diff --git a/web/src/test-utils/appModuleMocks.test.ts b/web/src/test-utils/appModuleMocks.test.ts
new file mode 100644
index 00000000..1359f45c
--- /dev/null
+++ b/web/src/test-utils/appModuleMocks.test.ts
@@ -0,0 +1,16 @@
+import { describe, expect, test, vi } from 'vitest';
+import { goto } from '$app/navigation';
+import { page } from '$app/state';
+
+// Guards the suite-wide $app mocks in vitest.setup.ts (#3943). If they are
+// removed, tests that never mock $app themselves go back to loading
+// SvelteKit's client runtime, which fails at module load on some CI runs.
+describe('suite-wide SvelteKit app-module mocks', () => {
+  test('$app/navigation is the mock, not the client runtime', () => {
+    expect(vi.isMockFunction(goto)).toBe(true);
+  });
+
+  test('$app/state gives a page at the root URL', () => {
+    expect(page.url.pathname).toBe('/');
+  });
+});
diff --git a/web/vitest.setup.ts b/web/vitest.setup.ts
index 8f28a4de..450f1133 100644
--- a/web/vitest.setup.ts
+++ b/web/vitest.setup.ts
@@ -15,6 +15,51 @@ vi.mock('@tanstack/svelte-query', async (orig) => {
   return { ...actual, useQueryClient: () => ({ invalidateQueries: vi.fn() }) };
 });
 
+// Default SvelteKit app-module mocks for the whole suite (#3943). Without
+// them, any page or component that reaches $app/navigation or $app/state —
+// the home page does, through AlbumCard → AlbumMenu — loads SvelteKit's
+// client runtime, whose $app/paths reads __SVELTEKIT_PAYLOAD__ at module
+// load. That global only exists when the kit plugin's define reaches the
+// module, which it does not reliably under vitest: page.test.ts failed to
+// load on one CI run and passed on the re-run of the same commit (#374
+// was the same class). Stubbing the modules keeps every test off the
+// client bootstrap. Tests that assert on goto or need a particular URL
+// still mock per file (pageUrlModule in test-utils/mocks/appState.ts),
+// and per-file mocks win over these.
+vi.mock('$app/navigation', () => ({
+  goto: vi.fn(() => Promise.resolve()),
+  invalidate: vi.fn(() => Promise.resolve()),
+  invalidateAll: vi.fn(() => Promise.resolve()),
+  preloadData: vi.fn(() => Promise.resolve()),
+  preloadCode: vi.fn(() => Promise.resolve()),
+  beforeNavigate: vi.fn(),
+  afterNavigate: vi.fn(),
+  onNavigate: vi.fn(),
+  pushState: vi.fn(),
+  replaceState: vi.fn(),
+  disableScrollHandling: vi.fn()
+}));
+vi.mock('$app/state', () => ({
+  page: {
+    url: new URL('http://localhost/'),
+    params: {},
+    route: { id: null },
+    status: 200,
+    error: null,
+    data: {},
+    form: null,
+    state: {}
+  },
+  navigating: null,
+  updated: { current: false, check: () => Promise.resolve(false) }
+}));
+vi.mock('$app/paths', () => ({
+  base: '',
+  assets: '',
+  resolve: (path: string) => path,
+  asset: (path: string) => path
+}));
+
 // Node 25 ships a partial localStorage global that lacks getItem/setItem/clear
 // unless launched with --localstorage-file. jsdom sees Node's global and skips
 // installing its own. Replace with an in-memory Storage-compatible polyfill so