diff --git a/components/contacts/contacts-sidebar.tsx b/components/contacts/contacts-sidebar.tsx
index 5831e1b24..c645f2d3d 100644
--- a/components/contacts/contacts-sidebar.tsx
+++ b/components/contacts/contacts-sidebar.tsx
@@ -846,6 +846,9 @@ function AddressBookItem({
       onDragOver={handleDragOver}
       onDragLeave={handleDragLeave}
       onDrop={handleDrop}
+      data-testid="address-book-item"
+      data-book-name={book.name}
+      data-account={book.accountName ?? ''}
       className={cn(
         "w-full flex items-center gap-2 ps-5 pe-3 text-sm transition-colors",
         isActive
diff --git a/integration/tests/12-multi-account-contacts.spec.ts b/integration/tests/12-multi-account-contacts.spec.ts
new file mode 100644
index 000000000..d275c7df0
--- /dev/null
+++ b/integration/tests/12-multi-account-contacts.spec.ts
@@ -0,0 +1,70 @@
+import { test, expect, type Page } from '@playwright/test';
+import { ACCOUNTS } from './helpers/config';
+import {
+  login,
+  addAccount,
+  enableProInterface,
+  openContacts,
+  addressBookRows,
+} from './helpers/app';
+
+/**
+ * Multi-account address-book aggregation across an account switch.
+ *
+ * Companion to the calendar spec for the cross-account id-namespacing fix.
+ * Contacts keep no persisted id-selection (unlike calendars'
+ * selectedCalendarIds), so there is no selection to reset; the observable here
+ * is that the aggregated cross-account address-book list stays correct and
+ * usable after switching the active account (the store now namespaces every
+ * account consistently, so ids no longer flip form on a switch).
+ */
+const { alice, bob } = ACCOUNTS;
+
+const switcher = (page: Page) => page.locator('[data-testid="account-switcher"]').first();
+
+async function accountId(page: Page, email: string): Promise<string> {
+  await switcher(page).click();
+  const id = await page
+    .locator(`[data-testid="account-option"][data-account-email="${email}"]`)
+    .first()
+    .getAttribute('data-account-id');
+  await page.keyboard.press('Escape');
+  if (!id) throw new Error(`no account id for ${email}`);
+  return id;
+}
+
+async function switchTo(page: Page, id: string): Promise<void> {
+  await switcher(page).click();
+  await page.locator(`[data-testid="account-option"][data-account-id="${id}"]`).first().click({ force: true });
+  await expect
+    .poll(async () => switcher(page).getAttribute('data-active-account-id'), { timeout: 30000 })
+    .toBe(id);
+}
+
+test.describe('Multi-account contacts', () => {
+  test('address books from both accounts stay aggregated across a switch', async ({ page }) => {
+    await login(page, alice);
+    await addAccount(page, bob);
+    const bobId = await accountId(page, bob.email);
+    await enableProInterface(page);
+
+    await openContacts(page);
+
+    // Both accounts' (default) address books aggregate into one list with
+    // distinct names.
+    const before = await addressBookRows(page);
+    expect(before.length).toBeGreaterThanOrEqual(2);
+    const namesBefore = new Set(before.map((b) => b.name));
+    expect(namesBefore.size).toBeGreaterThanOrEqual(2);
+
+    // Switch the active account: the previously-active account's ids flip
+    // raw<->namespaced. The aggregated list must remain the same set (no loss,
+    // no duplication, no corruption from the id-form change).
+    await switchTo(page, bobId);
+    await openContacts(page);
+
+    const after = await addressBookRows(page);
+    expect(after.length).toBe(before.length);
+    expect(new Set(after.map((b) => b.name))).toEqual(namesBefore);
+  });
+});
diff --git a/integration/tests/helpers/app.ts b/integration/tests/helpers/app.ts
index 34383dc75..b671cc899 100644
--- a/integration/tests/helpers/app.ts
+++ b/integration/tests/helpers/app.ts
@@ -428,3 +428,26 @@ export async function enableProInterface(page: Page): Promise<void> {
     await expect(toggle).toHaveAttribute('aria-checked', 'true');
   }
 }
+
+// --- Contacts / address books (multi-account) ------------------------------
+
+/** Navigate to contacts via in-app nav and wait for the address-book list. */
+export async function openContacts(page: Page): Promise<void> {
+  await page.locator('a[href$="/contacts"]').first().click();
+  await page
+    .locator('[data-testid="address-book-item"]')
+    .first()
+    .waitFor({ state: 'visible', timeout: 30000 });
+}
+
+/** All sidebar address books as {name, account}. */
+export async function addressBookRows(
+  page: Page,
+): Promise<Array<{ name: string; account: string }>> {
+  return page.locator('[data-testid="address-book-item"]').evaluateAll((els) =>
+    els.map((e) => ({
+      name: e.getAttribute('data-book-name') || '',
+      account: e.getAttribute('data-account') || '',
+    })),
+  );
+}