diff --git a/components/contacts/__tests__/contacts-sidebar.test.tsx b/components/contacts/__tests__/contacts-sidebar.test.tsx
index a47534d0a..11a15b9a3 100644
--- a/components/contacts/__tests__/contacts-sidebar.test.tsx
+++ b/components/contacts/__tests__/contacts-sidebar.test.tsx
@@ -2,7 +2,7 @@ import { render, screen } from '@testing-library/react';
 import { fireEvent } from '@testing-library/dom';
 import { describe, it, expect, vi, beforeEach } from 'vitest';
 import { ContactsSidebar } from '../contacts-sidebar';
-import type { ContactCard } from '@/lib/jmap/types';
+import type { AddressBook, ContactCard } from '@/lib/jmap/types';
 
 // next-intl + next/navigation are mocked globally in vitest.setup (t returns the key).
 vi.mock('@/stores/account-store', () => {
@@ -63,3 +63,51 @@ describe('ContactsSidebar — compose to group', () => {
     expect(onComposeGroup).toHaveBeenCalledWith('g1', 'cc');
   });
 });
+
+const books: AddressBook[] = [
+  { id: 'ab-1', name: 'Personal', isDefault: true },
+  { id: 'ab-2', name: 'Work' },
+  { id: 'ab-3', name: 'Team', isShared: true, accountId: 'account-2', accountName: 'Team account' },
+];
+
+function renderBooksSidebar(onSetDefaultAddressBook = vi.fn()) {
+  render(
+    <ContactsSidebar
+      groups={[]}
+      individuals={[]}
+      addressBooks={books}
+      activeCategory="all"
+      onSelectCategory={vi.fn()}
+      onCreateGroup={vi.fn()}
+      onCreateContact={vi.fn()}
+      onSetDefaultAddressBook={onSetDefaultAddressBook}
+    />,
+  );
+  return onSetDefaultAddressBook;
+}
+
+describe('ContactsSidebar - set default address book', () => {
+  beforeEach(() => vi.clearAllMocks());
+
+  it('offers "Set as default" for an owned, non-default book', () => {
+    const onSetDefaultAddressBook = renderBooksSidebar();
+    fireEvent.contextMenu(screen.getByText('Work'));
+
+    fireEvent.click(screen.getByText('address_books.set_default'));
+    expect(onSetDefaultAddressBook).toHaveBeenCalledWith(expect.objectContaining({ id: 'ab-2' }));
+  });
+
+  it('hides it for the book that is already the default', () => {
+    renderBooksSidebar();
+    fireEvent.contextMenu(screen.getByText('Personal'));
+
+    expect(screen.queryByText('address_books.set_default')).not.toBeInTheDocument();
+  });
+
+  it('hides it for a book shared from another account', () => {
+    renderBooksSidebar();
+    fireEvent.contextMenu(screen.getByText('Team'));
+
+    expect(screen.queryByText('address_books.set_default')).not.toBeInTheDocument();
+  });
+});
diff --git a/components/contacts/contact-form.tsx b/components/contacts/contact-form.tsx
index 566d53011..b8265c545 100644
--- a/components/contacts/contact-form.tsx
+++ b/components/contacts/contact-form.tsx
@@ -359,6 +359,12 @@ export function ContactForm({ contact, addressBooks, allKeywords, defaultAddress
     if (defaultAddressBookId && addressBooks?.some(b => b.id === defaultAddressBookId)) {
       return defaultAddressBookId;
     }
+    // New contacts land in the account's default book unless told otherwise -
+    // preselect it so the form shows where the contact will actually go.
+    if (!contact) {
+      const ownDefault = addressBooks?.find(b => b.isDefault && !b.isShared);
+      if (ownDefault) return ownDefault.id;
+    }
     return "";
   }, [contact, defaultAddressBookId, addressBooks]);
   const [selectedBookId, setSelectedBookId] = useState(currentBookId);
diff --git a/components/contacts/contacts-app.tsx b/components/contacts/contacts-app.tsx
index a9def7097..ff7292a0f 100644
--- a/components/contacts/contacts-app.tsx
+++ b/components/contacts/contacts-app.tsx
@@ -98,6 +98,7 @@ export function ContactsApp({ linkSegments }: ContactsAppProps = {}) {
     renameAddressBook,
     removeAddressBook,
     shareAddressBook,
+    setDefaultAddressBook,
     renameKeyword,
     importContacts,
   } = useContactStore();
@@ -950,6 +951,14 @@ export function ContactsApp({ linkSegments }: ContactsAppProps = {}) {
                       onDropContactsToCategory={handleDropContactsToCategory}
                       onRenameAddressBook={client ? (book) => setRenamingAddressBook(book) : undefined}
                       onShareAddressBook={client ? (book) => setSharingAddressBookId(book.id) : undefined}
+                      onSetDefaultAddressBook={client ? async (book) => {
+                        try {
+                          await setDefaultAddressBook(client, book);
+                          toast.success(t("address_books.default_updated"));
+                        } catch {
+                          toast.error(t("address_books.set_default_failed"));
+                        }
+                      } : undefined}
                       onCreateContactInBook={(book) => {
                         setDefaultBookIdForCreate(book.id);
                         handleCreateNew();
diff --git a/components/contacts/contacts-sidebar.tsx b/components/contacts/contacts-sidebar.tsx
index c645f2d3d..dc6ed5ad5 100644
--- a/components/contacts/contacts-sidebar.tsx
+++ b/components/contacts/contacts-sidebar.tsx
@@ -2,7 +2,7 @@
 
 import { useMemo, useState, useCallback, useEffect, useRef, type DragEvent } from "react";
 import { useTranslations } from "next-intl";
-import { BookUser, User, Users, Plus, Share2, Book, BookPlus, ChevronRight, ChevronDown, UserPlus, UsersRound, Upload, Tag, Pencil, Trash2, Settings, Mail } from "lucide-react";
+import { BookUser, User, Users, Plus, Share2, Book, BookPlus, ChevronRight, ChevronDown, UserPlus, UsersRound, Upload, Tag, Pencil, Trash2, Settings, Mail, Star } from "lucide-react";
 import { useRouter } from "next/navigation";
 import { Button } from "@/components/ui/button";
 import { ContextMenu, ContextMenuItem, ContextMenuSeparator, ContextMenuSubMenu } from "@/components/ui/context-menu";
@@ -31,6 +31,7 @@ interface ContactsSidebarProps {
   onDropContactsToCategory?: (contactIds: string[], keyword: string) => void;
   onRenameAddressBook?: (addressBook: AddressBook) => void;
   onShareAddressBook?: (addressBook: AddressBook) => void;
+  onSetDefaultAddressBook?: (addressBook: AddressBook) => void;
   onCreateContactInBook?: (addressBook: AddressBook) => void;
   onDeleteAddressBook?: (addressBook: AddressBook) => void;
   onRenameKeyword?: (keyword: string) => void;
@@ -101,6 +102,7 @@ export function ContactsSidebar({
   onDropContactsToCategory,
   onRenameAddressBook,
   onShareAddressBook,
+  onSetDefaultAddressBook,
   onCreateContactInBook,
   onDeleteAddressBook,
   onRenameKeyword,
@@ -111,6 +113,7 @@ export function ContactsSidebar({
   const router = useRouter();
   const { contextMenu: groupContextMenu, openContextMenu: openGroupContextMenu, closeContextMenu: closeGroupContextMenu, menuRef: groupMenuRef } = useContextMenu<ContactCard>();
   const { contextMenu: bookContextMenu, openContextMenu: openBookContextMenu, closeContextMenu: closeBookContextMenu, menuRef: bookMenuRef } = useContextMenu<AddressBook>();
+  const hasBookMenu = !!(onRenameAddressBook || onShareAddressBook || onSetDefaultAddressBook || onCreateContactInBook || onDeleteAddressBook);
   const { contextMenu: keywordContextMenu, openContextMenu: openKeywordContextMenu, closeContextMenu: closeKeywordContextMenu, menuRef: keywordMenuRef } = useContextMenu<string>();
 
   const [collapsed, setCollapsed] = useState<Record<string, boolean>>(loadCollapsed);
@@ -383,7 +386,7 @@ export function ContactsSidebar({
                             contactCount={contactCountByBook[book.id] || 0}
                             onSelect={() => onSelectCategory({ addressBookId: book.id })}
                             onDropContacts={onDropContacts}
-                            onContextMenu={(onRenameAddressBook || onShareAddressBook || onCreateContactInBook || onDeleteAddressBook) ? (e) => openBookContextMenu(e, book) : undefined}
+                            onContextMenu={hasBookMenu ? (e) => openBookContextMenu(e, book) : undefined}
                           />
                         ))}
                       </div>
@@ -402,7 +405,7 @@ export function ContactsSidebar({
                             contactCount={contactCountByBook[book.id] || 0}
                             onSelect={() => onSelectCategory({ addressBookId: book.id })}
                             onDropContacts={onDropContacts}
-                            onContextMenu={(onRenameAddressBook || onShareAddressBook || onCreateContactInBook || onDeleteAddressBook) ? (e) => openBookContextMenu(e, book) : undefined}
+                            onContextMenu={hasBookMenu ? (e) => openBookContextMenu(e, book) : undefined}
                           />
                         ))}
                       </div>
@@ -449,7 +452,7 @@ export function ContactsSidebar({
                   contactCount={contactCountByBook[book.id] || 0}
                   onSelect={() => onSelectCategory({ addressBookId: book.id })}
                   onDropContacts={onDropContacts}
-                  onContextMenu={(onRenameAddressBook || onShareAddressBook || onCreateContactInBook || onDeleteAddressBook) ? (e) => openBookContextMenu(e, book) : undefined}
+                  onContextMenu={hasBookMenu ? (e) => openBookContextMenu(e, book) : undefined}
                 />
               ))}
             </div>
@@ -593,7 +596,7 @@ export function ContactsSidebar({
                 contactCount={contactCountByBook[book.id] || 0}
                 onSelect={() => onSelectCategory({ addressBookId: book.id })}
                 onDropContacts={onDropContacts}
-                onContextMenu={(onRenameAddressBook || onShareAddressBook || onCreateContactInBook || onDeleteAddressBook) ? (e) => openBookContextMenu(e, book) : undefined}
+                onContextMenu={hasBookMenu ? (e) => openBookContextMenu(e, book) : undefined}
               />
             ))}
           </div>
@@ -601,13 +604,16 @@ export function ContactsSidebar({
       </div>
 
       {/* Address book context menu */}
-      {bookContextMenu.data && (onRenameAddressBook || onShareAddressBook || onCreateContactInBook || onDeleteAddressBook) && (() => {
+      {bookContextMenu.data && hasBookMenu && (() => {
         const book = bookContextMenu.data;
         const canCreate = onCreateContactInBook && book.myRights?.mayWrite !== false;
         const canRename = onRenameAddressBook && book.myRights?.mayWrite !== false;
         const canShare = onShareAddressBook && book.myRights?.mayShare && !book.isShared;
+        // The default only applies to books the account owns, and re-setting the
+        // current default is a no-op, so hide it in both cases.
+        const canSetDefault = onSetDefaultAddressBook && !book.isShared && !book.isDefault;
         const canDelete = onDeleteAddressBook && !book.isDefault && !book.isShared && book.myRights?.mayDelete !== false;
-        const showSeparator = (canCreate || canRename || canShare) && canDelete;
+        const showSeparator = (canCreate || canRename || canShare || canSetDefault) && canDelete;
         return (
           <ContextMenu
             ref={bookMenuRef}
@@ -645,6 +651,16 @@ export function ContactsSidebar({
                 }}
               />
             )}
+            {canSetDefault && (
+              <ContextMenuItem
+                icon={Star}
+                label={t("address_books.set_default")}
+                onClick={() => {
+                  closeBookContextMenu();
+                  onSetDefaultAddressBook(book);
+                }}
+              />
+            )}
             {showSeparator && <ContextMenuSeparator />}
             {canDelete && (
               <ContextMenuItem
diff --git a/components/settings/address-book-management-settings.tsx b/components/settings/address-book-management-settings.tsx
index da9a5d3ca..dbe72321d 100644
--- a/components/settings/address-book-management-settings.tsx
+++ b/components/settings/address-book-management-settings.tsx
@@ -2,7 +2,7 @@
 
 import { useEffect, useState } from "react";
 import { useTranslations } from "next-intl";
-import { Book, BookPlus, Pencil, Share2, Tag, Users } from "lucide-react";
+import { Book, BookPlus, Pencil, Share2, Star, Tag, Users } from "lucide-react";
 import { useContactStore } from "@/stores/contact-store";
 import { useAuthStore } from "@/stores/auth-store";
 import { useManagedAccountStore } from "@/stores/managed-account-store";
@@ -74,7 +74,7 @@ export function AddressBookManagementSettings() {
   const tSettings = useTranslations("settings.contacts");
   const { client } = useAuthStore();
   const managedAccountId = useManagedAccountStore((s) => s.managedAccountId);
-  const { addressBooks, contacts, supportsSync, fetchAddressBooks, createAddressBook, renameAddressBook, shareAddressBook, renameKeyword } = useContactStore();
+  const { addressBooks, contacts, supportsSync, fetchAddressBooks, createAddressBook, renameAddressBook, shareAddressBook, setDefaultAddressBook, renameKeyword } = useContactStore();
   const [editingId, setEditingId] = useState<string | null>(null);
   const [editingKeyword, setEditingKeyword] = useState<string | null>(null);
   const [sharingId, setSharingId] = useState<string | null>(null);
@@ -116,6 +116,19 @@ export function AddressBookManagementSettings() {
     }
   };
 
+  const handleSetDefault = async (book: AddressBook) => {
+    if (!client) return;
+    setIsLoading(true);
+    try {
+      await setDefaultAddressBook(client, book);
+      toast.success(t("default_updated"));
+    } catch {
+      toast.error(t("set_default_failed"));
+    } finally {
+      setIsLoading(false);
+    }
+  };
+
   // Group: personal first, then by shared account
   const personal = addressBooks.filter((b) => !b.isShared);
   const sharedGroups = new Map<string, { accountName: string; books: AddressBook[] }>();
@@ -141,6 +154,9 @@ export function AddressBookManagementSettings() {
     }
 
     const canRename = !book.isShared || book.myRights?.mayWrite !== false;
+    // The default only applies to books the account owns, and re-setting the
+    // current default is a no-op, so hide it in both cases.
+    const canSetDefault = !!client && !book.isShared && !book.isDefault;
 
     return (
       <div
@@ -182,6 +198,17 @@ export function AddressBookManagementSettings() {
               <Pencil className="w-3.5 h-3.5" />
             </button>
           )}
+          {canSetDefault && (
+            <button
+              type="button"
+              onClick={() => handleSetDefault(book)}
+              disabled={isLoading}
+              className="p-1.5 rounded-md hover:bg-muted text-muted-foreground hover:text-foreground transition-colors disabled:opacity-50"
+              title={t("set_default")}
+            >
+              <Star className="w-3.5 h-3.5" />
+            </button>
+          )}
           {!book.isShared && book.myRights?.mayShare && (
             <button
               type="button"
diff --git a/lib/__tests__/jmap-contact-client.test.ts b/lib/__tests__/jmap-contact-client.test.ts
index d3e234405..4c1fc9101 100644
--- a/lib/__tests__/jmap-contact-client.test.ts
+++ b/lib/__tests__/jmap-contact-client.test.ts
@@ -161,6 +161,69 @@ describe('JMAPClient contact methods', () => {
     });
   });
 
+  describe('setDefaultAddressBook', () => {
+    it('should send onSuccessSetIsDefault rather than an isDefault update', async () => {
+      const client = createClient();
+      const spy = mockFetch({
+        methodResponses: [['AddressBook/set', { updated: null }, '0']],
+      });
+
+      await client.setDefaultAddressBook('ab-2');
+
+      const body = JSON.parse((spy.mock.calls[0][1] as RequestInit).body as string);
+      expect(body.methodCalls[0][0]).toBe('AddressBook/set');
+      expect(body.methodCalls[0][1]).toMatchObject({
+        accountId: 'account-1',
+        onSuccessSetIsDefault: 'ab-2',
+      });
+      expect(body.methodCalls[0][1].update).toBeUndefined();
+    });
+
+    it('should target a shared account when one is given', async () => {
+      const client = createClient();
+      const spy = mockFetch({
+        methodResponses: [['AddressBook/set', { updated: null }, '0']],
+      });
+
+      await client.setDefaultAddressBook('ab-2', 'account-2');
+
+      const body = JSON.parse((spy.mock.calls[0][1] as RequestInit).body as string);
+      expect(body.methodCalls[0][1].accountId).toBe('account-2');
+    });
+
+    it('should throw on a JMAP method error', async () => {
+      const client = createClient();
+      mockFetch({
+        methodResponses: [['error', { type: 'forbidden', description: 'Not allowed' }, '0']],
+      });
+
+      await expect(client.setDefaultAddressBook('ab-2')).rejects.toThrow('Not allowed');
+    });
+
+    it('should throw for an unexpected response method', async () => {
+      const client = createClient();
+      mockFetch({
+        methodResponses: [['SomethingElse', {}, '0']],
+      });
+
+      await expect(client.setDefaultAddressBook('ab-2')).rejects.toThrow('Failed to set default address book');
+    });
+  });
+
+  describe('updateAddressBook', () => {
+    it('should drop isDefault, which AddressBook/set rejects as read-only', async () => {
+      const client = createClient();
+      const spy = mockFetch({
+        methodResponses: [['AddressBook/set', { updated: { 'ab-1': null } }, '0']],
+      });
+
+      await client.updateAddressBook('ab-1', { name: 'Work', isDefault: true });
+
+      const body = JSON.parse((spy.mock.calls[0][1] as RequestInit).body as string);
+      expect(body.methodCalls[0][1].update['ab-1']).toEqual({ name: 'Work' });
+    });
+  });
+
   describe('getContacts', () => {
     it('should return contacts from server', async () => {
       const client = createClient();
diff --git a/lib/demo/demo-client.ts b/lib/demo/demo-client.ts
index ee58c70cf..55c3c679c 100644
--- a/lib/demo/demo-client.ts
+++ b/lib/demo/demo-client.ts
@@ -779,6 +779,12 @@ export class DemoJMAPClient implements IJMAPClient {
     if (book) Object.assign(book, updates);
   }
 
+  async setDefaultAddressBook(addressBookId: string): Promise<void> {
+    for (const book of this.data.addressBooks) {
+      book.isDefault = book.id === addressBookId;
+    }
+  }
+
   async deleteAddressBook(addressBookId: string): Promise<void> {
     this.data.addressBooks = this.data.addressBooks.filter(b => b.id !== addressBookId);
     this.data.contacts = this.data.contacts.filter(c => !c.addressBookIds?.[addressBookId]);
diff --git a/lib/jmap/client-interface.ts b/lib/jmap/client-interface.ts
index fba655726..153dc8ad0 100644
--- a/lib/jmap/client-interface.ts
+++ b/lib/jmap/client-interface.ts
@@ -341,6 +341,7 @@ export interface IJMAPClient {
   getAllAddressBooks(): Promise<AddressBook[]>;
   createAddressBook(name: string): Promise<AddressBook>;
   updateAddressBook(addressBookId: string, updates: Partial<AddressBook>, targetAccountId?: string): Promise<void>;
+  setDefaultAddressBook(addressBookId: string, targetAccountId?: string): Promise<void>;
   deleteAddressBook(addressBookId: string, targetAccountId?: string): Promise<void>;
   getContacts(addressBookId?: string, options?: { throwOnError?: boolean }): Promise<ContactCard[]>;
   getAllContacts(): Promise<ContactCard[]>;
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index 43819a72f..f4e195a4f 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -4815,13 +4815,14 @@ export class JMAPClient implements IJMAPClient {
 
   async updateAddressBook(addressBookId: string, updates: Partial<AddressBook>, targetAccountId?: string): Promise<void> {
     const accountId = targetAccountId || this.getContactsAccountId();
-    // Only forward server-settable properties
-    const { name, description, sortOrder, isDefault, color } = updates as Record<string, unknown>;
+    // Only forward server-settable properties. `isDefault` is deliberately not
+    // among them: it is read-only in AddressBook/set and including it fails the
+    // whole update with invalidProperties - use setDefaultAddressBook instead.
+    const { name, description, sortOrder, color } = updates as Record<string, unknown>;
     const patch: Record<string, unknown> = {};
     if (name !== undefined) patch.name = name;
     if (description !== undefined) patch.description = description;
     if (sortOrder !== undefined) patch.sortOrder = sortOrder;
-    if (isDefault !== undefined) patch.isDefault = isDefault;
     if (color !== undefined) patch.color = color;
 
     const response = await this.request([
@@ -4842,6 +4843,31 @@ export class JMAPClient implements IJMAPClient {
     throw new Error("Failed to update address book");
   }
 
+  /**
+   * Mark an address book as the account default. `isDefault` is read-only in
+   * AddressBook/set - the default is changed via the `onSuccessSetIsDefault`
+   * request argument instead (same shape as Calendar/set).
+   */
+  async setDefaultAddressBook(addressBookId: string, targetAccountId?: string): Promise<void> {
+    const accountId = targetAccountId || this.getContactsAccountId();
+
+    const response = await this.request([
+      ["AddressBook/set", {
+        accountId,
+        onSuccessSetIsDefault: addressBookId,
+      }, "0"]
+    ], this.contactUsing());
+
+    const methodName = response.methodResponses?.[0]?.[0];
+    if (methodName === "error") {
+      const error = response.methodResponses?.[0]?.[1];
+      throw new Error(error?.description || error?.type || "Failed to set default address book");
+    }
+    if (methodName !== "AddressBook/set") {
+      throw new Error("Failed to set default address book");
+    }
+  }
+
   async deleteAddressBook(addressBookId: string, targetAccountId?: string): Promise<void> {
     const accountId = targetAccountId || this.getContactsAccountId();
     const response = await this.request([
diff --git a/locales/ar/common.json b/locales/ar/common.json
index 027949b9c..876e19a30 100644
--- a/locales/ar/common.json
+++ b/locales/ar/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "تمت إعادة تسمية دفتر العناوين",
       "rename_failed": "فشلت إعادة تسمية دفتر العناوين",
       "default": "افتراضي",
+      "set_default": "تعيين كافتراضي",
+      "default_updated": "تم تحديث دفتر العناوين الافتراضي",
+      "set_default_failed": "فشل تعيين دفتر العناوين الافتراضي",
       "manage": "إدارة دفاتر العناوين",
       "share": "مشاركة دفتر العناوين",
       "new_contact_in_book": "جهة اتصال جديدة في دفتر العناوين هذا",
diff --git a/locales/ca/common.json b/locales/ca/common.json
index b37db406b..42b8da91d 100644
--- a/locales/ca/common.json
+++ b/locales/ca/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "Nom de la llibreta d'adreces canviat",
       "rename_failed": "No s'ha pogut canviar el nom de la llibreta d'adreces",
       "default": "Predeterminada",
+      "set_default": "Estableix com a predeterminada",
+      "default_updated": "Llibreta d’adreces predeterminada actualitzada",
+      "set_default_failed": "No s’ha pogut establir la llibreta d’adreces predeterminada",
       "manage": "Gestiona les llibretes d'adreces",
       "share": "Comparteix la llibreta d'adreces",
       "new_contact_in_book": "Contacte nou en aquesta llibreta d'adreces",
diff --git a/locales/cs/common.json b/locales/cs/common.json
index dc3ee6ce5..b473debb1 100644
--- a/locales/cs/common.json
+++ b/locales/cs/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Adresář byl přejmenován",
       "rename_failed": "Přejmenování adresáře selhalo",
       "default": "Výchozí",
+      "set_default": "Nastavit jako výchozí",
+      "default_updated": "Výchozí adresář aktualizován",
+      "set_default_failed": "Nepodařilo se nastavit výchozí adresář",
       "manage": "Spravovat adresáře",
       "share": "Sdílet adresář",
       "new_contact_in_book": "Nový kontakt v tomto adresáři",
diff --git a/locales/da/common.json b/locales/da/common.json
index 621dffcd0..4126c75ce 100644
--- a/locales/da/common.json
+++ b/locales/da/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Adressebog omdøbt",
       "rename_failed": "Kunne ikke omdøbe adressebog",
       "default": "Standard",
+      "set_default": "Angiv som standard",
+      "default_updated": "Standardadressebog opdateret",
+      "set_default_failed": "Kunne ikke angive standardadressebog",
       "manage": "Administrér adressebøger",
       "share": "Del adressebog",
       "new_contact_in_book": "Ny kontakt i denne adressebog",
diff --git a/locales/de/common.json b/locales/de/common.json
index e72ed7d6f..931a0d555 100644
--- a/locales/de/common.json
+++ b/locales/de/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Adressbuch umbenannt",
       "rename_failed": "Adressbuch konnte nicht umbenannt werden",
       "default": "Standard",
+      "set_default": "Als Standard festlegen",
+      "default_updated": "Standardadressbuch aktualisiert",
+      "set_default_failed": "Standardadressbuch konnte nicht festgelegt werden",
       "manage": "Adressbücher verwalten",
       "share": "Adressbuch freigeben",
       "new_contact_in_book": "Neuer Kontakt in diesem Adressbuch",
diff --git a/locales/en/common.json b/locales/en/common.json
index 60bc1ab2d..72c55b54d 100644
--- a/locales/en/common.json
+++ b/locales/en/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "Address book renamed",
       "rename_failed": "Failed to rename address book",
       "default": "Default",
+      "set_default": "Set as default",
+      "default_updated": "Default address book updated",
+      "set_default_failed": "Failed to set default address book",
       "manage": "Manage address books",
       "share": "Share address book",
       "new_contact_in_book": "New contact in this address book",
diff --git a/locales/es/common.json b/locales/es/common.json
index 1da05735e..cd5f6a325 100644
--- a/locales/es/common.json
+++ b/locales/es/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Libreta de direcciones renombrada",
       "rename_failed": "Error al renombrar la libreta de direcciones",
       "default": "Predeterminada",
+      "set_default": "Establecer como predeterminada",
+      "default_updated": "Libreta de direcciones predeterminada actualizada",
+      "set_default_failed": "Error al establecer la libreta de direcciones predeterminada",
       "manage": "Administrar libretas de direcciones",
       "share": "Compartir libreta de direcciones",
       "new_contact_in_book": "Nuevo contacto en esta libreta",
diff --git a/locales/fa/common.json b/locales/fa/common.json
index dd71e891d..36b3698ec 100644
--- a/locales/fa/common.json
+++ b/locales/fa/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "نام دفترچه آدرس تغییر کرد",
       "rename_failed": "تغییر نام دفترچه آدرس ناموفق بود",
       "default": "پیش‌فرض",
+      "set_default": "تنظیم به عنوان پیش‌فرض",
+      "default_updated": "دفترچه آدرس پیش‌فرض به‌روزرسانی شد",
+      "set_default_failed": "تنظیم دفترچه آدرس پیش‌فرض ناموفق بود",
       "manage": "مدیریت دفترچه‌های آدرس",
       "share": "اشتراک‌گذاری دفترچه آدرس",
       "new_contact_in_book": "مخاطب جدید در این دفترچه آدرس",
diff --git a/locales/fr/common.json b/locales/fr/common.json
index 13a9f07a3..12d8ef9fc 100644
--- a/locales/fr/common.json
+++ b/locales/fr/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Carnet d'adresses renommé",
       "rename_failed": "Échec du renommage du carnet d'adresses",
       "default": "Par défaut",
+      "set_default": "Définir par défaut",
+      "default_updated": "Carnet d’adresses par défaut mis à jour",
+      "set_default_failed": "Échec de la définition du carnet d’adresses par défaut",
       "manage": "Gérer les carnets d'adresses",
       "share": "Partager le carnet d'adresses",
       "new_contact_in_book": "Nouveau contact dans ce carnet d'adresses",
diff --git a/locales/he/common.json b/locales/he/common.json
index 63b0d14f2..211bdaa12 100644
--- a/locales/he/common.json
+++ b/locales/he/common.json
@@ -2356,6 +2356,9 @@
       "renamed": "שם ספר הכתובות שונה",
       "rename_failed": "שינוי שם פנקס הכתובות נכשל",
       "default": "ברירת מחדל",
+      "set_default": "הגדר כברירת מחדל",
+      "default_updated": "פנקס הכתובות ברירת המחדל עודכן",
+      "set_default_failed": "נכשל בהגדרת פנקס כתובות ברירת מחדל",
       "manage": "ניהול פנקסי כתובות",
       "create": "ספר כתובות חדש",
       "created": "ספר כתובות נוצר",
diff --git a/locales/hu/common.json b/locales/hu/common.json
index 79d9a4573..9d86a88f3 100644
--- a/locales/hu/common.json
+++ b/locales/hu/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "Címjegyzék átnevezve",
       "rename_failed": "Nem sikerült átnevezni a címjegyzéket",
       "default": "Alapértelmezett",
+      "set_default": "Beállítás alapértelmezettként",
+      "default_updated": "Alapértelmezett címjegyzék frissítve",
+      "set_default_failed": "Nem sikerült beállítani az alapértelmezett címjegyzéket",
       "manage": "Címjegyzékek kezelése",
       "share": "Címjegyzék megosztása",
       "new_contact_in_book": "Új névjegy ebben a címjegyzékben",
diff --git a/locales/it/common.json b/locales/it/common.json
index c2ec7c017..6cab47dee 100644
--- a/locales/it/common.json
+++ b/locales/it/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Rubrica rinominata",
       "rename_failed": "Impossibile rinominare la rubrica",
       "default": "Predefinita",
+      "set_default": "Imposta come predefinita",
+      "default_updated": "Rubrica predefinita aggiornata",
+      "set_default_failed": "Impossibile impostare la rubrica predefinita",
       "manage": "Gestisci rubriche",
       "share": "Condividi rubrica",
       "new_contact_in_book": "Nuovo contatto in questa rubrica",
diff --git a/locales/ja/common.json b/locales/ja/common.json
index d00142e80..25f2fb731 100644
--- a/locales/ja/common.json
+++ b/locales/ja/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "アドレス帳の名前を変更しました",
       "rename_failed": "アドレス帳の名前変更に失敗しました",
       "default": "デフォルト",
+      "set_default": "デフォルトに設定",
+      "default_updated": "デフォルトのアドレス帳を更新しました",
+      "set_default_failed": "デフォルトのアドレス帳を設定できませんでした",
       "manage": "アドレス帳を管理",
       "share": "アドレス帳を共有",
       "new_contact_in_book": "このアドレス帳に新規連絡先",
diff --git a/locales/ko/common.json b/locales/ko/common.json
index 439a2826c..9a1b96920 100644
--- a/locales/ko/common.json
+++ b/locales/ko/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "주소록 이름이 변경되었습니다",
       "rename_failed": "주소록 이름 변경 실패",
       "default": "기본",
+      "set_default": "기본으로 설정",
+      "default_updated": "기본 주소록이 업데이트되었습니다",
+      "set_default_failed": "기본 주소록을 설정하지 못했습니다",
       "manage": "주소록 관리",
       "share": "주소록 공유",
       "new_contact_in_book": "이 주소록에 새 연락처",
diff --git a/locales/lv/common.json b/locales/lv/common.json
index 317e94667..e4fba1672 100644
--- a/locales/lv/common.json
+++ b/locales/lv/common.json
@@ -2430,6 +2430,9 @@
       "renamed": "Adrešu grāmata pārdēvēta",
       "rename_failed": "Neizdevās pārdēvēt adrešu grāmatu",
       "default": "Noklusējuma",
+      "set_default": "Iestatīt kā noklusējumu",
+      "default_updated": "Noklusējuma adrešu grāmata atjaunināta",
+      "set_default_failed": "Neizdevās iestatīt noklusējuma adrešu grāmatu",
       "manage": "Pārvaldīt adrešu grāmatas",
       "share": "Kopīgot adrešu grāmatu",
       "new_contact_in_book": "Jauns kontakts šajā adrešu grāmatā",
diff --git a/locales/mn/common.json b/locales/mn/common.json
index cceea1929..bed694034 100644
--- a/locales/mn/common.json
+++ b/locales/mn/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "Хаягийн дэвтрийн нэрийг өөрчилсөн",
       "rename_failed": "Хаягийн дэвтрийн нэрийг өөрчилж чадсангүй",
       "default": "Өгөгдмөл",
+      "set_default": "Өгөгдмөлөөр тохируулах",
+      "default_updated": "Өгөгдмөл хаягийн дэвтэр шинэчлэгдсэн",
+      "set_default_failed": "Өгөгдмөл хаягийн дэвтрийг тохируулж чадсангүй",
       "manage": "Хаягийн дэвтэр удирдах",
       "share": "Хаягийн дэвтэр хуваалцах",
       "new_contact_in_book": "Энэ хаягийн дэвтэрт шинэ харилцагч",
diff --git a/locales/nb/common.json b/locales/nb/common.json
index fee5432eb..20c89ee48 100644
--- a/locales/nb/common.json
+++ b/locales/nb/common.json
@@ -2433,6 +2433,9 @@
       "renamed": "Adresseboken har fått nytt navn",
       "rename_failed": "Kunne ikke gi adresseboken nytt navn",
       "default": "Standard",
+      "set_default": "Sett som standard",
+      "default_updated": "Standardadresseboken er oppdatert",
+      "set_default_failed": "Kunne ikke angi standardadressebok",
       "manage": "Administrer adressebøker",
       "share": "Del adressebok",
       "new_contact_in_book": "Ny kontakt i denne adresseboken",
diff --git a/locales/nl/common.json b/locales/nl/common.json
index e3962c6b1..3b715a709 100644
--- a/locales/nl/common.json
+++ b/locales/nl/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Adresboek hernoemd",
       "rename_failed": "Adresboek hernoemen mislukt",
       "default": "Standaard",
+      "set_default": "Als standaard instellen",
+      "default_updated": "Standaardadresboek bijgewerkt",
+      "set_default_failed": "Standaardadresboek instellen mislukt",
       "manage": "Adresboeken beheren",
       "share": "Adresboek delen",
       "new_contact_in_book": "Nieuw contact in dit adresboek",
diff --git a/locales/pl/common.json b/locales/pl/common.json
index e38a625b9..18fc48482 100644
--- a/locales/pl/common.json
+++ b/locales/pl/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Zmieniono nazwę książki adresowej",
       "rename_failed": "Nie udało się zmienić nazwy książki adresowej",
       "default": "Domyślna",
+      "set_default": "Ustaw jako domyślną",
+      "default_updated": "Zaktualizowano domyślną książkę adresową",
+      "set_default_failed": "Nie udało się ustawić domyślnej książki adresowej",
       "manage": "Zarządzaj książkami adresowymi",
       "share": "Udostępnij książkę adresową",
       "new_contact_in_book": "Nowy kontakt w tej książce adresowej",
diff --git a/locales/pt/common.json b/locales/pt/common.json
index 7cce29fd9..fe22f850a 100644
--- a/locales/pt/common.json
+++ b/locales/pt/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Catálogo de endereços renomeado",
       "rename_failed": "Falha ao renomear o catálogo de endereços",
       "default": "Padrão",
+      "set_default": "Definir como padrão",
+      "default_updated": "Catálogo de endereços padrão atualizado",
+      "set_default_failed": "Falha ao definir o catálogo de endereços padrão",
       "manage": "Gerenciar catálogos de endereços",
       "share": "Compartilhar lista de contatos",
       "new_contact_in_book": "Novo contato nesta lista",
diff --git a/locales/ro/common.json b/locales/ro/common.json
index 4e4402daf..84217d99e 100644
--- a/locales/ro/common.json
+++ b/locales/ro/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "Agenda de adrese a fost redenumită",
       "rename_failed": "Nu s-a putut redenumi agenda de adrese",
       "default": "Implicit",
+      "set_default": "Setați ca implicită",
+      "default_updated": "Agenda de adrese implicită a fost actualizată",
+      "set_default_failed": "Nu s-a reușit setarea agendei de adrese implicite",
       "manage": "Gestionați agendele de adrese",
       "share": "Partajați agenda de adrese",
       "new_contact_in_book": "Contact nou în această agendă",
diff --git a/locales/ru/common.json b/locales/ru/common.json
index 647e9aa13..42ac6e153 100644
--- a/locales/ru/common.json
+++ b/locales/ru/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Адресная книга переименована",
       "rename_failed": "Не удалось переименовать адресную книгу",
       "default": "По умолчанию",
+      "set_default": "Сделать по умолчанию",
+      "default_updated": "Адресная книга по умолчанию обновлена",
+      "set_default_failed": "Не удалось установить адресную книгу по умолчанию",
       "manage": "Управление адресными книгами",
       "share": "Поделиться адресной книгой",
       "new_contact_in_book": "Новый контакт в этой адресной книге",
diff --git a/locales/sk/common.json b/locales/sk/common.json
index 07a017b49..803160dcc 100644
--- a/locales/sk/common.json
+++ b/locales/sk/common.json
@@ -2435,6 +2435,9 @@
       "renamed": "Adresár bol premenovaný",
       "rename_failed": "Premenovanie adresára zlyhalo",
       "default": "Predvolený",
+      "set_default": "Nastaviť ako predvolený",
+      "default_updated": "Predvolený adresár aktualizovaný",
+      "set_default_failed": "Nepodarilo sa nastaviť predvolený adresár",
       "manage": "Spravovať adresáre",
       "share": "Zdieľať adresár",
       "new_contact_in_book": "Nový kontakt v tomto adresári",
diff --git a/locales/tr/common.json b/locales/tr/common.json
index 89fea83ad..c7db30fe9 100644
--- a/locales/tr/common.json
+++ b/locales/tr/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Adres defteri yeniden adlandırıldı",
       "rename_failed": "Adres defteri yeniden adlandırılamadı",
       "default": "Varsayılan",
+      "set_default": "Varsayılan olarak ayarla",
+      "default_updated": "Varsayılan adres defteri güncellendi",
+      "set_default_failed": "Varsayılan adres defteri ayarlanamadı",
       "manage": "Adres defterlerini yönet",
       "share": "Adres defterini paylaş",
       "new_contact_in_book": "Bu adres defterinde yeni kişi",
diff --git a/locales/uk/common.json b/locales/uk/common.json
index 2c4556053..10633d9cb 100644
--- a/locales/uk/common.json
+++ b/locales/uk/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "Адресну книгу перейменовано",
       "rename_failed": "Не вдалося перейменувати адресну книгу",
       "default": "За замовчуванням",
+      "set_default": "Зробити типовою",
+      "default_updated": "Типову адресну книгу оновлено",
+      "set_default_failed": "Не вдалося встановити типову адресну книгу",
       "manage": "Керуйте адресними книгами",
       "share": "Поділитися адресною книгою",
       "new_contact_in_book": "Новий контакт у цій адресній книзі",
diff --git a/locales/zh-TW/common.json b/locales/zh-TW/common.json
index aa4bafe2d..eaa223438 100644
--- a/locales/zh-TW/common.json
+++ b/locales/zh-TW/common.json
@@ -2428,6 +2428,9 @@
       "renamed": "已重新命名通訊錄",
       "rename_failed": "無法重新命名通訊錄",
       "default": "預設",
+      "set_default": "設為預設",
+      "default_updated": "預設通訊錄已更新",
+      "set_default_failed": "無法設定預設通訊錄",
       "manage": "管理通訊錄",
       "share": "分享通訊錄",
       "new_contact_in_book": "在此通訊錄中新增聯絡人",
diff --git a/locales/zh/common.json b/locales/zh/common.json
index 216bd3ecf..e8363bea9 100644
--- a/locales/zh/common.json
+++ b/locales/zh/common.json
@@ -2434,6 +2434,9 @@
       "renamed": "地址簿已重命名",
       "rename_failed": "重命名地址簿失败",
       "default": "默认",
+      "set_default": "设为默认",
+      "default_updated": "默认地址簿已更新",
+      "set_default_failed": "无法设置默认地址簿",
       "manage": "管理地址簿",
       "share": "共享通讯录",
       "new_contact_in_book": "在此通讯录中新建联系人",
diff --git a/stores/__tests__/contact-store.test.ts b/stores/__tests__/contact-store.test.ts
index 755185555..f20e6a4b3 100644
--- a/stores/__tests__/contact-store.test.ts
+++ b/stores/__tests__/contact-store.test.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { useContactStore, getContactPhotoUri, normalizeContactPhotoUri, TRUSTED_SENDERS_BOOK_NAME } from '../contact-store';
 import type { ContactCard } from '@/lib/jmap/types';
+import type { IJMAPClient } from '@/lib/jmap/client-interface';
 
 vi.stubGlobal('crypto', { randomUUID: () => '00000000-0000-0000-0000-000000000000' });
 
@@ -161,6 +162,78 @@ describe('contact-store', () => {
     });
   });
 
+  describe('setDefaultAddressBook', () => {
+    const makeClient = () => ({
+      setDefaultAddressBook: vi.fn().mockResolvedValue(undefined),
+    });
+
+    it('should move the default flag to the chosen book', async () => {
+      const books = [
+        { id: 'ab-1', name: 'Personal', isDefault: true },
+        { id: 'ab-2', name: 'Work', isDefault: false },
+      ];
+      useContactStore.setState({ addressBooks: books });
+      const client = makeClient();
+
+      await useContactStore.getState().setDefaultAddressBook(
+        client as unknown as IJMAPClient,
+        books[1],
+      );
+
+      expect(client.setDefaultAddressBook).toHaveBeenCalledWith('ab-2', undefined);
+      expect(useContactStore.getState().addressBooks.map((b) => b.isDefault)).toEqual([false, true]);
+    });
+
+    it('should send the un-namespaced id and leave other accounts alone', async () => {
+      const book = {
+        id: 'local-2::ab-9',
+        originalId: 'ab-9',
+        localAccountId: 'local-2',
+        name: 'Work',
+      };
+      useContactStore.setState({
+        addressBooks: [
+          { id: 'local-1::ab-1', originalId: 'ab-1', localAccountId: 'local-1', name: 'Personal', isDefault: true },
+          book,
+        ],
+      });
+      const client = makeClient();
+
+      await useContactStore.getState().setDefaultAddressBook(client as unknown as IJMAPClient, book);
+
+      expect(client.setDefaultAddressBook).toHaveBeenCalledWith('ab-9', undefined);
+      // The other account keeps its own default.
+      expect(useContactStore.getState().addressBooks.map((b) => b.isDefault)).toEqual([true, true]);
+    });
+
+    it('should target the owning account for a shared book', async () => {
+      const book = { id: 'ab-3', name: 'Team', isShared: true, accountId: 'account-2' };
+      useContactStore.setState({ addressBooks: [book] });
+      const client = makeClient();
+
+      await useContactStore.getState().setDefaultAddressBook(client as unknown as IJMAPClient, book);
+
+      expect(client.setDefaultAddressBook).toHaveBeenCalledWith('ab-3', 'account-2');
+    });
+
+    it('should surface the error and keep the previous default on failure', async () => {
+      const books = [
+        { id: 'ab-1', name: 'Personal', isDefault: true },
+        { id: 'ab-2', name: 'Work', isDefault: false },
+      ];
+      useContactStore.setState({ addressBooks: books });
+      const client = {
+        setDefaultAddressBook: vi.fn().mockRejectedValue(new Error('Not allowed')),
+      };
+
+      await expect(
+        useContactStore.getState().setDefaultAddressBook(client as unknown as IJMAPClient, books[1]),
+      ).rejects.toThrow('Not allowed');
+      expect(useContactStore.getState().error).toBe('Not allowed');
+      expect(useContactStore.getState().addressBooks.map((b) => b.isDefault)).toEqual([true, false]);
+    });
+  });
+
   describe('toggleContactSelection', () => {
     it('should add id to selection', () => {
       useContactStore.getState().toggleContactSelection('c1');
diff --git a/stores/contact-store.ts b/stores/contact-store.ts
index bebce70ef..767fd6c23 100644
--- a/stores/contact-store.ts
+++ b/stores/contact-store.ts
@@ -312,6 +312,7 @@ interface ContactStore {
   moveContactToAddressBook: (client: IJMAPClient, contactIds: string[], addressBook: AddressBook) => Promise<void>;
   createAddressBook: (client: IJMAPClient, name: string) => Promise<AddressBook>;
   renameAddressBook: (client: IJMAPClient, addressBook: AddressBook, newName: string) => Promise<void>;
+  setDefaultAddressBook: (client: IJMAPClient, addressBook: AddressBook) => Promise<void>;
   removeAddressBook: (client: IJMAPClient, addressBook: AddressBook) => Promise<void>;
   shareAddressBook: (client: IJMAPClient, addressBook: AddressBook, principalId: string, rights: AddressBookRights | null) => Promise<void>;
   renameKeyword: (client: IJMAPClient | null, oldKeyword: string, newKeyword: string) => Promise<void>;
@@ -1013,6 +1014,35 @@ export const useContactStore = create<ContactStore>()(
         }
       },
 
+      setDefaultAddressBook: async (client, addressBook) => {
+        set({ error: null });
+        try {
+          const originalId = addressBook.originalId || stripLocalAccountPrefix(addressBook.id, addressBook.localAccountId);
+          const accountId = addressBook.isShared ? addressBook.accountId : undefined;
+          client = resolveAccountClient(client, addressBook.localAccountId);
+          await client.setDefaultAddressBook(originalId, accountId);
+          set((state) => ({
+            addressBooks: state.addressBooks.map(b => {
+              if (b.id === addressBook.id) return { ...b, isDefault: true };
+              // Only one default per account - clear the flag on siblings in the
+              // same local account / shared-account scope.
+              if (
+                b.isDefault
+                && (b.localAccountId ?? null) === (addressBook.localAccountId ?? null)
+                && (b.accountId ?? null) === (addressBook.accountId ?? null)
+              ) {
+                return { ...b, isDefault: false };
+              }
+              return b;
+            }),
+          }));
+        } catch (error) {
+          const msg = error instanceof Error ? error.message : 'Failed to set default address book';
+          set({ error: msg });
+          throw error;
+        }
+      },
+
       removeAddressBook: async (client, addressBook) => {
         set({ error: null });
         try {