diff --git a/apps/readest-app/src/__tests__/components/settings/cloudSync.test.ts b/apps/readest-app/src/__tests__/components/settings/cloudSync.test.ts
index dcf36d87..cc09b521 100644
--- a/apps/readest-app/src/__tests__/components/settings/cloudSync.test.ts
+++ b/apps/readest-app/src/__tests__/components/settings/cloudSync.test.ts
@@ -32,6 +32,46 @@ describe('withActiveCloudProvider', () => {
     expect(next.webdav.serverUrl).toBe('https://dav');
     expect(next.googleDrive.accountLabel).toBe('a@b.com');
   });
+
+  // Selecting a third-party provider hands it the book-file channel:
+  // native Readest Cloud uploads gate off, so without syncBooks the books
+  // would back up nowhere. Activation therefore turns syncBooks on.
+  describe('syncBooks auto-enable on activation', () => {
+    const inactive = {
+      webdav: { enabled: false, serverUrl: 'https://dav', syncBooks: false },
+      googleDrive: { enabled: false, syncBooks: false },
+    } as unknown as SystemSettings;
+
+    test('activating a disabled provider turns its syncBooks on', () => {
+      const next = withActiveCloudProvider(inactive, 'webdav');
+      expect(next.webdav.syncBooks).toBe(true);
+      expect(next.googleDrive.syncBooks).toBe(false);
+    });
+
+    test('activating gdrive turns only gdrive syncBooks on', () => {
+      const next = withActiveCloudProvider(inactive, 'gdrive');
+      expect(next.googleDrive.syncBooks).toBe(true);
+      expect(next.webdav.syncBooks).toBe(false);
+    });
+
+    test('re-activating an already-active provider respects an explicit syncBooks opt-out', () => {
+      const active = {
+        webdav: { enabled: true, syncBooks: false },
+        googleDrive: { enabled: false },
+      } as unknown as SystemSettings;
+      const next = withActiveCloudProvider(active, 'webdav');
+      expect(next.webdav.syncBooks).toBe(false);
+    });
+
+    test('deactivating a provider leaves its syncBooks untouched', () => {
+      const active = {
+        webdav: { enabled: true, syncBooks: true },
+        googleDrive: { enabled: false, syncBooks: false },
+      } as unknown as SystemSettings;
+      const next = withActiveCloudProvider(active, null);
+      expect(next.webdav.syncBooks).toBe(true);
+    });
+  });
 });
 
 describe('isCloudSyncInPlan', () => {
diff --git a/apps/readest-app/src/__tests__/services/backup-settings.test.ts b/apps/readest-app/src/__tests__/services/backup-settings.test.ts
index f238a203..c8095779 100644
--- a/apps/readest-app/src/__tests__/services/backup-settings.test.ts
+++ b/apps/readest-app/src/__tests__/services/backup-settings.test.ts
@@ -59,6 +59,15 @@ function makeSettings(overrides: Partial<SystemSettings> = {}): SystemSettings {
       checksumMethod: 'binary',
       strategy: 'prompt',
     },
+    webdav: {
+      enabled: true,
+      serverUrl: 'https://dav.example',
+      username: 'wuser',
+      password: 'wpass',
+      rootPath: '/',
+      deviceId: 'webdav-device-id',
+      lastSyncedAt: 666,
+    },
     readwise: { enabled: true, accessToken: 'rw-token', lastSyncedAt: 999 },
     hardcover: { enabled: false, accessToken: 'hc-token', lastSyncedAt: 888 },
     googleDrive: {
@@ -104,6 +113,11 @@ describe('sanitizeSettingsForBackup - blacklist', () => {
     expect(rec(out['googleDrive'])['deviceId']).toBeUndefined();
     // Non-identity Drive settings still travel with the backup.
     expect(rec(out['googleDrive'])['enabled']).toBe(true);
+    // WebDAV device identity and cursor stay on the device; restoring
+    // them onto a second device would duplicate WebDAV sync identity.
+    expect(rec(out['webdav'])['deviceId']).toBeUndefined();
+    expect(rec(out['webdav'])['lastSyncedAt']).toBeUndefined();
+    expect(rec(out['webdav'])['serverUrl']).toBe('https://dav.example');
   });
 
   it('strips sync cursors', () => {
diff --git a/apps/readest-app/src/__tests__/services/sync/cloudSyncProvider.test.ts b/apps/readest-app/src/__tests__/services/sync/cloudSyncProvider.test.ts
new file mode 100644
index 00000000..1e6b1ff2
--- /dev/null
+++ b/apps/readest-app/src/__tests__/services/sync/cloudSyncProvider.test.ts
@@ -0,0 +1,158 @@
+import { describe, test, expect, beforeEach, vi } from 'vitest';
+import type { SystemSettings } from '@/types/settings';
+
+vi.mock('@/utils/access', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@/utils/access')>();
+  return {
+    ...actual,
+    isCloudSyncAllowed: vi.fn(actual.isCloudSyncAllowed),
+  };
+});
+
+import {
+  applySyncBooksAutoEnable,
+  getCloudSyncProvider,
+  isReadestCloudStorageActive,
+  resolveCloudSyncGate,
+  setCachedUserPlan,
+} from '@/services/sync/cloudSyncProvider';
+import { isCloudSyncAllowed } from '@/utils/access';
+
+const makeSettings = (overrides: Partial<SystemSettings> = {}): SystemSettings =>
+  ({
+    webdav: { enabled: false },
+    googleDrive: { enabled: false },
+    ...overrides,
+  }) as SystemSettings;
+
+beforeEach(() => {
+  vi.mocked(isCloudSyncAllowed).mockReturnValue(true);
+  setCachedUserPlan('free');
+});
+
+describe('getCloudSyncProvider', () => {
+  test('derives readest when no third-party provider is enabled', () => {
+    expect(getCloudSyncProvider(makeSettings())).toBe('readest');
+  });
+
+  test('derives webdav when webdav is enabled', () => {
+    const settings = makeSettings({ webdav: { enabled: true } } as Partial<SystemSettings>);
+    expect(getCloudSyncProvider(settings)).toBe('webdav');
+  });
+
+  test('derives gdrive when google drive is enabled', () => {
+    const settings = makeSettings({ googleDrive: { enabled: true } } as Partial<SystemSettings>);
+    expect(getCloudSyncProvider(settings)).toBe('gdrive');
+  });
+
+  test('webdav wins deterministically when both flags are enabled (corrupt state)', () => {
+    const settings = makeSettings({
+      webdav: { enabled: true },
+      googleDrive: { enabled: true },
+    } as Partial<SystemSettings>);
+    expect(getCloudSyncProvider(settings)).toBe('webdav');
+  });
+
+  test('defaults to readest for missing slices and missing settings', () => {
+    expect(getCloudSyncProvider({} as SystemSettings)).toBe('readest');
+    expect(getCloudSyncProvider(null)).toBe('readest');
+    expect(getCloudSyncProvider(undefined)).toBe('readest');
+  });
+});
+
+describe('resolveCloudSyncGate', () => {
+  test('readest provider is never paused, even when cloud sync is disallowed', () => {
+    vi.mocked(isCloudSyncAllowed).mockReturnValue(false);
+    expect(resolveCloudSyncGate(makeSettings(), 'free')).toEqual({
+      provider: 'readest',
+      paused: false,
+    });
+  });
+
+  test('third-party provider stays selected but paused when disallowed (no silent readest fallback)', () => {
+    vi.mocked(isCloudSyncAllowed).mockReturnValue(false);
+    const settings = makeSettings({ webdav: { enabled: true } } as Partial<SystemSettings>);
+    expect(resolveCloudSyncGate(settings, 'free')).toEqual({ provider: 'webdav', paused: true });
+  });
+
+  test('third-party provider is active when allowed', () => {
+    const settings = makeSettings({ googleDrive: { enabled: true } } as Partial<SystemSettings>);
+    expect(resolveCloudSyncGate(settings, 'plus')).toEqual({ provider: 'gdrive', paused: false });
+  });
+
+  test('falls back to the cached user plan when no plan argument is given', () => {
+    vi.mocked(isCloudSyncAllowed).mockImplementation((plan) => plan !== 'free');
+    const settings = makeSettings({ webdav: { enabled: true } } as Partial<SystemSettings>);
+
+    setCachedUserPlan('free');
+    expect(resolveCloudSyncGate(settings).paused).toBe(true);
+
+    setCachedUserPlan('pro');
+    expect(resolveCloudSyncGate(settings).paused).toBe(false);
+  });
+
+  test('undefined cached plan is treated as free', () => {
+    vi.mocked(isCloudSyncAllowed).mockImplementation((plan) => plan !== 'free');
+    const settings = makeSettings({ webdav: { enabled: true } } as Partial<SystemSettings>);
+    setCachedUserPlan(undefined);
+    expect(resolveCloudSyncGate(settings).paused).toBe(true);
+  });
+});
+
+describe('applySyncBooksAutoEnable (upgrade migration for already-enabled providers)', () => {
+  test('flips syncBooks on for an enabled webdav provider, mutating the given settings', () => {
+    const settings = makeSettings({
+      webdav: { enabled: true, syncBooks: false },
+    } as Partial<SystemSettings>);
+    expect(applySyncBooksAutoEnable(settings)).toBe(true);
+    expect(settings.webdav?.syncBooks).toBe(true);
+  });
+
+  test('flips syncBooks on for an enabled gdrive provider', () => {
+    const settings = makeSettings({
+      googleDrive: { enabled: true, syncBooks: false },
+    } as Partial<SystemSettings>);
+    expect(applySyncBooksAutoEnable(settings)).toBe(true);
+    expect(settings.googleDrive?.syncBooks).toBe(true);
+  });
+
+  test('no-op when readest is the provider', () => {
+    const settings = makeSettings();
+    expect(applySyncBooksAutoEnable(settings)).toBe(false);
+    expect(settings.webdav?.syncBooks).toBeUndefined();
+  });
+
+  test('no-op when syncBooks is already on', () => {
+    const settings = makeSettings({
+      webdav: { enabled: true, syncBooks: true },
+    } as Partial<SystemSettings>);
+    expect(applySyncBooksAutoEnable(settings)).toBe(false);
+  });
+
+  test('only the selected provider is flipped when both are enabled', () => {
+    const settings = makeSettings({
+      webdav: { enabled: true, syncBooks: false },
+      googleDrive: { enabled: true, syncBooks: false },
+    } as Partial<SystemSettings>);
+    expect(applySyncBooksAutoEnable(settings)).toBe(true);
+    expect(settings.webdav?.syncBooks).toBe(true);
+    expect(settings.googleDrive?.syncBooks).toBe(false);
+  });
+});
+
+describe('isReadestCloudStorageActive', () => {
+  test('true when readest is the derived provider', () => {
+    expect(isReadestCloudStorageActive(makeSettings())).toBe(true);
+  });
+
+  test('false when a third-party provider is selected', () => {
+    const settings = makeSettings({ webdav: { enabled: true } } as Partial<SystemSettings>);
+    expect(isReadestCloudStorageActive(settings)).toBe(false);
+  });
+
+  test('false while paused: uploads must not silently resume to Readest Cloud', () => {
+    vi.mocked(isCloudSyncAllowed).mockReturnValue(false);
+    const settings = makeSettings({ googleDrive: { enabled: true } } as Partial<SystemSettings>);
+    expect(isReadestCloudStorageActive(settings, 'free')).toBe(false);
+  });
+});
diff --git a/apps/readest-app/src/__tests__/services/transfer-manager-gating.test.ts b/apps/readest-app/src/__tests__/services/transfer-manager-gating.test.ts
new file mode 100644
index 00000000..e3d49d2a
--- /dev/null
+++ b/apps/readest-app/src/__tests__/services/transfer-manager-gating.test.ts
@@ -0,0 +1,445 @@
+import { describe, test, expect, beforeEach, afterEach, vi } from 'vitest';
+import { useTransferStore, TransferItem } from '@/store/transferStore';
+import { useSettingsStore } from '@/store/settingsStore';
+import type { SystemSettings } from '@/types/settings';
+
+vi.mock('@/utils/event', () => ({
+  eventDispatcher: {
+    dispatch: vi.fn(),
+    dispatchSync: vi.fn(),
+  },
+}));
+
+import { transferManager } from '@/services/transferManager';
+import { eventDispatcher } from '@/utils/event';
+import type { Book } from '@/types/book';
+
+function makeBook(overrides: Partial<Book> = {}): Book {
+  return {
+    hash: 'hash1',
+    format: 'EPUB',
+    title: 'Test Book',
+    author: 'Author',
+    createdAt: 1000,
+    updatedAt: 2000,
+    ...overrides,
+  };
+}
+
+function makeTransferItem(overrides: Partial<TransferItem> = {}): TransferItem {
+  return {
+    id: 't1',
+    kind: 'book',
+    bookHash: 'hash1',
+    bookTitle: 'Test Book',
+    type: 'upload',
+    status: 'pending',
+    progress: 0,
+    totalBytes: 0,
+    transferredBytes: 0,
+    transferSpeed: 0,
+    retryCount: 0,
+    maxRetries: 3,
+    createdAt: Date.now(),
+    priority: 10,
+    isBackground: false,
+    ...overrides,
+  };
+}
+
+const resetTransferManager = () => {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any -- test-only introspection
+  const mgr = transferManager as unknown as Record<string, unknown>;
+  mgr['isInitialized'] = false;
+  mgr['isProcessing'] = false;
+  mgr['appService'] = null;
+  mgr['getLibrary'] = null;
+  mgr['updateBook'] = null;
+  mgr['_'] = null;
+  (mgr['abortControllers'] as Map<string, AbortController>).clear();
+  let resolveReady: () => void = () => {};
+  mgr['readyPromise'] = new Promise<void>((res) => {
+    resolveReady = res;
+  });
+  mgr['readyResolve'] = resolveReady;
+};
+
+const resetTransferStore = () => {
+  useTransferStore.setState({
+    transfers: {},
+    isQueuePaused: false,
+    isTransferQueueOpen: false,
+    maxConcurrent: 2,
+    activeCount: 0,
+  });
+};
+
+const settingsLoaded = (overrides: Partial<SystemSettings> = {}): void => {
+  useSettingsStore.setState({
+    settings: {
+      version: 1,
+      webdav: { enabled: false },
+      googleDrive: { enabled: false },
+      ...overrides,
+    } as SystemSettings,
+  });
+};
+
+const settingsNotLoaded = (): void => {
+  useSettingsStore.setState({ settings: {} as SystemSettings });
+};
+
+const webdavSelected = (): void =>
+  settingsLoaded({ webdav: { enabled: true } } as Partial<SystemSettings>);
+
+function makeAppService(overrides: Record<string, unknown> = {}) {
+  return {
+    uploadBook: vi.fn().mockResolvedValue(undefined),
+    downloadBook: vi.fn().mockResolvedValue(undefined),
+    deleteBook: vi.fn().mockResolvedValue(undefined),
+    uploadReplicaFile: vi.fn().mockResolvedValue(undefined),
+    downloadReplicaFile: vi.fn().mockResolvedValue(undefined),
+    deleteReplicaBundle: vi.fn().mockResolvedValue(undefined),
+    isMacOSApp: false,
+    ...overrides,
+  } as Record<string, unknown>;
+}
+
+const translationFn = (key: string, params?: Record<string, string | number>) => {
+  if (params) {
+    return Object.entries(params).reduce((acc, [k, v]) => acc.replace(`{{${k}}}`, String(v)), key);
+  }
+  return key;
+};
+
+const initManager = async (appService = makeAppService(), library: Book[] = []) => {
+  await transferManager.initialize(appService as never, () => library, vi.fn(), translationFn);
+  return appService;
+};
+
+const flushAsync = async (ms = 5000) => {
+  await vi.advanceTimersByTimeAsync(ms);
+};
+
+beforeEach(() => {
+  vi.useFakeTimers({ shouldAdvanceTime: true });
+  resetTransferStore();
+  resetTransferManager();
+  vi.clearAllMocks();
+  localStorage.clear();
+  settingsLoaded();
+  vi.spyOn(console, 'warn').mockImplementation(() => {});
+  vi.spyOn(console, 'error').mockImplementation(() => {});
+  vi.spyOn(console, 'info').mockImplementation(() => {});
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe('provider gating of book uploads', () => {
+  test('queueUpload returns null and queues nothing when a third-party provider is selected', async () => {
+    webdavSelected();
+    await initManager();
+
+    const id = transferManager.queueUpload(makeBook());
+    expect(id).toBeNull();
+    expect(Object.keys(useTransferStore.getState().transfers)).toHaveLength(0);
+  });
+
+  test('queueUpload works when Readest Cloud is the provider', async () => {
+    await initManager();
+
+    const id = transferManager.queueUpload(makeBook());
+    expect(id).toBeTruthy();
+  });
+
+  test('queueBatchUploads returns empty when gated', async () => {
+    webdavSelected();
+    await initManager();
+
+    const ids = transferManager.queueBatchUploads([makeBook(), makeBook({ hash: 'hash2' })]);
+    expect(ids).toEqual([]);
+  });
+
+  test('downloads and replica transfers are never gated', async () => {
+    webdavSelected();
+    await initManager();
+
+    const downloadId = transferManager.queueDownload(makeBook());
+    expect(downloadId).toBeTruthy();
+
+    const replicaId = transferManager.queueReplicaUpload(
+      'font',
+      'font-1',
+      'A Font',
+      [{ logical: 'font.ttf', lfp: '/tmp/font.ttf', byteSize: 10 }],
+      'Data' as never,
+    );
+    expect(replicaId).toBeTruthy();
+  });
+});
+
+describe('settings-loaded barrier', () => {
+  test('a pending book upload does not execute before settings hydration', async () => {
+    settingsNotLoaded();
+    const persisted = {
+      transfers: { t1: makeTransferItem() },
+      isQueuePaused: false,
+    };
+    localStorage.setItem('readest_transfer_queue', JSON.stringify(persisted));
+
+    const appService = makeAppService();
+    await initManager(appService, [makeBook()]);
+    await flushAsync();
+
+    expect(appService['uploadBook']).not.toHaveBeenCalled();
+    expect(useTransferStore.getState().transfers['t1']?.status).toBe('pending');
+  });
+
+  test('the deferred upload executes once settings hydrate with readest selected', async () => {
+    settingsNotLoaded();
+    localStorage.setItem(
+      'readest_transfer_queue',
+      JSON.stringify({ transfers: { t1: makeTransferItem() }, isQueuePaused: false }),
+    );
+
+    const appService = makeAppService();
+    await initManager(appService, [makeBook()]);
+    await flushAsync();
+    expect(appService['uploadBook']).not.toHaveBeenCalled();
+
+    settingsLoaded();
+    await flushAsync();
+
+    expect(appService['uploadBook']).toHaveBeenCalledTimes(1);
+  });
+
+  test('replica transfers are not stalled by the barrier', async () => {
+    settingsNotLoaded();
+    const appService = makeAppService();
+    await initManager(appService);
+
+    transferManager.queueReplicaDownload(
+      'font',
+      'font-1',
+      'A Font',
+      [{ logical: 'font.ttf', lfp: '/tmp/font.ttf', byteSize: 10 }],
+      'Data' as never,
+    );
+    await flushAsync();
+
+    expect(appService['downloadReplicaFile']).toHaveBeenCalled();
+  });
+});
+
+describe('policy cancellation on restore/reconcile', () => {
+  test('pre-switch pending book uploads are policy-cancelled on restore; downloads and replicas survive', async () => {
+    webdavSelected();
+    const persisted = {
+      transfers: {
+        up1: makeTransferItem({ id: 'up1' }),
+        down1: makeTransferItem({ id: 'down1', type: 'download', bookHash: 'hash2' }),
+        rep1: makeTransferItem({
+          id: 'rep1',
+          kind: 'replica',
+          type: 'upload',
+          bookHash: '',
+          replicaKind: 'font',
+          replicaId: 'font-1',
+          replicaFiles: [{ logical: 'font.ttf', lfp: '/tmp/font.ttf', byteSize: 10 }],
+          replicaBase: 'Data' as never,
+        }),
+      },
+      isQueuePaused: true,
+    };
+    localStorage.setItem('readest_transfer_queue', JSON.stringify(persisted));
+
+    await initManager();
+    await flushAsync();
+
+    const transfers = useTransferStore.getState().transfers;
+    expect(transfers['up1']?.status).toBe('cancelled');
+    expect(transfers['up1']?.cancelReason).toBe('policy');
+    expect(transfers['down1']?.status).toBe('pending');
+    expect(transfers['rep1']?.status).toBe('pending');
+  });
+
+  test('switching providers after init policy-cancels pending book uploads', async () => {
+    await initManager();
+    useTransferStore.getState().pauseQueue();
+    const id = transferManager.queueUpload(makeBook())!;
+
+    webdavSelected();
+    await flushAsync();
+
+    const transfer = useTransferStore.getState().transfers[id];
+    expect(transfer?.status).toBe('cancelled');
+    expect(transfer?.cancelReason).toBe('policy');
+  });
+
+  test('policy-cancelled rows from a previous session are pruned on restore', async () => {
+    const persisted = {
+      transfers: {
+        old1: makeTransferItem({ id: 'old1', status: 'cancelled', cancelReason: 'policy' }),
+        user1: makeTransferItem({ id: 'user1', status: 'cancelled' }),
+      },
+      isQueuePaused: false,
+    };
+    localStorage.setItem('readest_transfer_queue', JSON.stringify(persisted));
+
+    await initManager();
+
+    const transfers = useTransferStore.getState().transfers;
+    expect(transfers['old1']).toBeUndefined();
+    expect(transfers['user1']).toBeDefined();
+  });
+
+  test('legacy persisted entries without kind or cancelReason restore cleanly', async () => {
+    const legacy = makeTransferItem({ id: 'l1', status: 'completed' }) as Partial<TransferItem>;
+    delete legacy.kind;
+    localStorage.setItem(
+      'readest_transfer_queue',
+      JSON.stringify({ transfers: { l1: legacy }, isQueuePaused: false }),
+    );
+
+    await initManager();
+
+    const restored = useTransferStore.getState().transfers['l1'];
+    expect(restored?.kind).toBe('book');
+    expect(restored?.status).toBe('completed');
+  });
+});
+
+describe('cancelled bucket accounting', () => {
+  test('policy-cancelled rows are excluded from failed stats and getFailedTransfers; user-cancelled stay', () => {
+    useTransferStore.setState({
+      transfers: {
+        p1: makeTransferItem({ id: 'p1', status: 'cancelled', cancelReason: 'policy' }),
+        u1: makeTransferItem({ id: 'u1', status: 'cancelled', cancelReason: 'user' }),
+        f1: makeTransferItem({ id: 'f1', status: 'failed' }),
+      },
+    });
+
+    const store = useTransferStore.getState();
+    const failedIds = store.getFailedTransfers().map((t) => t.id);
+    expect(failedIds).toContain('u1');
+    expect(failedIds).toContain('f1');
+    expect(failedIds).not.toContain('p1');
+    expect(store.getQueueStats().failed).toBe(2);
+  });
+
+  test('retryAllFailed does not resurrect policy-cancelled rows', async () => {
+    webdavSelected();
+    await initManager();
+    useTransferStore.setState({
+      transfers: {
+        p1: makeTransferItem({ id: 'p1', status: 'cancelled', cancelReason: 'policy' }),
+      },
+    });
+
+    transferManager.retryAllFailed();
+    await flushAsync();
+
+    expect(useTransferStore.getState().transfers['p1']?.status).toBe('cancelled');
+  });
+
+  test('retryTransfer no-ops on a policy-cancelled row', async () => {
+    webdavSelected();
+    await initManager();
+    useTransferStore.setState({
+      transfers: {
+        p1: makeTransferItem({ id: 'p1', status: 'cancelled', cancelReason: 'policy' }),
+      },
+    });
+
+    transferManager.retryTransfer('p1');
+    await flushAsync();
+
+    expect(useTransferStore.getState().transfers['p1']?.status).toBe('cancelled');
+  });
+
+  test('user cancelTransfer records cancelReason user', async () => {
+    await initManager();
+    useTransferStore.getState().pauseQueue();
+    const id = transferManager.queueUpload(makeBook())!;
+
+    transferManager.cancelTransfer(id);
+
+    const transfer = useTransferStore.getState().transfers[id];
+    expect(transfer?.status).toBe('cancelled');
+    expect(transfer?.cancelReason).toBe('user');
+  });
+
+  test('gate-off reconcile settles: a force-pended policy row is re-cancelled without looping', async () => {
+    webdavSelected();
+    await initManager();
+    useTransferStore.setState({
+      transfers: {
+        p1: makeTransferItem({ id: 'p1', status: 'pending', cancelReason: 'policy' }),
+      },
+    });
+
+    await (transferManager as unknown as { processQueue: () => Promise<void> }).processQueue();
+    await flushAsync();
+
+    const transfer = useTransferStore.getState().transfers['p1'];
+    expect(transfer?.status).toBe('cancelled');
+    expect(useTransferStore.getState().getPendingTransfers()).toHaveLength(0);
+  });
+});
+
+describe('quota failure handling', () => {
+  test('quota 403 fails immediately with zero retries', async () => {
+    const appService = makeAppService({
+      uploadBook: vi.fn().mockRejectedValue(new Error('Insufficient storage quota')),
+    });
+    const book = makeBook();
+    await initManager(appService, [book]);
+
+    transferManager.queueUpload(book);
+    await flushAsync();
+
+    const transfers = Object.values(useTransferStore.getState().transfers);
+    expect(transfers).toHaveLength(1);
+    expect(transfers[0]?.status).toBe('failed');
+    expect(transfers[0]?.retryCount).toBe(0);
+    expect(appService['uploadBook']).toHaveBeenCalledTimes(1);
+  });
+
+  test('a batch of quota failures produces one summary toast, not one per book', async () => {
+    const appService = makeAppService({
+      uploadBook: vi.fn().mockRejectedValue(new Error('Insufficient storage quota')),
+    });
+    const books = [makeBook(), makeBook({ hash: 'hash2' }), makeBook({ hash: 'hash3' })];
+    await initManager(appService, books);
+
+    transferManager.queueBatchUploads(books);
+    await flushAsync(10000);
+
+    const dispatched = vi.mocked(eventDispatcher.dispatch).mock.calls.filter(
+      ([event, payload]) =>
+        event === 'toast' &&
+        String((payload as { message?: string })?.message ?? '')
+          .toLowerCase()
+          .includes('quota'),
+    );
+    expect(dispatched).toHaveLength(1);
+    expect(String((dispatched[0]![1] as { message: string }).message)).toContain('3');
+  });
+
+  test('non-quota errors keep the existing retry behavior', async () => {
+    const appService = makeAppService({
+      uploadBook: vi.fn().mockRejectedValue(new Error('network boom')),
+    });
+    const book = makeBook();
+    await initManager(appService, [book]);
+
+    transferManager.queueUpload(book);
+    await flushAsync(60000);
+
+    const transfer = Object.values(useTransferStore.getState().transfers)[0];
+    expect(transfer?.status).toBe('failed');
+    expect(transfer?.retryCount).toBe(3);
+  });
+});
diff --git a/apps/readest-app/src/__tests__/services/transfer-manager.test.ts b/apps/readest-app/src/__tests__/services/transfer-manager.test.ts
index d94c55e0..eb25e494 100644
--- a/apps/readest-app/src/__tests__/services/transfer-manager.test.ts
+++ b/apps/readest-app/src/__tests__/services/transfer-manager.test.ts
@@ -1,5 +1,7 @@
 import { describe, test, expect, beforeEach, afterEach, vi, type Mock } from 'vitest';
 import { useTransferStore, TransferItem } from '@/store/transferStore';
+import { useSettingsStore } from '@/store/settingsStore';
+import type { SystemSettings } from '@/types/settings';
 
 // ── Mocks ────────────────────────────────────────────────────────────
 // The transferManager module is a singleton, so we need to mock its
@@ -105,6 +107,16 @@ beforeEach(() => {
   resetTransferManager();
   vi.clearAllMocks();
   localStorage.clear();
+  // Book uploads are gated on the selected cloud sync provider and
+  // deferred until settings hydrate; hydrate with Readest Cloud selected
+  // so the pre-gating behavior under test is preserved.
+  useSettingsStore.setState({
+    settings: {
+      version: 1,
+      webdav: { enabled: false },
+      googleDrive: { enabled: false },
+    } as SystemSettings,
+  });
   vi.spyOn(console, 'warn').mockImplementation(() => {});
   vi.spyOn(console, 'error').mockImplementation(() => {});
 });
diff --git a/apps/readest-app/src/app/library/components/TransferQueuePanel.tsx b/apps/readest-app/src/app/library/components/TransferQueuePanel.tsx
index 0ddefff1..4f8e025e 100644
--- a/apps/readest-app/src/app/library/components/TransferQueuePanel.tsx
+++ b/apps/readest-app/src/app/library/components/TransferQueuePanel.tsx
@@ -17,7 +17,12 @@ import { useTranslation } from '@/hooks/useTranslation';
 import { useResponsiveSize } from '@/hooks/useResponsiveSize';
 import { useKeyDownActions } from '@/hooks/useKeyDownActions';
 import { useLibraryStore } from '@/store/libraryStore';
-import { TransferItem, TransferStatus, useTransferStore } from '@/store/transferStore';
+import {
+  TransferItem,
+  TransferStatus,
+  isFailedLikeTransfer,
+  useTransferStore,
+} from '@/store/transferStore';
 
 const formatBytes = (bytes: number): string => {
   if (bytes === 0) return '0 B';
@@ -109,7 +114,10 @@ const TransferItemRow: React.FC<{
             <span className='text-error'>{transfer.error || _('Failed')}</span>
           )}
           {transfer.status === 'completed' && (completedLabel[transfer.type] || _('Completed'))}
-          {transfer.status === 'cancelled' && _('Cancelled')}
+          {transfer.status === 'cancelled' &&
+            (transfer.cancelReason === 'policy'
+              ? `${_('Cancelled')} · ${_('Cloud provider switched')}`
+              : _('Cancelled'))}
           {' · '}
           {formatDateTime(transfer.completedAt || transfer.startedAt || transfer.createdAt)}
         </div>
@@ -125,7 +133,7 @@ const TransferItemRow: React.FC<{
       </div>
 
       <div className='flex items-center gap-1'>
-        {(transfer.status === 'failed' || transfer.status === 'cancelled') && (
+        {isFailedLikeTransfer(transfer) && (
           <button
             onClick={() => onRetry(transfer.id)}
             className='btn btn-ghost btn-sm btn-circle'
@@ -197,7 +205,7 @@ const TransferQueuePanel: React.FC = () => {
         case 'completed':
           return t.status === 'completed';
         case 'failed':
-          return t.status === 'failed' || t.status === 'cancelled';
+          return isFailedLikeTransfer(t) || t.status === 'cancelled';
         default:
           return true;
       }
diff --git a/apps/readest-app/src/app/library/hooks/useLibraryFileSync.ts b/apps/readest-app/src/app/library/hooks/useLibraryFileSync.ts
index 2a69b765..8c28b3da 100644
--- a/apps/readest-app/src/app/library/hooks/useLibraryFileSync.ts
+++ b/apps/readest-app/src/app/library/hooks/useLibraryFileSync.ts
@@ -14,6 +14,7 @@ import {
   createFileSyncProvider,
   type FileSyncBackendKind,
 } from '@/services/sync/file/providerRegistry';
+import { getCloudSyncProvider } from '@/services/sync/cloudSyncProvider';
 
 /**
  * Library-scoped auto-sync for the active third-party cloud provider (WebDAV /
@@ -55,11 +56,8 @@ export const useLibraryFileSync = () => {
   const { userProfilePlan } = useQuotaStats();
 
   // The single active cloud provider (WebDAV and Google Drive are exclusive).
-  const activeKind: FileSyncBackendKind | null = settings.webdav?.enabled
-    ? 'webdav'
-    : settings.googleDrive?.enabled
-      ? 'gdrive'
-      : null;
+  const provider = getCloudSyncProvider(settings);
+  const activeKind: FileSyncBackendKind | null = provider === 'readest' ? null : provider;
 
   const isAllowed = isCloudSyncAllowed(userProfilePlan ?? 'free');
   const isReady = useMemo(() => {
diff --git a/apps/readest-app/src/app/library/page.tsx b/apps/readest-app/src/app/library/page.tsx
index c7109af1..f05ceeac 100644
--- a/apps/readest-app/src/app/library/page.tsx
+++ b/apps/readest-app/src/app/library/page.tsx
@@ -22,6 +22,10 @@ import { eventDispatcher } from '@/utils/event';
 import { ProgressPayload } from '@/utils/transfer';
 import { throttle } from '@/utils/throttle';
 import { transferManager } from '@/services/transferManager';
+import {
+  getCloudSyncProvider,
+  isReadestCloudStorageActive,
+} from '@/services/sync/cloudSyncProvider';
 import { getDirPath, getFilename, joinPaths } from '@/utils/path';
 import { parseOpenWithFiles } from '@/helpers/openWith';
 import { isTauriAppPlatform, isWebAppPlatform } from '@/services/environment';
@@ -954,6 +958,19 @@ const LibraryPageContent = ({ searchParams }: { searchParams: ReadonlyURLSearchP
         });
         return true;
       }
+      // An explicit Upload action must never silently no-op: explain the
+      // provider gate when it is the reason the queue refused the book.
+      const currentSettings = useSettingsStore.getState().settings;
+      if (!isReadestCloudStorageActive(currentSettings)) {
+        const provider = getCloudSyncProvider(currentSettings);
+        eventDispatcher.dispatch('toast', {
+          type: 'info',
+          timeout: 5000,
+          message: _('Uploads to Readest Cloud are paused while {{provider}} sync is selected', {
+            provider: provider === 'gdrive' ? 'Google Drive' : 'WebDAV',
+          }),
+        });
+      }
       return false;
     },
     // eslint-disable-next-line react-hooks/exhaustive-deps
diff --git a/apps/readest-app/src/app/opds/page.tsx b/apps/readest-app/src/app/opds/page.tsx
index d5362444..c4c08e51 100644
--- a/apps/readest-app/src/app/opds/page.tsx
+++ b/apps/readest-app/src/app/opds/page.tsx
@@ -18,6 +18,7 @@ import { useLibraryStore } from '@/store/libraryStore';
 import { useSettingsStore } from '@/store/settingsStore';
 import { useCustomOPDSStore } from '@/store/customOPDSStore';
 import { transferManager } from '@/services/transferManager';
+import { isReadestCloudStorageActive } from '@/services/sync/cloudSyncProvider';
 import { useTransferQueue } from '@/hooks/useTransferQueue';
 import { useTheme } from '@/hooks/useTheme';
 import { useLibrary } from '@/hooks/useLibrary';
@@ -625,7 +626,13 @@ export default function BrowserPage() {
                 console.error('OPDS: failed to update source map:', sourceMapError);
               }
             }
-            if (user && book && !book.uploadedAt && settings.autoUpload) {
+            if (
+              user &&
+              book &&
+              !book.uploadedAt &&
+              settings.autoUpload &&
+              isReadestCloudStorageActive(settings)
+            ) {
               setTimeout(() => {
                 transferManager.queueUpload(book);
               }, 3000);
diff --git a/apps/readest-app/src/app/reader/hooks/useFileSync.ts b/apps/readest-app/src/app/reader/hooks/useFileSync.ts
index bcb423c3..9c3f6234 100644
--- a/apps/readest-app/src/app/reader/hooks/useFileSync.ts
+++ b/apps/readest-app/src/app/reader/hooks/useFileSync.ts
@@ -17,6 +17,7 @@ import {
   createFileSyncProvider,
   type FileSyncBackendKind,
 } from '@/services/sync/file/providerRegistry';
+import { getCloudSyncProvider } from '@/services/sync/cloudSyncProvider';
 import { removeBookNoteOverlays } from '../utils/annotatorUtil';
 import { useWindowActiveChanged } from './useWindowActiveChanged';
 
@@ -79,11 +80,8 @@ export const useFileSync = (bookKey: string) => {
   const progress = useBookProgress(bookKey);
 
   // The single active cloud provider (WebDAV and Google Drive are exclusive).
-  const activeKind: FileSyncBackendKind | null = settings.webdav?.enabled
-    ? 'webdav'
-    : settings.googleDrive?.enabled
-      ? 'gdrive'
-      : null;
+  const provider = getCloudSyncProvider(settings);
+  const activeKind: FileSyncBackendKind | null = provider === 'readest' ? null : provider;
   const providerSettings = activeKind === 'gdrive' ? settings.googleDrive : settings.webdav;
 
   /** Flips true on the first local change after a push, false right before each push. */
diff --git a/apps/readest-app/src/components/settings/integrations/cloudSync.ts b/apps/readest-app/src/components/settings/integrations/cloudSync.ts
index 7b7147d4..e3da4a76 100644
--- a/apps/readest-app/src/components/settings/integrations/cloudSync.ts
+++ b/apps/readest-app/src/components/settings/integrations/cloudSync.ts
@@ -8,12 +8,26 @@ import type { FileSyncBackendKind } from '@/services/sync/file/providerRegistry'
  * (WebDAV creds, the Drive keychain token) is left untouched, so switching back
  * to a previously-configured provider needs no re-entry; only an explicit
  * Disconnect tears a provider's config down.
+ *
+ * Activating a provider (disabled -> enabled) also turns its `syncBooks` on:
+ * the selected provider owns the book-file channel — native Readest Cloud
+ * uploads gate off — so leaving syncBooks at its `false` default would back
+ * books up nowhere. An explicit opt-out while the provider stays active is
+ * respected (re-activation of an already-active provider changes nothing).
  */
 export const withActiveCloudProvider = (
   settings: SystemSettings,
   active: FileSyncBackendKind | null,
 ): SystemSettings => ({
   ...settings,
-  webdav: { ...settings.webdav, enabled: active === 'webdav' },
-  googleDrive: { ...settings.googleDrive, enabled: active === 'gdrive' },
+  webdav: {
+    ...settings.webdav,
+    enabled: active === 'webdav',
+    ...(active === 'webdav' && !settings.webdav?.enabled ? { syncBooks: true } : {}),
+  },
+  googleDrive: {
+    ...settings.googleDrive,
+    enabled: active === 'gdrive',
+    ...(active === 'gdrive' && !settings.googleDrive?.enabled ? { syncBooks: true } : {}),
+  },
 });
diff --git a/apps/readest-app/src/hooks/useOPDSSubscriptions.ts b/apps/readest-app/src/hooks/useOPDSSubscriptions.ts
index 0f43bc97..fe6e8775 100644
--- a/apps/readest-app/src/hooks/useOPDSSubscriptions.ts
+++ b/apps/readest-app/src/hooks/useOPDSSubscriptions.ts
@@ -3,6 +3,7 @@ import { useAuth } from '@/context/AuthContext';
 import { useEnv } from '@/context/EnvContext';
 import { useLibraryStore } from '@/store/libraryStore';
 import { useSettingsStore } from '@/store/settingsStore';
+import { isReadestCloudStorageActive } from '@/services/sync/cloudSyncProvider';
 import { useTranslation } from '@/hooks/useTranslation';
 import { syncSubscribedCatalogs } from '@/services/opds';
 import { AUTO_CHECK_INTERVAL_MS } from '@/services/opds/types';
@@ -51,7 +52,12 @@ export function useOPDSSubscriptions() {
           // autoUpload setting on. Delay so the transfer manager has a chance
           // to finish initializing if this fires right after libraryLoaded.
           const { settings: currentSettings } = useSettingsStore.getState();
-          if (user && currentSettings.autoUpload && uniqueNewBooks.length > 0) {
+          if (
+            user &&
+            currentSettings.autoUpload &&
+            isReadestCloudStorageActive(currentSettings) &&
+            uniqueNewBooks.length > 0
+          ) {
             const booksToUpload = uniqueNewBooks.filter((b) => !b.uploadedAt);
             if (booksToUpload.length > 0) {
               setTimeout(() => {
diff --git a/apps/readest-app/src/hooks/useQuotaStats.ts b/apps/readest-app/src/hooks/useQuotaStats.ts
index 927e95f2..dcea559e 100644
--- a/apps/readest-app/src/hooks/useQuotaStats.ts
+++ b/apps/readest-app/src/hooks/useQuotaStats.ts
@@ -2,6 +2,7 @@ import { useEffect, useState } from 'react';
 import { useAuth } from '@/context/AuthContext';
 import { QuotaType, UserPlan } from '@/types/quota';
 import { getStoragePlanData, getTranslationPlanData, getUserProfilePlan } from '@/utils/access';
+import { setCachedUserPlan } from '@/services/sync/cloudSyncProvider';
 import { useTranslation } from './useTranslation';
 
 export const useQuotaStats = (briefName = false) => {
@@ -41,7 +42,12 @@ export const useQuotaStats = (briefName = false) => {
       unit: 'K',
       resetAt: translationResetAt,
     };
-    setUserProfilePlan(getUserProfilePlan(token));
+    const profilePlan = getUserProfilePlan(token);
+    setUserProfilePlan(profilePlan);
+    // Non-React modules (transferManager, syncCategories) need the plan
+    // synchronously for the cloud-sync provider gate; cache it here, the
+    // one place the plan is resolved from the JWT.
+    setCachedUserPlan(profilePlan);
     setQuotas([storageQuota, translationQuota]);
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [token]);
diff --git a/apps/readest-app/src/hooks/useTransferQueue.ts b/apps/readest-app/src/hooks/useTransferQueue.ts
index 9601f279..5b66c6d2 100644
--- a/apps/readest-app/src/hooks/useTransferQueue.ts
+++ b/apps/readest-app/src/hooks/useTransferQueue.ts
@@ -2,10 +2,15 @@ import { useEffect, useCallback, useMemo } from 'react';
 import { useEnv } from '@/context/EnvContext';
 import { useTranslation } from './useTranslation';
 import { useLibraryStore } from '@/store/libraryStore';
-import { useTransferStore, TransferType } from '@/store/transferStore';
+import { useTransferStore, TransferType, isFailedLikeTransfer } from '@/store/transferStore';
 import { transferManager } from '@/services/transferManager';
 import { Book } from '@/types/book';
 
+// The `libraryLoaded = true` default lets surfaces like SettingsMenu and
+// TransferQueuePanel initialize the manager on mount, before settings
+// hydrate. That is safe: the manager defers book uploads until
+// settings.version is truthy and reconciles them against the selected
+// cloud sync provider once it is (see transferManager.isSettingsLoaded).
 export function useTransferQueue(libraryLoaded = true, delayInit = 0) {
   const { envConfig, appService } = useEnv();
   const _ = useTranslation();
@@ -92,7 +97,7 @@ export function useTransferQueue(libraryLoaded = true, delayInit = 0) {
       pending: transferList.filter((t) => t.status === 'pending').length,
       active: transferList.filter((t) => t.status === 'in_progress').length,
       completed: transferList.filter((t) => t.status === 'completed').length,
-      failed: transferList.filter((t) => t.status === 'failed' || t.status === 'cancelled').length,
+      failed: transferList.filter(isFailedLikeTransfer).length,
       total: transferList.length,
     };
   }, [transfers]);
@@ -106,9 +111,7 @@ export function useTransferQueue(libraryLoaded = true, delayInit = 0) {
   }, [transfers]);
 
   const failedTransfers = useMemo(() => {
-    return Object.values(transfers).filter(
-      (t) => t.status === 'failed' || t.status === 'cancelled',
-    );
+    return Object.values(transfers).filter(isFailedLikeTransfer);
   }, [transfers]);
 
   const completedTransfers = useMemo(() => {
diff --git a/apps/readest-app/src/services/appService.ts b/apps/readest-app/src/services/appService.ts
index 5482e617..aa9c9475 100644
--- a/apps/readest-app/src/services/appService.ts
+++ b/apps/readest-app/src/services/appService.ts
@@ -1,4 +1,5 @@
 import { SystemSettings } from '@/types/settings';
+import { applySyncBooksAutoEnable } from '@/services/sync/cloudSyncProvider';
 import {
   AppPlatform,
   AppService,
@@ -68,7 +69,7 @@ export abstract class BaseAppService implements AppService {
   storefrontRegionCode: string | null = null;
   isOnlineCatalogsAccessible = true;
 
-  protected CURRENT_MIGRATION_VERSION = 20251124;
+  protected CURRENT_MIGRATION_VERSION = 20260706;
 
   protected abstract fs: FileSystem;
   protected abstract resolvePath(fp: string, base: BaseDir): ResolvedPath;
@@ -100,7 +101,10 @@ export abstract class BaseAppService implements AppService {
     opts?: DatabaseOpts,
   ): Promise<DatabaseService>;
 
-  protected async runMigrations(lastMigrationVersion: number): Promise<void> {
+  protected async runMigrations(
+    lastMigrationVersion: number,
+    settings?: SystemSettings,
+  ): Promise<void> {
     if (lastMigrationVersion < 20251124) {
       try {
         await this.migrate20251124();
@@ -108,6 +112,26 @@ export abstract class BaseAppService implements AppService {
         console.error('Error migrating to version 20251124:', error);
       }
     }
+    if (lastMigrationVersion < 20260706 && settings) {
+      try {
+        this.migrate20260706(settings);
+      } catch (error) {
+        console.error('Error migrating to version 20260706:', error);
+      }
+    }
+  }
+
+  /**
+   * Users with WebDAV/Drive already enabled become "third-party selected"
+   * when cloud sync provider selection ships, gating native Readest Cloud
+   * uploads off; flip the selected provider's syncBooks on once so their
+   * books keep backing up somewhere. Mutates the caller's settings
+   * snapshot, which the caller persists together with migrationVersion.
+   */
+  private migrate20260706(settings: SystemSettings): void {
+    if (applySyncBooksAutoEnable(settings)) {
+      console.log('Migration 20260706: enabled syncBooks for the selected cloud sync provider.');
+    }
   }
 
   private async migrate20251124(): Promise<void> {
diff --git a/apps/readest-app/src/services/backupService.ts b/apps/readest-app/src/services/backupService.ts
index 68b38352..bbb6b715 100644
--- a/apps/readest-app/src/services/backupService.ts
+++ b/apps/readest-app/src/services/backupService.ts
@@ -51,6 +51,8 @@ export const BACKUP_SETTINGS_BLACKLIST = [
   'hardcover.lastSyncedAt',
   'googleDrive.deviceId',
   'googleDrive.lastSyncedAt',
+  'webdav.deviceId',
+  'webdav.lastSyncedAt',
   // Transient runtime state — book keys may not exist post-restore; screen
   // brightness is live device state.
   'lastOpenBooks',
diff --git a/apps/readest-app/src/services/ingestService.ts b/apps/readest-app/src/services/ingestService.ts
index 82f438a8..c2966a5f 100644
--- a/apps/readest-app/src/services/ingestService.ts
+++ b/apps/readest-app/src/services/ingestService.ts
@@ -2,6 +2,7 @@ import type { Book, BookLookupIndex } from '@/types/book';
 import type { AppService, OsPlatform } from '@/types/system';
 import type { SystemSettings } from '@/types/settings';
 import { transferManager } from '@/services/transferManager';
+import { isReadestCloudStorageActive } from '@/services/sync/cloudSyncProvider';
 import { normalizeFilePathForIndex } from '@/services/bookService';
 import { isContentURI, isValidURL } from '@/utils/misc';
 import { isPseStreamFileName } from '@/services/opds/pseStream';
@@ -237,11 +238,16 @@ export async function ingestFile(
   // they are equivalent to a hash-copy book — only the local storage
   // location differs. uploadBook reads straight from book.filePath in that
   // case; downloads on other devices land in Books/<hash>/ as a normal copy.
+  // When a third-party provider (WebDAV/Drive) is the selected cloud sync
+  // backend, Readest Cloud storage is not written to at all — the file-sync
+  // engine mirrors the import instead (including Sent books, which then reach
+  // other devices via that provider when its syncBooks toggle is on).
   if (
     !opts.transient &&
     isLoggedIn &&
     !book.uploadedAt &&
-    (opts.forceUpload || settings.autoUpload)
+    (opts.forceUpload || settings.autoUpload) &&
+    isReadestCloudStorageActive(settings)
   ) {
     transferManager.queueUpload(book);
   }
diff --git a/apps/readest-app/src/services/nativeAppService.ts b/apps/readest-app/src/services/nativeAppService.ts
index be97d14b..57e4a290 100644
--- a/apps/readest-app/src/services/nativeAppService.ts
+++ b/apps/readest-app/src/services/nativeAppService.ts
@@ -658,7 +658,7 @@ export class NativeAppService extends BaseAppService {
       const settings = await this.loadSettings();
       const lastMigrationVersion = settings.migrationVersion || 0;
 
-      await super.runMigrations(lastMigrationVersion);
+      await super.runMigrations(lastMigrationVersion, settings);
 
       if (lastMigrationVersion < 20251029) {
         try {
diff --git a/apps/readest-app/src/services/sync/cloudSyncProvider.ts b/apps/readest-app/src/services/sync/cloudSyncProvider.ts
new file mode 100644
index 00000000..cbf0151c
--- /dev/null
+++ b/apps/readest-app/src/services/sync/cloudSyncProvider.ts
@@ -0,0 +1,98 @@
+import type { SystemSettings } from '@/types/settings';
+import type { UserPlan } from '@/types/quota';
+import { isCloudSyncAllowed } from '@/utils/access';
+import type { FileSyncBackendKind } from '@/services/sync/file/providerRegistry';
+
+/**
+ * The user's selected cloud sync provider for library data (book files,
+ * book rows, progress, notes). 'readest' is the native Readest Cloud;
+ * the others are the third-party file-sync backends. Account-level data
+ * (settings replicas, reading stats, dictionaries/fonts, translations)
+ * always syncs via Readest Cloud regardless of this selection.
+ *
+ * The selection is DERIVED from the existing per-device enabled flags —
+ * there is no separate persisted field, so it inherits the device-local
+ * semantics of `webdav.enabled` / `googleDrive.enabled` and needs no
+ * migration. `withActiveCloudProvider` keeps the flags mutually
+ * exclusive; if both are ever enabled (hand-edited or restored
+ * settings), WebDAV wins deterministically.
+ */
+export type CloudSyncProviderKind = 'readest' | FileSyncBackendKind;
+
+export interface CloudSyncGate {
+  provider: CloudSyncProviderKind;
+  /**
+   * True when a third-party provider is selected but cloud sync is not
+   * allowed for the user's plan. Paused means paused: Readest Cloud
+   * uploads do NOT silently resume (that would push a possibly-private
+   * library to Readest servers without consent and reintroduce the
+   * #4959 quota path); the UI surfaces the paused state instead.
+   */
+  paused: boolean;
+}
+
+export const getCloudSyncProvider = (
+  settings: SystemSettings | null | undefined,
+): CloudSyncProviderKind =>
+  settings?.webdav?.enabled ? 'webdav' : settings?.googleDrive?.enabled ? 'gdrive' : 'readest';
+
+/**
+ * `isCloudSyncAllowed` needs the UserPlan, which comes from the async
+ * auth JWT — non-React modules (transferManager, syncCategories) cannot
+ * resolve it synchronously. The plan-resolution flow (auth / quota
+ * refresh) writes the latest plan here; gate checks read it back.
+ * Defaults to 'free', the most restrictive plan, so a gate evaluated
+ * before the first auth resolution can only be too cautious, never too
+ * permissive.
+ */
+let cachedUserPlan: UserPlan = 'free';
+
+export const setCachedUserPlan = (plan: UserPlan | undefined): void => {
+  cachedUserPlan = plan ?? 'free';
+};
+
+export const getCachedUserPlan = (): UserPlan => cachedUserPlan;
+
+export const resolveCloudSyncGate = (
+  settings: SystemSettings | null | undefined,
+  plan: UserPlan = cachedUserPlan,
+): CloudSyncGate => {
+  const provider = getCloudSyncProvider(settings);
+  if (provider !== 'readest' && !isCloudSyncAllowed(plan)) {
+    return { provider, paused: true };
+  }
+  return { provider, paused: false };
+};
+
+/**
+ * One-time upgrade migration helper (appService migrate20260706): users
+ * who already had WebDAV/Drive enabled before provider selection shipped
+ * become "third-party selected" on upgrade, which gates native Readest
+ * Cloud uploads off — with syncBooks at its old `false` default their
+ * books would back up nowhere. Flip syncBooks on for the SELECTED
+ * provider only. Mutates `settings` in place (the migration runner saves
+ * the same snapshot afterwards) and returns whether anything changed.
+ */
+export const applySyncBooksAutoEnable = (settings: SystemSettings): boolean => {
+  const provider = getCloudSyncProvider(settings);
+  if (provider === 'webdav' && settings.webdav && !settings.webdav.syncBooks) {
+    settings.webdav = { ...settings.webdav, syncBooks: true };
+    return true;
+  }
+  if (provider === 'gdrive' && settings.googleDrive && !settings.googleDrive.syncBooks) {
+    settings.googleDrive = { ...settings.googleDrive, syncBooks: true };
+    return true;
+  }
+  return false;
+};
+
+/**
+ * Whether Readest Cloud storage may be written to (book file uploads).
+ * Strictly: only when Readest Cloud is the selected provider. A selected
+ * third-party provider — active or paused — means no Readest Cloud
+ * uploads.
+ */
+export const isReadestCloudStorageActive = (
+  settings: SystemSettings | null | undefined,
+  plan?: UserPlan,
+): boolean => resolveCloudSyncGate(settings, plan).provider === 'readest';
diff --git a/apps/readest-app/src/services/transferManager.ts b/apps/readest-app/src/services/transferManager.ts
index b2e77a94..d428af44 100644
--- a/apps/readest-app/src/services/transferManager.ts
+++ b/apps/readest-app/src/services/transferManager.ts
@@ -1,6 +1,8 @@
 import { Book } from '@/types/book';
 import { AppService, BaseDir } from '@/types/system';
 import { useTransferStore, TransferItem, ReplicaTransferFile } from '@/store/transferStore';
+import { useSettingsStore } from '@/store/settingsStore';
+import { isReadestCloudStorageActive } from '@/services/sync/cloudSyncProvider';
 import { TranslationFunc } from '@/hooks/useTranslation';
 import { ProgressHandler, ProgressPayload } from '@/utils/transfer';
 import { eventDispatcher } from '@/utils/event';
@@ -8,12 +10,18 @@ import { getTransferMessages } from './transferMessages';
 
 const TRANSFER_QUEUE_KEY = 'readest_transfer_queue';
 const RETRY_DELAY_BASE_MS = 2000;
+// Quota failures in a batch import arrive one per book as transfers drain;
+// collapse them into one summary toast per burst instead of N identical toasts.
+const QUOTA_TOAST_FLUSH_MS = 1500;
 
 interface PersistedQueueData {
+  schemaVersion?: number;
   transfers: Record<string, TransferItem>;
   isQueuePaused: boolean;
 }
 
+const QUEUE_SCHEMA_VERSION = 1;
+
 class TransferManager {
   private static instance: TransferManager;
   private appService: AppService | null = null;
@@ -23,6 +31,9 @@ class TransferManager {
   private getLibrary: (() => Book[]) | null = null;
   private updateBook: ((book: Book) => Promise<void>) | null = null;
   private _: TranslationFunc | null = null;
+  private settingsUnsub: (() => void) | null = null;
+  private quotaFailureCount = 0;
+  private quotaToastTimer: ReturnType<typeof setTimeout> | null = null;
   private readyResolve: () => void = () => {};
   private readyPromise: Promise<void> = new Promise<void>((resolve) => {
     this.readyResolve = resolve;
@@ -30,6 +41,52 @@ class TransferManager {
 
   private constructor() {}
 
+  /**
+   * Settings hydrate asynchronously at app start (`settings` begins as
+   * `{}`); `settings.version` truthiness is the loaded signal (same
+   * convention as useSync). Before hydration the provider is unknown, so
+   * book uploads are deferred rather than judged — acting on unknown
+   * settings could both mis-cancel and mis-allow. Replica transfers and
+   * downloads are never deferred: they are not provider-gated and the
+   * boot-time replica pull relies on waitUntilReady().
+   */
+  private isSettingsLoaded(): boolean {
+    return !!useSettingsStore.getState().settings?.version;
+  }
+
+  private isBookUploadAllowed(): boolean {
+    return isReadestCloudStorageActive(useSettingsStore.getState().settings);
+  }
+
+  private isDeferredBookUpload(t: TransferItem): boolean {
+    return t.kind === 'book' && t.type === 'upload' && !this.isSettingsLoaded();
+  }
+
+  /**
+   * Cancel pending book uploads when Readest Cloud is not the selected
+   * provider. Idempotent (acts on pending rows only) — safe to run on
+   * every processQueue pass, which also re-settles rows another window
+   * or a rogue retry re-pended. Cancellation is visible ('cancelled'
+   * with cancelReason 'policy'), never a silent drop.
+   */
+  private reconcileUploadsWithProvider(): void {
+    if (!this.isSettingsLoaded() || this.isBookUploadAllowed()) return;
+
+    const store = useTransferStore.getState();
+    const gated = store
+      .getPendingTransfers()
+      .filter((t) => t.kind === 'book' && t.type === 'upload');
+    if (gated.length === 0) return;
+
+    gated.forEach((t) => {
+      store.setTransferStatus(t.id, 'cancelled', undefined, 'policy');
+    });
+    console.info(
+      `[cloudSync] cancelled ${gated.length} pending Readest Cloud upload(s): third-party provider selected`,
+    );
+    this.persistQueue();
+  }
+
   static getInstance(): TransferManager {
     if (!TransferManager.instance) {
       TransferManager.instance = new TransferManager();
@@ -50,9 +107,18 @@ class TransferManager {
     this.updateBook = updateBook;
     this._ = translationFn;
     await this.loadPersistedQueue();
+    this.reconcileUploadsWithProvider();
     this.isInitialized = true;
     this.readyResolve();
 
+    // Re-gate when settings hydrate or the selected provider changes.
+    this.settingsUnsub?.();
+    this.settingsUnsub = useSettingsStore.subscribe((state, prev) => {
+      if (state.settings === prev.settings) return;
+      this.reconcileUploadsWithProvider();
+      this.processQueue();
+    });
+
     // Start processing queue
     this.processQueue();
   }
@@ -77,6 +143,13 @@ class TransferManager {
       return null;
     }
 
+    // Readest Cloud storage is not written to while a third-party
+    // provider is selected. Before settings hydrate the entry is queued
+    // and deferred; the reconcile on hydration decides its fate.
+    if (this.isSettingsLoaded() && !this.isBookUploadAllowed()) {
+      return null;
+    }
+
     const store = useTransferStore.getState();
 
     // Check if already queued or in progress
@@ -222,7 +295,7 @@ class TransferManager {
       this.abortControllers.delete(transferId);
     }
 
-    useTransferStore.getState().setTransferStatus(transferId, 'cancelled');
+    useTransferStore.getState().setTransferStatus(transferId, 'cancelled', undefined, 'user');
     this.persistQueue();
   }
 
@@ -287,11 +360,13 @@ class TransferManager {
   }
 
   private async _processQueueInternal(): Promise<void> {
+    this.reconcileUploadsWithProvider();
+
     const store = useTransferStore.getState();
 
     if (store.isQueuePaused) return;
 
-    const pending = store.getPendingTransfers();
+    const pending = store.getPendingTransfers().filter((t) => !this.isDeferredBookUpload(t));
     const activeCount = store.getActiveTransfers().length;
     const maxConcurrent = store.maxConcurrent;
 
@@ -308,9 +383,12 @@ class TransferManager {
 
     await Promise.all(toProcess.map((transfer) => this.executeTransfer(transfer)));
 
-    // Check if more items to process
+    // Check if more items to process. Deferred book uploads (settings
+    // not yet hydrated) don't count — re-looping on them every 100ms
+    // would busy-wait; the settings subscription wakes them instead.
     const newStore = useTransferStore.getState();
-    if (newStore.getPendingTransfers().length > 0 && !newStore.isQueuePaused) {
+    const processable = newStore.getPendingTransfers().filter((t) => !this.isDeferredBookUpload(t));
+    if (processable.length > 0 && !newStore.isQueuePaused) {
       setTimeout(() => this.processQueue(), 100);
     }
   }
@@ -373,7 +451,15 @@ class TransferManager {
       const currentStore = useTransferStore.getState();
       const currentTransfer = currentStore.transfers[transfer.id];
 
-      if (currentTransfer && currentTransfer.retryCount < currentTransfer.maxRetries) {
+      // Quota exhaustion is permanent for this account state; retrying
+      // burns three backoff rounds per book for the same 403.
+      const isQuotaError = errorMessage.includes('Insufficient storage quota');
+
+      if (
+        !isQuotaError &&
+        currentTransfer &&
+        currentTransfer.retryCount < currentTransfer.maxRetries
+      ) {
         // Schedule retry with exponential backoff
         const delay = RETRY_DELAY_BASE_MS * Math.pow(2, currentTransfer.retryCount);
         currentStore.incrementRetryCount(transfer.id);
@@ -392,11 +478,8 @@ class TransferManager {
             type: 'error',
             message: _('Please log in to continue'),
           });
-        } else if (errorMessage.includes('Insufficient storage quota')) {
-          eventDispatcher.dispatch('toast', {
-            type: 'error',
-            message: _('Insufficient storage quota'),
-          });
+        } else if (isQuotaError) {
+          this.recordQuotaFailure();
         } else {
           const errorMessages = getTransferMessages(transfer, _).failure;
 
@@ -420,6 +503,28 @@ class TransferManager {
     }
   }
 
+  private recordQuotaFailure(): void {
+    this.quotaFailureCount += 1;
+    if (this.quotaToastTimer) clearTimeout(this.quotaToastTimer);
+    this.quotaToastTimer = setTimeout(() => this.flushQuotaToast(), QUOTA_TOAST_FLUSH_MS);
+  }
+
+  private flushQuotaToast(): void {
+    const _ = this._;
+    const count = this.quotaFailureCount;
+    this.quotaFailureCount = 0;
+    this.quotaToastTimer = null;
+    if (!count || !_) return;
+
+    eventDispatcher.dispatch('toast', {
+      type: 'error',
+      message:
+        count === 1
+          ? _('Insufficient storage quota')
+          : _('{{count}} uploads failed: insufficient storage quota', { count }),
+    });
+  }
+
   private async executeBookTransfer(
     transfer: TransferItem,
     progressHandler: (p: ProgressPayload) => void,
@@ -561,6 +666,7 @@ class TransferManager {
 
       // Persist all transfers including completed (for history)
       const data: PersistedQueueData = {
+        schemaVersion: QUEUE_SCHEMA_VERSION,
         transfers: store.transfers,
         isQueuePaused: store.isQueuePaused,
       };
diff --git a/apps/readest-app/src/services/webAppService.ts b/apps/readest-app/src/services/webAppService.ts
index b18aa06a..6a10213f 100644
--- a/apps/readest-app/src/services/webAppService.ts
+++ b/apps/readest-app/src/services/webAppService.ts
@@ -302,7 +302,7 @@ export class WebAppService extends BaseAppService {
       const settings = await this.loadSettings();
       const lastMigrationVersion = settings.migrationVersion || 0;
 
-      await super.runMigrations(lastMigrationVersion);
+      await super.runMigrations(lastMigrationVersion, settings);
 
       if (lastMigrationVersion < this.CURRENT_MIGRATION_VERSION) {
         await this.saveSettings({
diff --git a/apps/readest-app/src/store/transferStore.ts b/apps/readest-app/src/store/transferStore.ts
index 3496d2be..0396c789 100644
--- a/apps/readest-app/src/store/transferStore.ts
+++ b/apps/readest-app/src/store/transferStore.ts
@@ -4,6 +4,15 @@ import type { BaseDir } from '@/types/system';
 export type TransferType = 'upload' | 'download' | 'delete';
 export type TransferStatus = 'pending' | 'in_progress' | 'completed' | 'failed' | 'cancelled';
 export type TransferKind = 'book' | 'replica';
+/**
+ * Why a transfer was cancelled. 'user' = an explicit cancel action;
+ * 'policy' = the app cancelled it because Readest Cloud is not the
+ * selected sync provider. Policy cancellations are not failures: they
+ * are excluded from the failed bucket, Retry All, and per-item retry
+ * (retrying would either no-op against the provider gate or loop
+ * cancel-retry-cancel), and they are pruned on the next restore.
+ */
+export type TransferCancelReason = 'user' | 'policy';
 
 export interface ReplicaTransferFile {
   logical: string;
@@ -28,6 +37,7 @@ export interface TransferItem {
   transferredBytes: number;
   transferSpeed: number; // bytes per second
   error?: string;
+  cancelReason?: TransferCancelReason;
   retryCount: number;
   maxRetries: number;
   createdAt: number;
@@ -76,7 +86,12 @@ interface TransferState {
     total: number,
     speed: number,
   ) => void;
-  setTransferStatus: (transferId: string, status: TransferStatus, error?: string) => void;
+  setTransferStatus: (
+    transferId: string,
+    status: TransferStatus,
+    error?: string,
+    cancelReason?: TransferCancelReason,
+  ) => void;
   retryTransfer: (transferId: string) => void;
   incrementRetryCount: (transferId: string) => void;
 
@@ -118,6 +133,16 @@ const generateTransferId = (): string => {
   return `${Date.now()}-${Math.random().toString(36).substring(2, 9)}`;
 };
 
+/**
+ * The single failed-bucket predicate. Every surface that shows, counts,
+ * or retries "failed" transfers (store getters, useTransferQueue stats,
+ * TransferQueuePanel filter/retry) must use this instead of matching
+ * statuses inline, so policy cancellations stay out of the bucket
+ * everywhere at once.
+ */
+export const isFailedLikeTransfer = (t: TransferItem): boolean =>
+  t.status === 'failed' || (t.status === 'cancelled' && t.cancelReason !== 'policy');
+
 export const useTransferStore = create<TransferState>((set, get) => ({
   transfers: {},
   isQueuePaused: false,
@@ -225,12 +250,15 @@ export const useTransferStore = create<TransferState>((set, get) => ({
     });
   },
 
-  setTransferStatus: (transferId, status, error) => {
+  setTransferStatus: (transferId, status, error, cancelReason) => {
     set((state) => {
       const transfer = state.transfers[transferId];
       if (!transfer) return state;
 
       const updates: Partial<TransferItem> = { status, error };
+      if (status === 'cancelled') {
+        updates.cancelReason = cancelReason ?? transfer.cancelReason ?? 'user';
+      }
 
       if (status === 'in_progress' && !transfer.startedAt) {
         updates.startedAt = Date.now();
@@ -253,6 +281,9 @@ export const useTransferStore = create<TransferState>((set, get) => ({
     set((state) => {
       const transfer = state.transfers[transferId];
       if (!transfer) return state;
+      // Policy cancellations are not retryable: the provider gate would
+      // re-cancel the row immediately (cancel-retry-cancel loop).
+      if (transfer.status === 'cancelled' && transfer.cancelReason === 'policy') return state;
 
       return {
         transfers: {
@@ -264,6 +295,7 @@ export const useTransferStore = create<TransferState>((set, get) => ({
             transferredBytes: 0,
             transferSpeed: 0,
             error: undefined,
+            cancelReason: undefined,
             startedAt: undefined,
             completedAt: undefined,
           },
@@ -339,9 +371,7 @@ export const useTransferStore = create<TransferState>((set, get) => ({
   },
 
   getFailedTransfers: () => {
-    return Object.values(get().transfers).filter(
-      (t) => t.status === 'failed' || t.status === 'cancelled',
-    );
+    return Object.values(get().transfers).filter(isFailedLikeTransfer);
   },
 
   getCompletedTransfers: () => {
@@ -375,7 +405,7 @@ export const useTransferStore = create<TransferState>((set, get) => ({
       pending: transfers.filter((t) => t.status === 'pending').length,
       active: transfers.filter((t) => t.status === 'in_progress').length,
       completed: transfers.filter((t) => t.status === 'completed').length,
-      failed: transfers.filter((t) => t.status === 'failed' || t.status === 'cancelled').length,
+      failed: transfers.filter(isFailedLikeTransfer).length,
       total: transfers.length,
     };
   },
@@ -386,6 +416,11 @@ export const useTransferStore = create<TransferState>((set, get) => ({
     // Legacy rows persisted before the kind discriminator default to 'book'.
     const restoredTransfers: Record<string, TransferItem> = {};
     Object.entries(transfers).forEach(([id, transfer]) => {
+      // Policy-cancelled rows are session-scoped history: a large
+      // pre-switch queue must not leave hundreds of permanent
+      // "Cancelled" rows in the panel and localStorage. Prune on
+      // restore.
+      if (transfer.status === 'cancelled' && transfer.cancelReason === 'policy') return;
       const withKind: TransferItem = { ...transfer, kind: transfer.kind ?? 'book' };
       if (withKind.status === 'in_progress') {
         restoredTransfers[id] = {