diff --git a/components/calendar/calendar-app.tsx b/components/calendar/calendar-app.tsx
index f674984c0..29acf97fc 100644
--- a/components/calendar/calendar-app.tsx
+++ b/components/calendar/calendar-app.tsx
@@ -669,7 +669,7 @@ export function CalendarApp({ linkSegments }: CalendarAppProps = {}) {
     const { dateRange: currentRange } = useCalendarStore.getState();
     if (!currentRange) return;
     if (multiAccountEnabled && accountClients.length > 0) {
-      await fetchAllAccountsEventsFn(accountClients, activeAccountId, currentRange.start, currentRange.end);
+      await fetchAllAccountsEventsFn(accountClients, currentRange.start, currentRange.end);
       return;
     }
     await fetchEvents(client, currentRange.start, currentRange.end);
@@ -682,7 +682,7 @@ export function CalendarApp({ linkSegments }: CalendarAppProps = {}) {
     onRefresh: async () => {
       if (!client) return;
       const calendarRefresh = multiAccountEnabled && accountClients.length > 0 && activeAccountId
-        ? fetchAllAccountsCalendarsFn(accountClients, activeAccountId)
+        ? fetchAllAccountsCalendarsFn(accountClients)
         : fetchCalendars(client);
       await Promise.all([
         calendarRefresh,
diff --git a/components/calendar/calendar-sidebar-panel.tsx b/components/calendar/calendar-sidebar-panel.tsx
index 38dc2dccd..7dd844510 100644
--- a/components/calendar/calendar-sidebar-panel.tsx
+++ b/components/calendar/calendar-sidebar-panel.tsx
@@ -242,6 +242,10 @@ export function CalendarSidebarPanel({
         <button
           onClick={() => onToggleVisibility(cal.id)}
           onContextMenu={hasMenu ? (e) => openContextMenu(e, cal) : undefined}
+          data-testid="calendar-item"
+          data-calendar-name={cal.name}
+          data-account={cal.accountName ?? ''}
+          data-visible={isVisible}
           className={cn(
             "flex items-center gap-2 w-full px-1.5 py-1 rounded-md text-sm transition-colors duration-150",
             "hover:bg-muted"
diff --git a/components/contacts/contacts-app.tsx b/components/contacts/contacts-app.tsx
index 7788fb016..12d828749 100644
--- a/components/contacts/contacts-app.tsx
+++ b/components/contacts/contacts-app.tsx
@@ -224,8 +224,8 @@ export function ContactsApp({ linkSegments }: ContactsAppProps = {}) {
         if (activeId) {
           const { fetchAllAccountsContacts, fetchAllAccountsAddressBooks } = useContactStore.getState();
           await Promise.all([
-            fetchAllAccountsAddressBooks(accountClients, activeId),
-            fetchAllAccountsContacts(accountClients, activeId),
+            fetchAllAccountsAddressBooks(accountClients),
+            fetchAllAccountsContacts(accountClients),
           ]);
           return;
         }
@@ -331,7 +331,7 @@ export function ContactsApp({ linkSegments }: ContactsAppProps = {}) {
       const activeId = useAuthStore.getState().activeAccountId;
       if (activeId) {
         const { fetchAllAccountsAddressBooks } = useContactStore.getState();
-        await fetchAllAccountsAddressBooks(accountClients, activeId);
+        await fetchAllAccountsAddressBooks(accountClients);
         return;
       }
     }
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/components/settings/layout-settings.tsx b/components/settings/layout-settings.tsx
index 9cc305d1d..505b3e49f 100644
--- a/components/settings/layout-settings.tsx
+++ b/components/settings/layout-settings.tsx
@@ -354,6 +354,7 @@ export function LayoutSettings() {
         <ToggleSwitch
           checked={proInterface}
           onChange={(v) => updateSetting('proInterface', v)}
+          testId="setting-pro-interface"
         />
       </SettingItem>
     </SettingsSection>
diff --git a/components/settings/settings-app.tsx b/components/settings/settings-app.tsx
index 0fc31ba62..f3f527fdc 100644
--- a/components/settings/settings-app.tsx
+++ b/components/settings/settings-app.tsx
@@ -932,6 +932,7 @@ export function SettingsApp({ linkSegments }: SettingsAppProps = {}) {
                   return (
                     <div key={tab.id}>
                       <button
+                        data-testid={`settings-tab-${tab.id}`}
                         onClick={() => handleTabSelect(tab.id)}
                         className="w-full flex items-center justify-between px-5 py-3.5 text-sm text-foreground hover:bg-muted transition-colors duration-150"
                       >
@@ -1072,6 +1073,7 @@ export function SettingsApp({ linkSegments }: SettingsAppProps = {}) {
                   return (
                     <div key={tab.id}>
                       <button
+                        data-testid={`settings-tab-${tab.id}`}
                         onClick={() => handleTabSelect(tab.id)}
                         className={cn(
                           'w-full text-start px-3 py-2 rounded-md text-sm transition-colors duration-150 flex items-center gap-2.5',
diff --git a/components/settings/settings-section.tsx b/components/settings/settings-section.tsx
index c05d8fc74..9169d9a5a 100644
--- a/components/settings/settings-section.tsx
+++ b/components/settings/settings-section.tsx
@@ -67,14 +67,17 @@ interface ToggleSwitchProps {
   disabled?: boolean;
   /** Overrides the enclosing SettingItem label as the accessible name. */
   ariaLabel?: string;
+  /** Test hook (data-testid) for integration tests. */
+  testId?: string;
 }
 
-export function ToggleSwitch({ checked, onChange, disabled, ariaLabel }: ToggleSwitchProps) {
+export function ToggleSwitch({ checked, onChange, disabled, ariaLabel, testId }: ToggleSwitchProps) {
   const labelledBy = useContext(SettingLabelContext);
   return (
     <button
       type="button"
       role="switch"
+      data-testid={testId}
       aria-checked={checked}
       aria-label={ariaLabel}
       aria-labelledby={ariaLabel ? undefined : labelledBy}
diff --git a/hooks/use-pro-multi-account-calendars.ts b/hooks/use-pro-multi-account-calendars.ts
index 7bc225ad8..2acac5ff7 100644
--- a/hooks/use-pro-multi-account-calendars.ts
+++ b/hooks/use-pro-multi-account-calendars.ts
@@ -49,14 +49,14 @@ export function useProMultiAccountCalendars(start: string | null, end: string |
   // bootstrapping).
   useEffect(() => {
     if (!enabled || !activeAccountId || accountClients.length === 0) return;
-    void fetchAllAccountsCalendars(accountClients, activeAccountId);
+    void fetchAllAccountsCalendars(accountClients);
   }, [enabled, activeAccountId, accountClients, fetchAllAccountsCalendars]);
 
   // Fetch events for the current visible date range across all accounts.
   useEffect(() => {
     if (!enabled || !activeAccountId || accountClients.length === 0) return;
     if (!start || !end) return;
-    void fetchAllAccountsEvents(accountClients, activeAccountId, start, end);
+    void fetchAllAccountsEvents(accountClients, start, end);
   }, [enabled, activeAccountId, accountClients, start, end, fetchAllAccountsEvents]);
 
   return { enabled, accountClients };
diff --git a/hooks/use-pro-multi-account-contacts.ts b/hooks/use-pro-multi-account-contacts.ts
index a69b7c0d4..5a77950d4 100644
--- a/hooks/use-pro-multi-account-contacts.ts
+++ b/hooks/use-pro-multi-account-contacts.ts
@@ -40,8 +40,8 @@ export function useProMultiAccountContacts(): {
 
   useEffect(() => {
     if (!enabled || !activeAccountId || accountClients.length === 0) return;
-    void fetchAllAccountsAddressBooks(accountClients, activeAccountId);
-    void fetchAllAccountsContacts(accountClients, activeAccountId);
+    void fetchAllAccountsAddressBooks(accountClients);
+    void fetchAllAccountsContacts(accountClients);
   }, [enabled, activeAccountId, accountClients, fetchAllAccountsAddressBooks, fetchAllAccountsContacts]);
 
   return { enabled, accountClients };
diff --git a/integration/tests/11-multi-account-calendar.spec.ts b/integration/tests/11-multi-account-calendar.spec.ts
new file mode 100644
index 000000000..9e6bc259a
--- /dev/null
+++ b/integration/tests/11-multi-account-calendar.spec.ts
@@ -0,0 +1,89 @@
+import { test, expect, type Page } from '@playwright/test';
+import { ACCOUNTS } from './helpers/config';
+import {
+  login,
+  addAccount,
+  enableProInterface,
+  openCalendar,
+  calendarItem,
+  calendarRows,
+} from './helpers/app';
+
+/**
+ * Multi-account calendar aggregation across an account switch.
+ *
+ * Regression for the cross-account id-namespacing bug: the Pro shell used to
+ * leave the ACTIVE account's calendar ids raw while namespacing the others, so
+ * switching accounts flipped the id form and reset the persisted calendar
+ * visibility selection (and broke subscription/mutation routing). All accounts
+ * are now namespaced consistently, so the selection survives a switch.
+ *
+ * Mail is unaffected (it keys per account + uses stable virtual ids), which is
+ * why the user saw this only in Calendar and Address Book.
+ */
+const { alice, bob } = ACCOUNTS;
+
+const switcher = (page: Page) => page.locator('[data-testid="account-switcher"]').first();
+
+/** Read an account's id from the (stable, standard-shell) switcher popover. */
+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 ${email}`);
+  return id;
+}
+
+/**
+ * Switch active account robustly in the Pro shell. The switcher popover is a
+ * portalled, repositioning element, so a normal click can race its layout;
+ * force the click and verify via the switcher's data-active-account-id.
+ */
+async function switchTo(page: Page, id: string): Promise<void> {
+  await switcher(page).click();
+  await page.locator(`[data-testid="account-option"][data-account-id="${id}"]`).first().click({ force: true });
+  await expect
+    .poll(async () => switcher(page).getAttribute('data-active-account-id'), { timeout: 30000 })
+    .toBe(id);
+}
+
+test.describe('Multi-account calendar', () => {
+  test('an account switch preserves the calendar visibility selection', async ({ page }) => {
+    // Both accounts in the standard shell (stable switcher), capture their ids,
+    // then flip on the Pro interface so the calendar aggregates across accounts.
+    await login(page, alice);
+    await addAccount(page, bob);
+    const bobId = await accountId(page, bob.email);
+    await enableProInterface(page);
+
+    await openCalendar(page);
+
+    // Both accounts' (default) calendars aggregate into one list with distinct
+    // names, and every row is initially visible.
+    const rows = await calendarRows(page);
+    expect(rows.length).toBeGreaterThanOrEqual(2);
+    expect(new Set(rows.map((r) => r.name)).size).toBeGreaterThanOrEqual(2);
+
+    // Deselect one calendar; remember it by name.
+    const target = rows.find((r) => r.visible);
+    expect(target, 'expected at least one visible calendar').toBeTruthy();
+    const name = target!.name;
+    await calendarItem(page, name).click();
+    await expect(calendarItem(page, name)).toHaveAttribute('data-visible', 'false');
+
+    // Switch the active account. This alone triggered the bug: the just-
+    // deselected calendar belonged to the previously-active account, whose ids
+    // flipped raw->namespaced on the switch, so the reconciler dropped the
+    // selection and reset visibility to "all".
+    await switchTo(page, bobId);
+    await openCalendar(page);
+
+    // The deselection survived the switch (the calendar is still aggregated in
+    // and still hidden), and the list is still the cross-account set.
+    await expect(calendarItem(page, name)).toHaveAttribute('data-visible', 'false');
+    expect((await calendarRows(page)).length).toBeGreaterThanOrEqual(2);
+  });
+});
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 f9859b960..b671cc899 100644
--- a/integration/tests/helpers/app.ts
+++ b/integration/tests/helpers/app.ts
@@ -378,3 +378,76 @@ export async function emailContextAction(page: Page, subject: string, testId: st
   }).toPass({ timeout: 15000 });
   await item.click();
 }
+
+// --- Calendar (multi-account) ---------------------------------------------
+
+/**
+ * Navigate to the calendar via the in-app nav (client-side routing) and wait
+ * for the sidebar calendar list. A hard `page.goto('/calendar')` redirects to
+ * login because the deep-link SSR path runs before client auth hydrates.
+ */
+export async function openCalendar(page: Page): Promise<void> {
+  await page.locator('a[href$="/calendar"]').first().click();
+  await page
+    .locator('[data-testid="calendar-item"]')
+    .first()
+    .waitFor({ state: 'visible', timeout: 30000 });
+}
+
+/** A sidebar calendar row by its (exact) name. */
+export function calendarItem(page: Page, name: string): Locator {
+  return page.locator(`[data-testid="calendar-item"][data-calendar-name="${name}"]`).first();
+}
+
+/** All sidebar calendars as {name, account, visible}. */
+export async function calendarRows(
+  page: Page,
+): Promise<Array<{ name: string; account: string; visible: boolean }>> {
+  return page.locator('[data-testid="calendar-item"]').evaluateAll((els) =>
+    els.map((e) => ({
+      name: e.getAttribute('data-calendar-name') || '',
+      account: e.getAttribute('data-account') || '',
+      visible: e.getAttribute('data-visible') === 'true',
+    })),
+  );
+}
+
+/**
+ * Turn on the Pro (multi-account) interface via the settings UI, so the
+ * calendar/contacts aggregate across accounts. Done through the real toggle
+ * (not a reload/seed) because the app doesn't survive a hard reload in tests
+ * and seeding proInterface before login destabilises the add-account popover.
+ */
+export async function enableProInterface(page: Page): Promise<void> {
+  await page.locator('a[href$="/settings"]').first().click();
+  await page.locator('[data-testid="settings-tab-layout"]').first().click();
+  const toggle = page.locator('[data-testid="setting-pro-interface"]').first();
+  await toggle.waitFor({ state: 'visible', timeout: 30000 });
+  if ((await toggle.getAttribute('aria-checked')) !== 'true') {
+    await toggle.click();
+    await expect(toggle).toHaveAttribute('aria-checked', 'true');
+  }
+}
+
+// --- Contacts / address books (multi-account) ------------------------------
+
+/** Navigate to contacts via in-app nav and wait for the address-book list. */
+export async function openContacts(page: Page): Promise<void> {
+  await page.locator('a[href$="/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') || '',
+    })),
+  );
+}
diff --git a/stores/__tests__/calendar-store-namespacing.test.ts b/stores/__tests__/calendar-store-namespacing.test.ts
new file mode 100644
index 000000000..96780c315
--- /dev/null
+++ b/stores/__tests__/calendar-store-namespacing.test.ts
@@ -0,0 +1,93 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { useCalendarStore, reconcileSelectedIds } from '../calendar-store';
+import type { Calendar } from '@/lib/jmap/types';
+
+function cal(overrides: Partial<Calendar>): Calendar {
+  return {
+    id: 'c', name: 'Cal', color: null, sortOrder: 0, isSubscribed: true, isVisible: true,
+    isDefault: false, includeInAvailability: 'all', timeZone: null, shareWith: null,
+    myRights: {
+      mayReadFreeBusy: true, mayReadItems: true, mayWriteAll: true, mayWriteOwn: true,
+      mayUpdatePrivate: true, mayRSVP: true, mayShare: true, mayDelete: true,
+    },
+    ...overrides,
+  } as Calendar;
+}
+
+describe('reconcileSelectedIds (account-switch selection stability)', () => {
+  it('keeps a selection whose id form flipped raw -> namespaced', () => {
+    // Persisted while account A was active (raw "b"); after the fix A is
+    // namespaced. The calendar is the same, only the id string changed.
+    const calendars = [cal({ id: 'A@h::b', originalId: 'b', localAccountId: 'A@h' })];
+    expect(reconcileSelectedIds(['b'], calendars)).toEqual(['A@h::b']);
+  });
+
+  it('keeps an already-namespaced selection unchanged across a switch', () => {
+    const calendars = [
+      cal({ id: 'A@h::b', originalId: 'b', localAccountId: 'A@h' }),
+      cal({ id: 'B@h::b', originalId: 'b', localAccountId: 'B@h' }),
+    ];
+    // Both accounts have raw id "b"; the namespaced selection is unambiguous.
+    expect(reconcileSelectedIds(['B@h::b'], calendars)).toEqual(['B@h::b']);
+  });
+
+  it('remaps a just-created raw id to its namespaced form after refetch', () => {
+    const calendars = [
+      cal({ id: 'A@h::b', originalId: 'b', localAccountId: 'A@h' }),
+      cal({ id: 'A@h::f', originalId: 'f', localAccountId: 'A@h' }),
+    ];
+    // createCalendar added the raw "f"; the refetch namespaced it.
+    expect(reconcileSelectedIds(['A@h::b', 'f'], calendars)).toEqual(['A@h::b', 'A@h::f']);
+  });
+
+  it('drops ids that resolve to no calendar and de-dupes', () => {
+    const calendars = [cal({ id: 'A@h::b', originalId: 'b', localAccountId: 'A@h' })];
+    expect(reconcileSelectedIds(['b', 'A@h::b', 'gone'], calendars)).toEqual(['A@h::b']);
+  });
+
+  it('preserves the birthday calendar id verbatim', () => {
+    const calendars = [cal({ id: 'A@h::b', originalId: 'b', localAccountId: 'A@h' })];
+    expect(reconcileSelectedIds(['__birthday-calendar__', 'b'], calendars)).toEqual(['__birthday-calendar__', 'A@h::b']);
+  });
+
+  it('leaves single-account (raw, no localAccountId) selections untouched', () => {
+    const calendars = [cal({ id: 'b' }), cal({ id: 'c' })];
+    expect(reconcileSelectedIds(['b', 'c'], calendars)).toEqual(['b', 'c']);
+  });
+});
+
+describe('isSubscriptionCalendar (namespace-robust, collision-safe)', () => {
+  beforeEach(() => {
+    useCalendarStore.setState({ calendars: [], icalSubscriptions: [] });
+  });
+
+  it('matches a subscription across the raw->namespaced boundary', () => {
+    useCalendarStore.setState({
+      calendars: [cal({ id: 'A@h::sub1', originalId: 'sub1', localAccountId: 'A@h', accountId: 'acctA' })],
+      // subs store the raw JMAP calendar id
+      icalSubscriptions: [{ id: 's', url: 'x', calendarId: 'sub1', accountId: 'acctA', name: 'Feed', color: '#000', refreshInterval: 60, lastRefreshed: null }],
+    });
+    expect(useCalendarStore.getState().isSubscriptionCalendar('A@h::sub1')).toBe(true);
+  });
+
+  it('does NOT false-positive when another account has the same raw id', () => {
+    useCalendarStore.setState({
+      calendars: [
+        cal({ id: 'A@h::b', originalId: 'b', localAccountId: 'A@h', accountId: 'acctA' }), // A's subscription calendar
+        cal({ id: 'B@h::b', originalId: 'b', localAccountId: 'B@h', accountId: 'acctB' }), // B's normal calendar, same raw id
+      ],
+      icalSubscriptions: [{ id: 's', url: 'x', calendarId: 'b', accountId: 'acctA', name: 'Feed', color: '#000', refreshInterval: 60, lastRefreshed: null }],
+    });
+    expect(useCalendarStore.getState().isSubscriptionCalendar('A@h::b')).toBe(true);
+    expect(useCalendarStore.getState().isSubscriptionCalendar('B@h::b')).toBe(false);
+  });
+
+  it('matches a single-account (raw) subscription exactly', () => {
+    useCalendarStore.setState({
+      calendars: [cal({ id: 'sub1' })],
+      icalSubscriptions: [{ id: 's', url: 'x', calendarId: 'sub1', name: 'Feed', color: '#000', refreshInterval: 60, lastRefreshed: null }],
+    });
+    expect(useCalendarStore.getState().isSubscriptionCalendar('sub1')).toBe(true);
+    expect(useCalendarStore.getState().isSubscriptionCalendar('other')).toBe(false);
+  });
+});
diff --git a/stores/calendar-store.ts b/stores/calendar-store.ts
index 62e8a1936..5346bf1b5 100644
--- a/stores/calendar-store.ts
+++ b/stores/calendar-store.ts
@@ -42,6 +42,42 @@ function stripLocalAccountPrefix(id: string, localAccountId?: string): string {
   return id.startsWith(prefix) ? id.slice(prefix.length) : id;
 }
 
+/** The raw identity of a store id, independent of any `<localAccountId>::` prefix. */
+function rawIdentityOf(id: string): string {
+  const idx = id.indexOf(CROSS_ACCOUNT_ID_DELIMITER);
+  return idx >= 0 ? id.slice(idx + CROSS_ACCOUNT_ID_DELIMITER.length) : id;
+}
+
+/**
+ * Reconcile persisted selectedCalendarIds against a freshly fetched calendars
+ * list. A selected id can be stale in id-*form* (not existence): the raw->
+ * namespaced transition (aggregation switched on, an account switch, or a
+ * just-created calendar added with its raw id) changes the id string while the
+ * calendar is the same. Remap by raw identity so the selection survives instead
+ * of silently resetting to "all". Keeps BIRTHDAY_CALENDAR_ID; drops ids that
+ * resolve to no calendar.
+ */
+export function reconcileSelectedIds(selectedCalendarIds: string[], calendars: Calendar[]): string[] {
+  const byStoreId = new Set(calendars.map((c) => c.id));
+  const rawToStoreId = new Map<string, string>();
+  for (const c of calendars) {
+    const raw = c.originalId ?? stripLocalAccountPrefix(c.id, c.localAccountId);
+    if (!rawToStoreId.has(raw)) rawToStoreId.set(raw, c.id);
+  }
+  const out: string[] = [];
+  const seen = new Set<string>();
+  for (const id of selectedCalendarIds) {
+    let mapped: string | undefined;
+    if (id === BIRTHDAY_CALENDAR_ID || byStoreId.has(id)) mapped = id;
+    else mapped = rawToStoreId.get(rawIdentityOf(id));
+    if (mapped && !seen.has(mapped)) {
+      out.push(mapped);
+      seen.add(mapped);
+    }
+  }
+  return out;
+}
+
 // In-flight refresh dedup. Concurrent callers (auto-interval +
 // manual refresh, two account-switch reloads, etc.) share the same
 // promise instead of double-fetching and racing the diff/import phase.
@@ -56,10 +92,11 @@ export function isCalendarViewMode(value: unknown): value is CalendarViewMode {
 }
 
 /**
- * Prefix used to namespace calendar/event IDs that belong to a non-active
- * JMAP account when the Pro shell aggregates across accounts. The active
- * account's IDs are left untouched so existing single-account code paths
- * (links, deep-links, JMAP mutations) keep working unchanged.
+ * Prefix used to namespace calendar/event IDs when the Pro shell aggregates
+ * across accounts. EVERY aggregated account is namespaced (including the active
+ * one) so an id stably identifies (account, entity) regardless of which account
+ * is active - otherwise switching accounts flipped the id form. Single-account
+ * (non-aggregated) code paths carry no localAccountId and keep raw ids.
  */
 const CROSS_ACCOUNT_ID_DELIMITER = '::';
 
@@ -67,14 +104,18 @@ function buildCrossAccountIdPrefix(localAccountId: string): string {
   return `${localAccountId}${CROSS_ACCOUNT_ID_DELIMITER}`;
 }
 
+// EVERY aggregated account is namespaced, including the active one. Leaving the
+// active account's IDs raw made an entity's ID depend on *which* account is
+// active, so switching accounts flipped the ID form and invalidated persisted
+// references (selectedCalendarIds, colors, subscriptions) - the account-switch
+// bug. The invariant is now: an entity is namespaced iff it carries a
+// `localAccountId`; `originalId` always holds the raw JMAP ID. Mutation/consumer
+// code already resolves the raw ID via `originalId || stripLocalAccountPrefix`,
+// so it keeps working unchanged.
 function prefixCalendarsWithLocalAccount(
   calendars: Calendar[],
   localAccountId: string,
-  isActiveAccount: boolean,
 ): Calendar[] {
-  if (isActiveAccount) {
-    return calendars.map((cal) => ({ ...cal, localAccountId }));
-  }
   const prefix = buildCrossAccountIdPrefix(localAccountId);
   // Preserve each calendar's original `isShared` flag - it distinguishes
   // the user's own calendars on the other account from calendars shared
@@ -83,6 +124,7 @@ function prefixCalendarsWithLocalAccount(
   return calendars.map((cal) => ({
     ...cal,
     id: `${prefix}${cal.id}`,
+    originalId: cal.originalId ?? cal.id,
     localAccountId,
   }));
 }
@@ -90,15 +132,12 @@ function prefixCalendarsWithLocalAccount(
 function prefixEventsWithLocalAccount(
   events: CalendarEvent[],
   localAccountId: string,
-  isActiveAccount: boolean,
 ): CalendarEvent[] {
-  if (isActiveAccount) {
-    return events.map((event) => ({ ...event, localAccountId }));
-  }
   const prefix = buildCrossAccountIdPrefix(localAccountId);
   return events.map((event) => ({
     ...event,
     id: `${prefix}${event.id}`,
+    originalId: event.originalId ?? event.id,
     localAccountId,
     calendarIds: event.calendarIds
       ? Object.fromEntries(
@@ -143,15 +182,21 @@ function mapServerEventToStoreEvent(
     .find((calendar): calendar is Calendar => Boolean(calendar));
   const resolvedAccountId = matchedCalendar?.accountId || targetAccountId;
   const isShared = matchedCalendar?.isShared || false;
+  // Namespace the event id with the owning account when the store is aggregated
+  // (the matched calendar carries a localAccountId). `calendarIds` are already
+  // mapped to store ids above; `originalId` keeps the raw id for mutations.
+  const localAccountId = matchedCalendar?.localAccountId;
+  const baseId = isShared && resolvedAccountId ? `${resolvedAccountId}:${event.id}` : event.id;
 
   return {
     ...event,
-    id: isShared && resolvedAccountId ? `${resolvedAccountId}:${event.id}` : event.id,
+    id: localAccountId ? `${localAccountId}${CROSS_ACCOUNT_ID_DELIMITER}${baseId}` : baseId,
     originalId: event.id,
     originalCalendarIds: event.calendarIds,
     calendarIds: mappedCalendarIds,
     accountId: resolvedAccountId,
     accountName: matchedCalendar?.accountName,
+    localAccountId,
     isShared,
   };
 }
@@ -224,8 +269,8 @@ interface CalendarStore {
   setSupported: (supported: boolean) => void;
   fetchCalendars: (client: IJMAPClient) => Promise<void>;
   fetchEvents: (client: IJMAPClient, start: string, end: string) => Promise<void>;
-  fetchAllAccountsCalendars: (accounts: CalendarAccountClient[], activeLocalAccountId: string) => Promise<void>;
-  fetchAllAccountsEvents: (accounts: CalendarAccountClient[], activeLocalAccountId: string, start: string, end: string) => Promise<void>;
+  fetchAllAccountsCalendars: (accounts: CalendarAccountClient[]) => Promise<void>;
+  fetchAllAccountsEvents: (accounts: CalendarAccountClient[], start: string, end: string) => Promise<void>;
   createEvent: (client: IJMAPClient, event: Partial<CalendarEvent>, sendSchedulingMessages?: boolean) => Promise<CalendarEvent | null>;
   updateEvent: (client: IJMAPClient, id: string, updates: Partial<CalendarEvent>, sendSchedulingMessages?: boolean) => Promise<void>;
   deleteEvent: (client: IJMAPClient, id: string, sendSchedulingMessages?: boolean) => Promise<void>;
@@ -284,12 +329,11 @@ export const useCalendarStore = create<CalendarStore>()(
         try {
           const calendars = await client.getAllCalendars();
           const { selectedCalendarIds } = get();
-          const validIds = calendars.map(c => c.id);
-          const stillValid = selectedCalendarIds.filter(id => validIds.includes(id) || id === BIRTHDAY_CALENDAR_ID);
+          const stillValid = reconcileSelectedIds(selectedCalendarIds, calendars);
           set({
             calendars,
             isLoading: false,
-            selectedCalendarIds: stillValid.length > 0 ? stillValid : validIds,
+            selectedCalendarIds: stillValid.length > 0 ? stillValid : calendars.map(c => c.id),
           });
         } catch (error) {
           debug.error('Failed to fetch calendars:', error);
@@ -332,18 +376,14 @@ export const useCalendarStore = create<CalendarStore>()(
         }
       },
 
-      fetchAllAccountsCalendars: async (accounts, activeLocalAccountId) => {
+      fetchAllAccountsCalendars: async (accounts) => {
         set({ isLoading: true, error: null });
         try {
           const results = await Promise.all(
             accounts.map(async ({ client, localAccountId }) => {
               try {
                 const list = await client.getAllCalendars();
-                return prefixCalendarsWithLocalAccount(
-                  list,
-                  localAccountId,
-                  localAccountId === activeLocalAccountId,
-                );
+                return prefixCalendarsWithLocalAccount(list, localAccountId);
               } catch (error) {
                 debug.error(`Failed to fetch calendars for account ${localAccountId}:`, error);
                 return [] as Calendar[];
@@ -352,12 +392,11 @@ export const useCalendarStore = create<CalendarStore>()(
           );
           const calendars = results.flat();
           const { selectedCalendarIds } = get();
-          const validIds = calendars.map(c => c.id);
-          const stillValid = selectedCalendarIds.filter(id => validIds.includes(id) || id === BIRTHDAY_CALENDAR_ID);
+          const stillValid = reconcileSelectedIds(selectedCalendarIds, calendars);
           set({
             calendars,
             isLoading: false,
-            selectedCalendarIds: stillValid.length > 0 ? stillValid : validIds,
+            selectedCalendarIds: stillValid.length > 0 ? stillValid : calendars.map(c => c.id),
           });
         } catch (error) {
           debug.error('Failed to fetch all-account calendars:', error);
@@ -365,7 +404,7 @@ export const useCalendarStore = create<CalendarStore>()(
         }
       },
 
-      fetchAllAccountsEvents: async (accounts, activeLocalAccountId, start, end) => {
+      fetchAllAccountsEvents: async (accounts, start, end) => {
         set({ isLoadingEvents: true, error: null });
         try {
           const results = await Promise.all(
@@ -376,11 +415,7 @@ export const useCalendarStore = create<CalendarStore>()(
                   typeof e.start === 'string' && e.start && !isNaN(parseISO(e.start).getTime())
                 );
                 const expanded = expandRecurringEvents(valid, start, end);
-                return prefixEventsWithLocalAccount(
-                  expanded,
-                  localAccountId,
-                  localAccountId === activeLocalAccountId,
-                );
+                return prefixEventsWithLocalAccount(expanded, localAccountId);
               } catch (error) {
                 debug.error(`Failed to fetch events for account ${localAccountId}:`, error);
                 return [] as CalendarEvent[];
@@ -884,6 +919,8 @@ export const useCalendarStore = create<CalendarStore>()(
         set({ error: null });
         try {
           const created = await client.createCalendar(calendar);
+          // Added with its raw id; the next aggregated refetch namespaces it and
+          // reconcileSelectedIds() remaps the selection so it stays visible.
           set((state) => ({
             calendars: [...state.calendars, created],
             selectedCalendarIds: [...state.selectedCalendarIds, created.id],
@@ -997,7 +1034,20 @@ export const useCalendarStore = create<CalendarStore>()(
 
       // iCal subscriptions
       isSubscriptionCalendar: (calendarId) => {
-        return get().icalSubscriptions.some(s => s.calendarId === calendarId);
+        const subs = get().icalSubscriptions;
+        // Fast path: exact match (single-account, or a sub stored with the same
+        // id form as the query).
+        if (subs.some((s) => s.calendarId === calendarId)) return true;
+        // Aggregated: the query is a namespaced store id while subs store the raw
+        // JMAP calendar id. Resolve the calendar and match by raw id, scoped to
+        // the owning JMAP account so raw ids that collide across accounts (e.g.
+        // Stalwart's "b"/"c") don't cause false positives.
+        const cal = get().calendars.find((c) => c.id === calendarId);
+        if (!cal) return false;
+        const rawId = cal.originalId ?? stripLocalAccountPrefix(cal.id, cal.localAccountId);
+        return subs.some(
+          (s) => s.calendarId === rawId && (!s.accountId || !cal.accountId || s.accountId === cal.accountId),
+        );
       },
 
       addICalSubscription: async (client, url, name, color, refreshInterval = 60) => {
diff --git a/stores/contact-store.ts b/stores/contact-store.ts
index 9243fe573..bebce70ef 100644
--- a/stores/contact-store.ts
+++ b/stores/contact-store.ts
@@ -55,10 +55,9 @@ export interface ContactAccountClient {
 }
 
 /**
- * Prefix used to namespace contact/address-book IDs that belong to a
- * non-active JMAP account when the Pro shell aggregates across accounts.
- * The active account's IDs are left untouched so existing single-account
- * code paths keep working unchanged.
+ * Prefix used to namespace contact/address-book IDs when the Pro shell
+ * aggregates across accounts. EVERY aggregated account is namespaced (including
+ * the active one); single-account paths carry no localAccountId and stay raw.
  */
 const CROSS_ACCOUNT_ID_DELIMITER = '::';
 
@@ -66,18 +65,21 @@ function buildCrossAccountIdPrefix(localAccountId: string): string {
   return `${localAccountId}${CROSS_ACCOUNT_ID_DELIMITER}`;
 }
 
+// EVERY aggregated account is namespaced, including the active one, so an id
+// stably identifies (account, entity) regardless of which account is active -
+// otherwise switching accounts flipped the id form and broke mutation routing /
+// selection. Invariant: namespaced iff `localAccountId` is set; `originalId`
+// holds the raw id. Mutations already resolve raw via `originalId ||
+// stripLocalAccountPrefix`, so they keep working. Mirrors the calendar store.
 function prefixAddressBooksWithLocalAccount(
   books: AddressBook[],
   localAccountId: string,
-  isActiveAccount: boolean,
 ): AddressBook[] {
-  if (isActiveAccount) {
-    return books.map((b) => ({ ...b, localAccountId }));
-  }
   const prefix = buildCrossAccountIdPrefix(localAccountId);
   return books.map((b) => ({
     ...b,
     id: `${prefix}${b.id}`,
+    originalId: b.originalId ?? b.id,
     localAccountId,
   }));
 }
@@ -85,15 +87,12 @@ function prefixAddressBooksWithLocalAccount(
 function prefixContactsWithLocalAccount(
   contacts: ContactCard[],
   localAccountId: string,
-  isActiveAccount: boolean,
 ): ContactCard[] {
-  if (isActiveAccount) {
-    return contacts.map((c) => ({ ...c, localAccountId }));
-  }
   const prefix = buildCrossAccountIdPrefix(localAccountId);
   return contacts.map((c) => ({
     ...c,
     id: `${prefix}${c.id}`,
+    originalId: c.originalId ?? c.id,
     localAccountId,
     addressBookIds: c.addressBookIds
       ? Object.fromEntries(
@@ -277,8 +276,8 @@ interface ContactStore {
   fetchContacts: (client: IJMAPClient) => Promise<void>;
   fetchDirectory: (client: IJMAPClient) => Promise<void>;
   fetchAddressBooks: (client: IJMAPClient) => Promise<void>;
-  fetchAllAccountsContacts: (accounts: ContactAccountClient[], activeLocalAccountId: string) => Promise<void>;
-  fetchAllAccountsAddressBooks: (accounts: ContactAccountClient[], activeLocalAccountId: string) => Promise<void>;
+  fetchAllAccountsContacts: (accounts: ContactAccountClient[]) => Promise<void>;
+  fetchAllAccountsAddressBooks: (accounts: ContactAccountClient[]) => Promise<void>;
   createContact: (client: IJMAPClient, contact: Partial<ContactCard>) => Promise<void>;
   updateContact: (client: IJMAPClient, id: string, updates: Partial<ContactCard>) => Promise<void>;
   deleteContact: (client: IJMAPClient, id: string) => Promise<void>;
@@ -429,18 +428,14 @@ export const useContactStore = create<ContactStore>()(
         }
       },
 
-      fetchAllAccountsContacts: async (accounts, activeLocalAccountId) => {
+      fetchAllAccountsContacts: async (accounts) => {
         set({ isLoading: true, error: null });
         try {
           const results = await Promise.all(
             accounts.map(async ({ client, localAccountId }) => {
               try {
                 const list = await client.getAllContacts();
-                return prefixContactsWithLocalAccount(
-                  list,
-                  localAccountId,
-                  localAccountId === activeLocalAccountId,
-                );
+                return prefixContactsWithLocalAccount(list, localAccountId);
               } catch (error) {
                 debug.error(`Failed to fetch contacts for account ${localAccountId}:`, error);
                 return [] as ContactCard[];
@@ -454,17 +449,13 @@ export const useContactStore = create<ContactStore>()(
         }
       },
 
-      fetchAllAccountsAddressBooks: async (accounts, activeLocalAccountId) => {
+      fetchAllAccountsAddressBooks: async (accounts) => {
         try {
           const results = await Promise.all(
             accounts.map(async ({ client, localAccountId }) => {
               try {
                 const list = await client.getAllAddressBooks();
-                return prefixAddressBooksWithLocalAccount(
-                  list,
-                  localAccountId,
-                  localAccountId === activeLocalAccountId,
-                );
+                return prefixAddressBooksWithLocalAccount(list, localAccountId);
               } catch (error) {
                 debug.error(`Failed to fetch address books for account ${localAccountId}:`, error);
                 return [] as AddressBook[];