diff --git a/src/main/runtime/device-registry.ts b/src/main/runtime/device-registry.ts
index e3d848405f0..612b5fcada1 100644
--- a/src/main/runtime/device-registry.ts
+++ b/src/main/runtime/device-registry.ts
@@ -11,6 +11,7 @@ import {
   writeSecureJsonFile
 } from '../../shared/secure-file'
 import type { DeviceScope } from '../../shared/runtime-types'
+import { removeStaleDurableWriteTempFiles } from '../durable-file-write'
 import { DEVICE_REGISTRY_FILENAME } from './mobile-pairing-files'
 import type { RelayDeviceBinding } from './relay/relay-revoke-outbox'
 import type { MobilePairingConnectionMode } from '../../shared/mobile-pairing-connection-mode'
@@ -61,6 +62,7 @@ function validRelayBinding(value: unknown, deviceId: string): RelayDeviceBinding
 // Why: a lastSeen refresh is pure bookkeeping, so coalesce reconnect bursts into one write instead of
 // paying a secure-file rewrite (two synchronous PowerShell ACL spawns on Windows) per connection.
 const LAST_SEEN_FLUSH_DELAY_MS = 250
+const STALE_WRITE_TEMP_AGE_MS = 24 * 60 * 60 * 1000
 
 export class DeviceRegistry {
   private readonly registryPath: string
@@ -71,6 +73,10 @@ export class DeviceRegistry {
 
   constructor(userDataPath: string) {
     this.registryPath = join(userDataPath, DEVICE_REGISTRY_FILENAME)
+    // Why: a write killed between writeFile and rename (e.g. a hung icacls, #20497) orphans its temp forever.
+    void removeStaleDurableWriteTempFiles(this.registryPath, {
+      minimumAgeMs: STALE_WRITE_TEMP_AGE_MS
+    })
     this.load()
   }
 
diff --git a/src/main/runtime/mobile-notification-dismissal-store.ts b/src/main/runtime/mobile-notification-dismissal-store.ts
index 984a8c9a1e0..0be613b792b 100644
--- a/src/main/runtime/mobile-notification-dismissal-store.ts
+++ b/src/main/runtime/mobile-notification-dismissal-store.ts
@@ -5,6 +5,7 @@ import {
   hardenExistingSecureFile,
   isUnreadableError
 } from '../../shared/secure-file'
+import { removeStaleDurableWriteTempFiles } from '../durable-file-write'
 import type { MobileNotificationEvent } from './runtime-mobile-notification-controller'
 
 export type DeliveredNotificationIdentity = {
@@ -15,6 +16,7 @@ export type DeliveredNotificationIdentity = {
 type RecordEntry = DeliveredNotificationIdentity & { dismissedThrough: number; expiresAt: number }
 const LIMIT = 4096
 const RETENTION_MS = 7 * 86400_000
+const STALE_WRITE_TEMP_AGE_MS = 86400_000
 
 export class MobileNotificationDismissalStore {
   private readonly path: string
@@ -22,6 +24,8 @@ export class MobileNotificationDismissalStore {
   private unreadable = false
   constructor(userDataPath: string) {
     this.path = join(userDataPath, 'mobile-notification-dismissals.json')
+    // Why: a write killed between writeFile and rename (e.g. a hung icacls, #20497) orphans its temp forever.
+    void removeStaleDurableWriteTempFiles(this.path, { minimumAgeMs: STALE_WRITE_TEMP_AGE_MS })
     try {
       hardenExistingSecureFile(this.path)
       const value: unknown = JSON.parse(readFileSync(this.path, 'utf8'))
diff --git a/src/main/runtime/mobile-store-orphaned-temp-cleanup.test.ts b/src/main/runtime/mobile-store-orphaned-temp-cleanup.test.ts
new file mode 100644
index 00000000000..0eb1690a1a3
--- /dev/null
+++ b/src/main/runtime/mobile-store-orphaned-temp-cleanup.test.ts
@@ -0,0 +1,78 @@
+import { existsSync, mkdtempSync, readFileSync, rmSync, utimesSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterEach, expect, it, vi } from 'vitest'
+import * as durableFileWrite from '../durable-file-write'
+import { DeviceRegistry } from './device-registry'
+import { MobileNotificationDismissalStore } from './mobile-notification-dismissal-store'
+import { DEVICE_REGISTRY_FILENAME } from './mobile-pairing-files'
+
+const dirs: string[] = []
+afterEach(() => {
+  vi.restoreAllMocks()
+  dirs.splice(0).forEach((dir) => rmSync(dir, { recursive: true, force: true }))
+})
+
+const stores = [
+  [
+    'dismissal store',
+    'mobile-notification-dismissals.json',
+    (dir: string) => new MobileNotificationDismissalStore(dir)
+  ],
+  ['device registry', DEVICE_REGISTRY_FILENAME, (dir: string) => new DeviceRegistry(dir)]
+] as const
+
+it.each(stores)(
+  '%s reclaims orphaned temps while preserving active writes and unrelated files',
+  async (_, fileName, open) => {
+    const dir = mkdtempSync(join(tmpdir(), 'orca-orphaned-temp-'))
+    dirs.push(dir)
+    // Other-process PIDs: the sweep always spares this process's own temps.
+    const orphaned = join(dir, `${fileName}.${process.pid + 1}.1784108697605.b306bb91.tmp`)
+    const recent = join(dir, `${fileName}.${process.pid + 2}.1784108697605.cafef00d.tmp`)
+    const own = join(dir, `${fileName}.${process.pid}.1784108697605.aaaaaaaa.tmp`)
+    const otherStore = join(
+      dir,
+      `${fileName === DEVICE_REGISTRY_FILENAME ? 'mobile-notification-dismissals.json' : DEVICE_REGISTRY_FILENAME}.${process.pid + 1}.0.bbbbbbbb.tmp`
+    )
+    const similarName = join(dir, `${fileName}-backup.${process.pid + 1}.0.cccccccc.tmp`)
+    const wrongSuffix = join(dir, `${fileName}.${process.pid + 1}.0.dddddddd.tmp.backup`)
+    const finalPath = join(dir, fileName)
+    writeFileSync(orphaned, '[]')
+    writeFileSync(recent, '[]')
+    writeFileSync(finalPath, '[]')
+    const twoDaysAgo = (Date.now() - 2 * 86400_000) / 1000
+    for (const path of [orphaned, own, otherStore, similarName, wrongSuffix]) {
+      writeFileSync(path, '[]')
+      utimesSync(path, twoDaysAgo, twoDaysAgo)
+    }
+
+    open(dir)
+
+    await vi.waitFor(() => expect(existsSync(orphaned)).toBe(false))
+    for (const path of [recent, own, otherStore, similarName, wrongSuffix, finalPath]) {
+      expect(readFileSync(path, 'utf8')).toBe('[]')
+    }
+  }
+)
+
+it.each(stores)('%s loads while startup cleanup is still pending', async (_, fileName, open) => {
+  const dir = mkdtempSync(join(tmpdir(), 'orca-orphaned-temp-pending-'))
+  dirs.push(dir)
+  writeFileSync(join(dir, fileName), '[]')
+  let finishCleanup = () => {}
+  const pending = new Promise<void>((resolve) => {
+    finishCleanup = resolve
+  })
+  const cleanup = vi
+    .spyOn(durableFileWrite, 'removeStaleDurableWriteTempFiles')
+    .mockReturnValue(pending)
+  try {
+    const store = open(dir)
+    expect(cleanup).toHaveBeenCalledOnce()
+    expect(store instanceof DeviceRegistry ? store.listDevices() : store.reconcile([])).toEqual([])
+  } finally {
+    finishCleanup()
+    await pending
+  }
+})