diff --git a/extension/src/shared/__tests__/state-vault-locked.test.ts b/extension/src/shared/__tests__/state-vault-locked.test.ts
new file mode 100644
index 0000000..16003ad
--- /dev/null
+++ b/extension/src/shared/__tests__/state-vault-locked.test.ts
@@ -0,0 +1,66 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { registerHost, __resetHostForTests, sendMessage } from '../state';
+import type { StateHost } from '../state';
+import type { Response } from '../messages';
+
+function makeHost(response: { ok: boolean; error?: string }): StateHost {
+  return {
+    getState: () => ({ view: 'list' } as never),
+    setState: vi.fn(),
+    navigate: vi.fn(),
+    sendMessage: vi.fn().mockResolvedValue(response as Response),
+    escapeHtml: (s) => s,
+    popOutToTab: vi.fn(),
+    isInTab: () => false,
+    openVaultTab: vi.fn(),
+  };
+}
+
+describe('shared/state sendMessage vault_locked intercept', () => {
+  beforeEach(() => __resetHostForTests());
+
+  it('navigates to the lock screen on a vault_locked response', async () => {
+    const host = makeHost({ ok: false, error: 'vault_locked' });
+    registerHost(host);
+    await sendMessage({ type: 'list_items' });
+    expect(host.navigate).toHaveBeenCalledWith(
+      'locked',
+      expect.objectContaining({ error: expect.any(String) }),
+    );
+  });
+
+  it('does NOT intercept the unlock request itself', async () => {
+    const host = makeHost({ ok: false, error: 'vault_locked' });
+    registerHost(host);
+    await sendMessage({ type: 'unlock', passphrase: 'x' });
+    expect(host.navigate).not.toHaveBeenCalled();
+  });
+
+  it('does NOT intercept is_unlocked', async () => {
+    const host = makeHost({ ok: false, error: 'vault_locked' });
+    registerHost(host);
+    await sendMessage({ type: 'is_unlocked' });
+    expect(host.navigate).not.toHaveBeenCalled();
+  });
+
+  it('does not intercept a successful response', async () => {
+    const host = makeHost({ ok: true });
+    registerHost(host);
+    await sendMessage({ type: 'list_items' });
+    expect(host.navigate).not.toHaveBeenCalled();
+  });
+
+  it('does not intercept a non-vault_locked error', async () => {
+    const host = makeHost({ ok: false, error: 'something_else' });
+    registerHost(host);
+    await sendMessage({ type: 'list_items' });
+    expect(host.navigate).not.toHaveBeenCalled();
+  });
+
+  it('returns the response unchanged', async () => {
+    const host = makeHost({ ok: false, error: 'vault_locked' });
+    registerHost(host);
+    const resp = await sendMessage({ type: 'list_items' });
+    expect(resp).toEqual({ ok: false, error: 'vault_locked' });
+  });
+});
diff --git a/extension/src/shared/state.ts b/extension/src/shared/state.ts
index 30deeaf..68dfa04 100644
--- a/extension/src/shared/state.ts
+++ b/extension/src/shared/state.ts
@@ -50,13 +50,33 @@ export function navigate(view: View, extras?: Partial<PopupState>): void {
   host.navigate(view, extras);
 }
 
+// Requests that must NOT trigger the lock screen on a vault_locked response:
+// they run during cold start / unlock, before a session exists, so a
+// vault_locked here is expected rather than a lost session.
+const VAULT_LOCKED_BYPASS: ReadonlySet<Request['type']> = new Set([
+  'unlock', 'is_unlocked',
+]);
+
 /**
- * Phase 4 will add a vault_locked intercept here. For now, this is a pure
- * pass-through so the signature is stable for Phase 4 to fill.
+ * Dispatches a request to the service worker and intercepts the `vault_locked`
+ * response. MV3 evicts the service worker after ~30s idle, wiping the in-memory
+ * session/manifest; the next RPC comes back `vault_locked`. Any surface (popup
+ * or vault tab) that gets that on a non-bypassed request treats it as "session
+ * lost" and navigates to the lock screen so the user can re-enter their
+ * passphrase. Lifted here from vault.ts's local sendMessage in Plan C Phase 4
+ * so both surfaces share one channel.
  */
 export async function sendMessage(request: Request): Promise<Response> {
   if (!host) throw new Error('No state host registered');
-  return host.sendMessage(request);
+  const response = await host.sendMessage(request);
+  if (
+    !response.ok &&
+    response.error === 'vault_locked' &&
+    !VAULT_LOCKED_BYPASS.has(request.type)
+  ) {
+    host.navigate('locked', { error: 'Session expired — please unlock again.' });
+  }
+  return response;
 }
 
 export function escapeHtml(s: string): string {
diff --git a/extension/src/vault/vault.ts b/extension/src/vault/vault.ts
index f07434c..82b11e1 100644
--- a/extension/src/vault/vault.ts
+++ b/extension/src/vault/vault.ts
@@ -5,7 +5,7 @@
 /// vault tab's pane area.
 
 import type { Request, Response } from '../shared/messages';
-import { registerHost } from '../shared/state';
+import { registerHost, sendMessage } from '../shared/state';
 import { type ErrorCta } from '../shared/error-copy';
 import {
   type VaultController, type VaultState, type VaultView,
@@ -27,32 +27,12 @@ import { parseHash, setHash, renderPane, loadManifest, selectItem } from './vaul
 // Helpers
 // ---------------------------------------------------------------------------
 
-function sendMessage(request: Request): Promise<Response> {
+// Plain transport to the service worker, registered as the host's sendMessage.
+// The shared sendMessage() wrapper (shared/state.ts) layers the session-lost
+// → lock-screen intercept on top of this for every UI RPC.
+function postToServiceWorker(request: Request): Promise<Response> {
   return new Promise((resolve) => {
-    chrome.runtime.sendMessage(request, (response: Response) => {
-      // MV3 service workers are evicted after ~30s idle, which wipes the
-      // in-memory session/manifest/gitHost. The fullscreen tab stays open
-      // and has no signal that the SW restarted — the next RPC just comes
-      // back `vault_locked`. Treat that as "session lost" and force the
-      // lock screen so the user can re-enter their passphrase. Skip for
-      // is_unlocked / unlock themselves to avoid loops on cold start.
-      if (
-        response &&
-        !response.ok &&
-        response.error === 'vault_locked' &&
-        request.type !== 'is_unlocked' &&
-        request.type !== 'unlock' &&
-        state.unlocked
-      ) {
-        state.unlocked = false;
-        state.selectedId = null;
-        state.selectedItem = null;
-        state.entries = [];
-        state.error = 'Session expired — please unlock again.';
-        render(ctx);
-      }
-      resolve(response);
-    });
+    chrome.runtime.sendMessage(request, (response: Response) => resolve(response));
   });
 }
 
@@ -115,6 +95,17 @@ registerHost({
     renderPane(ctx);
   },
   navigate: (view, extras) => {
+    if (view === 'locked') {
+      // Session lost (SW evicted mid-session). The vault shows its lock
+      // screen off state.unlocked, so flip it and drop the in-memory data.
+      state.unlocked = false;
+      state.selectedId = null;
+      state.selectedItem = null;
+      state.entries = [];
+      state.error = (extras?.error as string | undefined) ?? null;
+      render(ctx);
+      return;
+    }
     Object.assign(state, { view, error: null, loading: false, ...extras });
     setHash(view as VaultView);
     applyShellViewClass(ctx);
@@ -122,7 +113,7 @@ registerHost({
     if (state.view === 'list') renderListPane(ctx);
     renderPane(ctx);
   },
-  sendMessage,
+  sendMessage: postToServiceWorker,
   escapeHtml,
   popOutToTab: () => {},
   isInTab: () => true,