vitest-v4

Vitest 4 Testing Skill

Write, configure, and debug Vitest 4 test suites with Vite-native patterns.

Before You Start

This skill prevents 7+ common Vitest 4 mistakes and saves ~50% tokens.

Metric	Without Skill	With Skill
Setup Time	~90 min	~30 min
Common Errors	7+	0
Token Usage	High (trial/error)	Low (known patterns)

Known Issues This Skill Prevents

1. Hanging agent runs from using watch mode instead of vitest run
2. Broken coverage configs from using removed coverage.all or coverage.extensions
3. Browser Mode spying failures from sealed ESM namespace objects
4. Mock leakage between tests from missing restore/reset config
5. Invalid multi-project setup from using deprecated workspace terminology
6. Wrong APIs from mixing Jest helpers into Vitest tests
7. Flaky browser interactions from using synthetic helpers instead of vitest/browser
8. Slow or unstable large suites from choosing the wrong execution pool or isolation mode

Quick Start

Step 1: Configure Vitest 4 for agent-safe runs

// vitest.config.ts
import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    environment: 'node',
    restoreMocks: true,
    clearMocks: true,
    coverage: {
      provider: 'v8',
      include: ['src/**/*.{ts,tsx}'],
    },
  },
});

Why this matters: Vitest 4 removed coverage.all and coverage.extensions, and agent/CI environments need one-shot execution plus automatic mock cleanup.

Step 2: Write tests with Vitest APIs, not Jest APIs

import { describe, expect, it, vi } from 'vitest';
import { addUser } from './add-user';
import * as api from './api';

describe('addUser', () => {
  it('returns the created user', async () => {
    vi.spyOn(api, 'createUser').mockResolvedValue({ id: '1', name: 'Ada' });

    await expect(addUser('Ada')).resolves.toEqual({ id: '1', name: 'Ada' });
  });
});

Why this matters: vi is the supported mocking API. Mixing jest.fn() or Jest-only patterns causes confusing failures and poor autocomplete.