diff --git a/src/tests/setup.ts b/src/tests/setup.ts
index b210af53..7120c2ca 100644
--- a/src/tests/setup.ts
+++ b/src/tests/setup.ts
@@ -1,4 +1,11 @@
 import matchers from '@testing-library/jest-dom/matchers';
-import { expect } from 'vitest';
+import { expect, beforeAll, vi } from 'vitest';
 
 expect.extend(matchers);
+
+// TODO: Remove once https://github.com/sveltejs/kit/issues/6259 is closed.
+beforeAll(() => {
+	vi.mock('$app/env', () => ({
+		browser: 'window' in globalThis
+	}));
+});