Build the API layer before the UI suite

UI setup is the slowest part of a UI test

A checkout test that registers a user, verifies an email, adds three products and applies a coupon through the interface spends ninety percent of its runtime on setup that is already covered elsewhere. Create that state through the API and start the test at the step you actually want to verify.

A minimal seeding helper

Wrap your service calls in typed helpers and expose them as fixtures. Each test gets isolated data, so tests can run in parallel without stepping on each other.

export const test = base.extend<{ user: User }>({
  user: async ({ request }, use) => {
    const res = await request.post('/api/users', { data: newUser() });
    const user = await res.json();
    await use(user);
    await request.delete(`/api/users/${user.id}`);
  },
});

Validate contracts, not just status codes

Assert the schema of every response you depend on. A 200 with a renamed field breaks the UI just as badly as a 500, and schema assertions catch it in seconds.