lib/shutdown-checkpoint-guard'
+import { runShutdownCheckpointPersist } from './shutdown-checkpoint-persist'
 import { shutdownBufferCaptures } from '../components/terminal-pane/shutdown-buffer-captures'
 import { dispatchWindowCloseRequest } from '../components/window-close-request-coordinator'
 import {
@@ -117,51 +118,40 @@ export function useAppSessionPersistence(): void {
     // two firings, PTY exit events can arrive and unmount TerminalPanes,
     // emptying shutdownBufferCaptures. The guard prevents the second call
     // from overwriting the good session data with an empty snapshot.
-    const shutdownCheckpoint = createShutdownCheckpointGuard(() => {
-      const shouldCaptureSession = shouldPersistWorkspaceSession(useAppStore.getState())
-      if (shouldCaptureSession) {
-        for (const capture of shutdownBufferCaptures.values()) {
-          try {
-            capture({ includeLocalBuffers: false })
-          } catch {
-            // Don't let one pane's failure block the rest.
+    const shutdownCheckpoint = createShutdownCheckpointGuard(() =>
+      runShutdownCheckpointPersist({
+        shouldCaptureSession: () => shouldPersistWorkspaceSession(useAppStore.getState()),
+        captureTerminalBuffers: () => {
+          for (const capture of shutdownBufferCaptures.values()) {
+            try {
+              capture({ includeLocalBuffers: false })
+            } catch {
+              // Don't let one pane's failure block the rest.
+            }
           }
-        }
+        },
         // Why: agent provider session ids live only in agentStatusByPaneKey,
         // which is in-memory. Capture them into the persisted sleeping-session
         // map so a daemon/session death while the app is closed can still
         // cold-restore via the agent's resume command (#5232).
-        useAppStore.getState().captureAllSleepingAgentSessions('quit')
-      }
-      // Why: re-read state after capture() calls populated scrollback buffers
-      // into the store via Zustand setters. The earlier read is only for the
-      // gating flags and would miss those updates.
-      const freshState = useAppStore.getState()
-      let sessionSnapshots: ReturnType<typeof buildWorkspaceSessionHostSnapshots> = []
-      try {
-        sessionSnapshots = shouldCaptureSession
-          ? buildWorkspaceSessionHostSnapshots(buildWorkspaceSessionPayload(freshState), freshState)
-          : []
-      } catch (error) {
-        // Why: dirty drafts exist only in the full session snapshot.
-        if (
-          !isIntentionalAppRestartInProgress() ||
-          freshState.openFiles.some((file) => file.isDirty)
-        ) {
-          throw error
-        }
-        console.error('[app] Full renderer session snapshot failed; using durable session', error)
-        window.api.app.stageBeforeUnloadSync({
-          sessions: [],
-          ui: buildActiveViewUnloadPatch(freshState)
-        })
-        return
-      }
-      window.api.app.stageBeforeUnloadSync({
-        sessions: sessionSnapshots,
-        ui: buildActiveViewUnloadPatch(freshState)
+        captureSleepingAgentSessions: () =>
+          useAppStore.getState().captureAllSleepingAgentSessions('quit'),
+        // Why: re-read state after capture() calls populated scrollback buffers
+        // into the store via Zustand setters. The shouldCaptureSession read is
+        // only for the gating flags and would miss those updates.
+        buildSessionSnapshots: () => {
+          const freshState = useAppStore.getState()
+          return buildWorkspaceSessionHostSnapshots(
+            buildWorkspaceSessionPayload(freshState),
+            freshState
+          )
+        },
+        buildUiPatch: () => buildActiveViewUnloadPatch(useAppStore.getState()),
+        hasDirtyOpenFiles: () => useAppStore.getState().openFiles.some((file) => file.isDirty),
+        isIntentionalAppRestartInProgress,
+        stageBeforeUnloadSync: (args) => window.api.app.stageBeforeUnloadSync(args)
       })
-    })
+    )
     const persistBeforeUnload = createShutdownCheckpointBeforeUnloadHandler(shutdownCheckpoint)
     window.addEventListener('beforeunload', persistBeforeUnload)
     window.addEventListener(ORCA_APP_RESTART_ABORTED_EVENT, shutdownCheckpoint.reset)
diff --git a/src/renderer/src/app-startup-routing.test.ts b/src/renderer/src/app-startup-routing.test.ts
index 99ef97bb211..e4add37080e 100644
--- a/src/renderer/src/app-startup-routing.test.ts
+++ b/src/renderer/src/app-startup-routing.test.ts
@@ -498,20 +498,17 @@ describe('renderer startup runtime routing', () => {
     expect(checkpointEnd).toBeGreaterThan(checkpointStart)
     const checkpointBlock = source.slice(checkpointStart, checkpointEnd)
 
+    expect(checkpointBlock).toContain('runShutdownCheckpointPersist({')
     expect(checkpointBlock).toContain(
-      'let sessionSnapshots: ReturnType<typeof buildWorkspaceSessionHostSnapshots> = []'
+      'buildWorkspaceSessionHostSnapshots(\n            buildWorkspaceSessionPayload(freshState),\n            freshState\n          )'
+    )
+    expect(checkpointBlock).toContain('buildUiPatch: () => buildActiveViewUnloadPatch(')
+    expect(checkpointBlock).toContain('isIntentionalAppRestartInProgress')
+    expect(checkpointBlock).toContain(
+      'useAppStore.getState().openFiles.some((file) => file.isDirty)'
     )
     expect(checkpointBlock).toContain(
-      'buildWorkspaceSessionHostSnapshots(buildWorkspaceSessionPayload(freshState), freshState)'
-    )
-    expect(checkpointBlock).toContain('window.api.app.stageBeforeUnloadSync({')
-    expect(checkpointBlock).toContain('sessions: sessionSnapshots')
-    expect(checkpointBlock).toContain('ui: buildActiveViewUnloadPatch(freshState)')
-    expect(checkpointBlock).toContain('!isIntentionalAppRestartInProgress()')
-    expect(checkpointBlock).toContain('freshState.openFiles.some((file) => file.isDirty)')
-    expect(checkpointBlock).toContain('sessions: []')
-    expect(checkpointBlock).toContain(
-      'return\n      }\n      window.api.app.stageBeforeUnloadSync({\n        sessions: sessionSnapshots'
+      'stageBeforeUnloadSync: (args) => window.api.app.stageBeforeUnloadSync(args)'
     )
     expect(source).toContain(
       'window.addEventListener(ORCA_APP_RESTART_ABORTED_EVENT, shutdownCheckpoint.reset)'
diff --git a/src/renderer/src/lib/shutdown-checkpoint-guard.test.ts b/src/renderer/src/lib/shutdown-checkpoint-guard.test.ts
index 688d396d3b8..2adf838a6d5 100644
--- a/src/renderer/src/lib/shutdown-checkpoint-guard.test.ts
+++ b/src/renderer/src/lib/shutdown-checkpoint-guard.test.ts
@@ -1,17 +1,25 @@
+// @vitest-environment happy-dom
 import { readFileSync } from 'node:fs'
 import { join } from 'node:path'
-import { describe, expect, it, vi } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import {
   createShutdownCheckpointBeforeUnloadHandler,
   createShutdownCheckpointGuard,
   preventUnloadAndScheduleShutdownCheckpointReset
 } from './shutdown-checkpoint-guard'
 import {
+  consumeShutdownCheckpointFailureReason,
   ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT,
   ORCA_RENDERER_UNLOAD_PREVENTED_EVENT
 } from '../../../shared/renderer-shutdown-events'
 
 describe('createShutdownCheckpointGuard', () => {
+  afterEach(() => {
+    vi.restoreAllMocks()
+    delete (window as unknown as { api?: unknown }).api
+    consumeShutdownCheckpointFailureReason()
+  })
+
   it('dedupes the synthetic and native unload events in one close attempt', () => {
     const persist = vi.fn()
     const guard = createShutdownCheckpointGuard(persist)
@@ -45,6 +53,37 @@ describe('createShutdownCheckpointGuard', () => {
     expect(persist).toHaveBeenCalledTimes(2)
   })
 
+  it('publishes the failure cause and records a crash breadcrumb (STA-5505)', () => {
+    const recordBreadcrumb = vi.fn()
+    ;(window as unknown as { api: unknown }).api = { crashReports: { recordBreadcrumb } }
+    vi.spyOn(console, 'error').mockImplementation(() => {})
+    const guard = createShutdownCheckpointGuard(() => {
+      throw new Error('sendSync payload rejected')
+    })
+
+    expect(guard.persistOnce()).toBe(false)
+
+    expect(consumeShutdownCheckpointFailureReason()).toBe('sendSync payload rejected')
+    expect(recordBreadcrumb).toHaveBeenCalledWith({
+      name: 'renderer_shutdown_checkpoint_failed',
+      data: { message: 'sendSync payload rejected' }
+    })
+  })
+
+  it('clears a stale failure cause once a later checkpoint succeeds (STA-5505)', () => {
+    vi.spyOn(console, 'error').mockImplementation(() => {})
+    const guard = createShutdownCheckpointGuard(
+      vi.fn().mockImplementationOnce(() => {
+        throw new Error('disk full')
+      })
+    )
+
+    expect(guard.persistOnce()).toBe(false)
+    expect(guard.persistOnce()).toBe(true)
+
+    expect(consumeShutdownCheckpointFailureReason()).toBeNull()
+  })
+
   it('reports checkpoint failure separately from the unload verdict', () => {
     const eventTarget = new EventTarget()
     const failed = vi.fn()
diff --git a/src/renderer/src/lib/shutdown-checkpoint-guard.ts b/src/renderer/src/lib/shutdown-checkpoint-guard.ts
index 129c67406d0..a5505894c60 100644
--- a/src/renderer/src/lib/shutdown-checkpoint-guard.ts
+++ b/src/renderer/src/lib/shutdown-checkpoint-guard.ts
@@ -1,13 +1,25 @@
 import {
+  clearShutdownCheckpointFailureReason,
   ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT,
-  ORCA_RENDERER_UNLOAD_PREVENTED_EVENT
+  ORCA_RENDERER_UNLOAD_PREVENTED_EVENT,
+  publishShutdownCheckpointFailureReason
 } from '../../../shared/renderer-shutdown-events'
+import { recordRendererCrashBreadcrumb } from './crash-breadcrumb-recorder'
 
 export type ShutdownCheckpointGuard = {
   persistOnce: () => boolean
   reset: () => void
 }
 
+// Why: without this, a reproducible checkpoint failure strands the user on an old
+// build behind an error that names the symptom while the cause is swallowed (STA-5505).
+function reportShutdownCheckpointFailure(error: unknown): void {
+  console.error('[app] Shutdown checkpoint persist failed:', error)
+  const message = error instanceof Error ? error.message : String(error)
+  publishShutdownCheckpointFailureReason(message)
+  recordRendererCrashBreadcrumb('renderer_shutdown_checkpoint_failed', { message })
+}
+
 export function createShutdownCheckpointGuard(persist: () => void): ShutdownCheckpointGuard {
   let persisted = false
   return {
@@ -17,12 +29,14 @@ export function createShutdownCheckpointGuard(persist: () => void): ShutdownChec
       }
       try {
         persist()
-      } catch {
+      } catch (error) {
         // Why: browser event targets swallow listener exceptions. Returning a
         // failure lets the caller cancel unload and keep this attempt retryable.
+        reportShutdownCheckpointFailure(error)
         return false
       }
       persisted = true
+      clearShutdownCheckpointFailureReason()
       return true
     },
     reset(): void {
diff --git a/src/shared/renderer-restart-preparation.test.ts b/src/shared/renderer-restart-preparation.test.ts
index ca690ab3cc6..78a2d17dfcd 100644
--- a/src/shared/renderer-restart-preparation.test.ts
+++ b/src/shared/renderer-restart-preparation.test.ts
@@ -1,6 +1,10 @@
+// @vitest-environment happy-dom
 import { describe, expect, it, vi } from 'vitest'
 import type { UpdateStatus } from './update-status-types'
-import { ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT } from './renderer-shutdown-events'
+import {
+  ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT,
+  publishShutdownCheckpointFailureReason
+} from './renderer-shutdown-events'
 import {
   createUpdaterQuitAbortRelay,
   prepareRendererForAppRestart
@@ -32,6 +36,24 @@ describe('prepareRendererForAppRestart', () => {
     expect(aborted).toHaveBeenCalledTimes(1)
   })
 
+  it('names the published failure cause in the thrown checkpoint error (STA-5505)', async () => {
+    const eventTarget = new EventTarget()
+    eventTarget.addEventListener('beforeunload', (event) => {
+      // Mirrors the checkpoint guard: publish the cause, then fail the checkpoint.
+      publishShutdownCheckpointFailureReason('sendSync payload rejected')
+      event.currentTarget?.dispatchEvent(new Event(ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT))
+      event.preventDefault()
+    })
+
+    await expect(
+      prepareRendererForAppRestart(eventTarget, {
+        startedEventName: 'restart-started',
+        abortedEventName: 'restart-aborted',
+        awaitCheckpoint: () => Promise.resolve()
+      })
+    ).rejects.toThrow('Renderer shutdown checkpoint was not completed: sendSync payload rejected')
+  })
+
   it('does not mistake an unrelated unload veto for checkpoint failure', async () => {
     const eventTarget = new EventTarget()
     const veto = vi.fn((event: Event) => event.preventDefault())
diff --git a/src/shared/renderer-restart-preparation.ts b/src/shared/renderer-restart-preparation.ts
index 356008e0f62..b8e4d802fa1 100644
--- a/src/shared/renderer-restart-preparation.ts
+++ b/src/shared/renderer-restart-preparation.ts
@@ -2,7 +2,10 @@ import {
   ORCA_EDITOR_PREPARE_HOT_EXIT_EVENT,
   type EditorPrepareHotExitDetail
 } from './editor-save-events'
-import { ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT } from './renderer-shutdown-events'
+import {
+  consumeShutdownCheckpointFailureReason,
+  ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT
+} from './renderer-shutdown-events'
 import type { UpdateStatus } from './update-status-types'
 
 export type AppRestartPrepOptions = {
@@ -63,7 +66,14 @@ export async function prepareRendererForAppRestart(
       )
     }
     if (checkpointFailed) {
-      throw new Error('Renderer shutdown checkpoint was not completed.')
+      // Why: the guard publishes the swallowed persist error out-of-band; naming it
+      // here is the only way the update-error dialog can say what actually failed.
+      const reason = consumeShutdownCheckpointFailureReason()
+      throw new Error(
+        reason
+          ? `Renderer shutdown checkpoint was not completed: ${reason}`
+          : 'Renderer shutdown checkpoint was not completed.'
+      )
     }
     // Why: the checkpoint only stages synchronously. Navigating before that
     // write lands loses the session snapshot to a crash or power loss.
diff --git a/src/shared/renderer-shutdown-events.ts b/src/shared/renderer-shutdown-events.ts
index 3aa3dbcc25d..b55831b818b 100644
--- a/src/shared/renderer-shutdown-events.ts
+++ b/src/shared/renderer-shutdown-events.ts
@@ -1,3 +1,45 @@
 export const ORCA_RENDERER_UNLOAD_PREVENTED_EVENT = 'orca:renderer-unload-prevented'
 export const ORCA_RENDERER_SHUTDOWN_CHECKPOINT_FAILED_EVENT =
   'orca:renderer-shutdown-checkpoint-failed'
+
+// Why a DOM attribute: the checkpoint guard runs in the renderer's main world while
+// prepareRendererForAppRestart runs in the context-isolated preload world. Events
+// cross worlds but their JS payloads don't; document attributes are shared platform
+// state, so this is the one channel that carries the failure reason to the thrower.
+export const ORCA_SHUTDOWN_CHECKPOINT_FAILURE_REASON_ATTRIBUTE =
+  'data-orca-shutdown-checkpoint-failure'
+
+export function publishShutdownCheckpointFailureReason(reason: string): void {
+  try {
+    globalThis.document?.documentElement?.setAttribute(
+      ORCA_SHUTDOWN_CHECKPOINT_FAILURE_REASON_ATTRIBUTE,
+      reason
+    )
+  } catch {
+    // Best-effort diagnostics; the checkpoint verdict itself is carried by the event.
+  }
+}
+
+export function clearShutdownCheckpointFailureReason(): void {
+  try {
+    globalThis.document?.documentElement?.removeAttribute(
+      ORCA_SHUTDOWN_CHECKPOINT_FAILURE_REASON_ATTRIBUTE
+    )
+  } catch {
+    // Best-effort diagnostics only.
+  }
+}
+
+/** Read and clear the published reason so a stale one can't label a later failure. */
+export function consumeShutdownCheckpointFailureReason(): string | null {
+  try {
+    const root = globalThis.document?.documentElement
+    const reason = root?.getAttribute(ORCA_SHUTDOWN_CHECKPOINT_FAILURE_REASON_ATTRIBUTE)
+    if (reason) {
+      root?.removeAttribute(ORCA_SHUTDOWN_CHECKPOINT_FAILURE_REASON_ATTRIBUTE)
+    }
+    return reason || null
+  } catch {
+    return null
+  }
+}