diff --git a/src/preload/app-restart-checkpoint-routing.test.ts b/src/preload/app-restart-checkpoint-routing.test.ts
index 19eb6948c9a..e809d421b93 100644
--- a/src/preload/app-restart-checkpoint-routing.test.ts
+++ b/src/preload/app-restart-checkpoint-routing.test.ts
@@ -92,6 +92,20 @@ describe('native preload destructive app actions', () => {
     })
   }
 
+  // Why: the lazy-chunk recovery reload joins this member through `?.()`, so leaving
+  // it off the bridge reads as "checkpoint fine" and reloads over unsaved buffers.
+  it('exposes the durable checkpoint join the recovery reload refuses on', async () => {
+    const api = await loadApi()
+    invoke.mockImplementation(async (channel: string) =>
+      channel === 'app:await-before-unload-checkpoint' ? { ok: false } : undefined
+    )
+
+    expect(typeof api.app.awaitBeforeUnloadCheckpoint).toBe('function')
+    await expect(api.app.awaitBeforeUnloadCheckpoint()).rejects.toThrow(
+      'Failed to persist renderer state before unload.'
+    )
+  })
+
   it('preserves both macOS keyboard preload adapters', async () => {
     const api = await loadApi()
     invoke.mockResolvedValue(undefined)
diff --git a/src/preload/index.ts b/src/preload/index.ts
index fe11e19f0e0..df8f2fb8a07 100644
--- a/src/preload/index.ts
+++ b/src/preload/index.ts
@@ -594,6 +594,7 @@ const api = {
         throw new Error('Failed to stage renderer state before unload.')
       }
     },
+    awaitBeforeUnloadCheckpoint,
     awaitFirstWindowStartupServices: (): Promise<void> =>
       ipcRenderer.invoke('app:awaitFirstWindowStartupServices'),
     prepareTerminalStartupRestoration: (): Promise<void> =>
@@ -5328,6 +5329,9 @@ const api = {
   }
 }
 
+/** The bridge object the renderer receives; checked against PreloadApi in preload-api-implementation-completeness.ts. */
+export type PreloadApiImplementation = typeof api
+
 // Expose Electron APIs via contextBridge when context-isolated, otherwise attach to the DOM global.
 if (process.contextIsolated) {
   try {
diff --git a/src/preload/preload-api-implementation-completeness.ts b/src/preload/preload-api-implementation-completeness.ts
new file mode 100644
index 00000000000..2b7b09551e0
--- /dev/null
+++ b/src/preload/preload-api-implementation-completeness.ts
@@ -0,0 +1,47 @@
+import type { PreloadApi } from './api-types'
+import type { PreloadApiImplementation } from './index'
+
+/**
+ * Compile-time proof that the preload bridge implements every member the
+ * renderer is allowed to call.
+ *
+ * Why this is not redundant with the annotations in index.ts: `api` is handed to
+ * `contextBridge.exposeInMainWorld`, which takes `any`, and the renderer types
+ * every call as `PreloadApi` (api-types.ts). Nothing compared the two, so a
+ * member declared in `PreloadApi` and never implemented reached the renderer as
+ * `undefined` — and callers guard with `?.()`, which turns the absence into a
+ * confident default rather than a failure.
+ *
+ * Names only, deliberately: `ipcRenderer.invoke` returns `Promise<unknown>`, so
+ * full assignability reports pre-existing return-type widenings that the call
+ * sites already cast. Presence is the half that fails silently.
+ */
+type AssertNoMissingMembers<Missing extends never> = Missing
+
+type UnimplementedGroups = Exclude<keyof PreloadApi, keyof PreloadApiImplementation>
+
+type ObjectMembers<T> = T extends object
+  ? T extends (...args: never[]) => unknown
+    ? never
+    : T
+  : never
+
+/** Recurses through nested API records while treating callable leaves as terminals. */
+type UnimplementedMembers<Expected, Actual, Prefix extends string = ''> =
+  | `${Prefix}${Extract<Exclude<keyof Expected, keyof Actual>, string>}`
+  | {
+      [Key in keyof Expected & keyof Actual]: ObjectMembers<Expected[Key]> extends never
+        ? never
+        : ObjectMembers<Actual[Key]> extends never
+          ? `${Prefix}${Extract<Key, string>}`
+          : UnimplementedMembers<
+              ObjectMembers<Expected[Key]>,
+              ObjectMembers<Actual[Key]>,
+              `${Prefix}${Extract<Key, string>}.`
+            >
+    }[keyof Expected & keyof Actual]
+
+export type PreloadApiGroupsAreImplemented = AssertNoMissingMembers<UnimplementedGroups>
+export type PreloadApiMembersAreImplemented = AssertNoMissingMembers<
+  UnimplementedMembers<PreloadApi, PreloadApiImplementation>
+>