MCPInstall

vitest

by bobmatnycTesting & QA

Vitest - Modern TypeScript testing framework with Vite-native performance, ESM support, and TypeScript-first design

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Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/bobmatnyc/claude-mpm-skills --skill vitest
  1. 01

    Install

    Run the command in your project. The CLI asks which agents to install for.

  2. 02

    Describe the task

    Ask your agent for something this skill covers. It loads vitest automatically when relevant.

  3. 03

    Review the result

    The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

Vitest - Modern TypeScript Testing

Overview

Vitest is a next-generation test framework powered by Vite, designed for modern TypeScript/JavaScript projects. It provides blazing-fast test execution through HMR-based test running, native ESM support, and first-class TypeScript integration.

Key Features:

  • ⚡ Vite-native: Instant HMR-based test execution (10-100x faster than Jest)
  • 🎯 TypeScript-first: Built-in TypeScript support, no configuration needed
  • 🔄 ESM-native: Native ES modules, async/await, top-level await
  • 🧪 Jest-compatible: Compatible API for easy migration
  • 📸 Snapshot testing: Built-in snapshot support
  • 🎨 Component testing: React Testing Library, Vue Test Utils integration
  • 📊 Coverage: Built-in v8/c8 coverage (faster than Istanbul)
  • 🌐 UI mode: Beautiful web UI for test debugging

Installation:

npm install -D vitest
# TypeScript types (usually auto-detected)
npm install -D @vitest/ui  # Optional: UI mode

Basic Setup

1. Configure Vitest

vitest.config.ts:

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true,           // Use describe/it/expect globally
    environment: 'node',     // or 'jsdom' for DOM testing
    coverage: {
      provider: 'v8',        // or 'istanbul'
      reporter: ['text', 'json', 'html'],
      exclude: [
        'node_modules/',
        'dist/',
        '**/*.test.ts',
        '**/*.spec.ts',
      ],
    },
    include: ['**/*.{test,spec}.{ts,tsx}'],
    exclude: ['node_modules', 'dist', '.idea', '.git', '.cache'],
  },
});

2. TypeScript Configuration

tsconfig.json:

{
  "compilerOptions": {
    "types": ["vitest/globals"]  // For global describe/it/expect
  }
}

Alternative (without globals):

import { describe, it, expect } from 'vitest';

3. Package.json Scripts

{
  "scripts": {
    "test": "vitest run",              // CI mode (single run)
    "test:watch": "vitest",            // Watch mode (default)
    "test:ui": "vitest --ui",          // UI mode
    "test:coverage": "vitest run --coverage"
  }
}

Core Testing Patterns

Basic Test Structure

import { describe, it, expect, beforeEach, afterEach } from 'vitest';

describe('Calculator', () => {
  let calculator: Calculator;

  beforeEach(() => {
    calculator = new Calculator();
  });

  it('adds two numbers correctly', () => {
    const result = calculator.add(2, 3);
    expect(result).toBe(5);
  });

  it('handles negative numbers', () => {
    expect(calculator.add(-5, 3)).toBe(-2);
  });
});

TypeScript Type Testing

import { describe, it, expectTypeOf, assertType } from 'vitest';

interface User {
  id: number;
  name: string;
  email: string;
}

describe('Type Safety', () => {
  it('ensures correct types', () => {
    const user: User = {
      id: 1,
      name: 'Alice',
      email: 'alice@example.com',
    };

    // Type assertions
    expectTypeOf(user.id).toBeNumber();
    expectTypeOf(user.name).toBeString();
    expectTypeOf(user).toMatchTypeOf<User>();

    // Assert type at compile time
    assertType<User>(user);
  });

  it('checks function return types', () => {
    function getUser(): User {
      return { id: 1, name: 'Bob', email: 'bob@example.com' };
    }

    expectTypeOf(getUser).returns.toMatchTypeOf<User>();
  });
});

Mocking and Spies

vi.mock for Module Mocking

import { describe, it, expect, vi } from 'vitest';
import { fetchUser } from './api';
import { UserService } from './UserService';

// Mock entire module
vi.mock('./api', () => ({
  fetchUser: vi.fn(),
}));

describe('UserService', () => {
  it('fetches user data', async () => {
    const mockUser = { id: 1, name: 'Alice' };
    vi.mocked(fetchUser).mockResolvedValue(mockUser);

    const service = new UserService();
    const user = await service.getUser(1);

    expect(fetchUser).toHaveBeenCalledWith(1);
    expect(user).toEqual(mockUser);
  });
});

vi.spyOn for Method Spying

import { describe, it, expect, vi } from 'vitest';

class Logger {
  log(message: string) {
    console.log(message);
  }
}

describe('Logger Spy', () => {
  it('tracks method calls', () => {
    const logger = new Logger();
    const spy = vi.spyOn(logger, 'log');

    logger.log('Hello');
    logger.log('World');

    expect(spy).toHaveBeenCalledTimes(2);
    expect(spy).toHaveBeenCalledWith('Hello');
    expect(spy).toHaveBeenLastCalledWith('World');

    spy.mockRestore(); // Restore original implementation
  });
});

Mock Implementation

import { describe, it, expect, vi } from 'vitest';

describe('Mock Implementation', () => {
  it('provides custom mock implementation', () => {
    const mockFn = vi.fn((x: number) => x * 2);

    expect(mockFn(5)).toBe(10);
    expect(mockFn).toHaveBeenCalledWith(5);

    // Change implementation
    mockFn.mockImplementation((x: number) => x + 10);
    expect(mockFn(5)).toBe(15);

    // One-time implementation
    mockFn.mockImplementationOnce((x: number) => 100);
    expect(mockFn(5)).toBe(100);
    expect(mockFn(5)).toBe(15); // Back to default
  });
});

Mocking Timers

import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';

describe('Timer Mocking', () => {
  beforeEach(() => {
    vi.useFakeTimers();
  });

  afterEach(() => {
    vi.restoreAllMocks();
  });

  it('fast-forwards time', () => {
    const callback = vi.fn();
    setTimeout(callback, 1000);

    vi.advanceTimersByTime(500);
    expect(callback).not.toHaveBeenCalled();

    vi.advanceTimersByTime(500);
    expect(callback).toHaveBeenCalledTimes(1);
  });

  it('runs all timers', async () => {
    const callback = vi.fn();
    setTimeout(callback, 1000);
    setTimeout(callback, 2000);

    await vi.runAllTimersAsync();
    expect(callback).toHaveBeenCalledTimes(2);
  });
});

React Testing Integration

Setup React Testing Library

npm install -D @testing-library/react @testing-library/jest-dom @testing-library/user-event
npm install -D jsdom  # For DOM environment

vitest.config.ts (React):

import { defineConfig } from 'vitest/config';
import react from '@vitejs/plugin-react';

export default defineConfig({
  plugins: [react()],
  test: {
    globals: true,
    environment: 'jsdom',
    setupFiles: './src/test/setup.ts',
  },
});

src/test/setup.ts:

import '@testing-library/jest-dom';
import { expect, afterEach } from 'vitest';
import { cleanup } from '@testing-library/react';
import * as matchers from '@testing-library/jest-dom/matchers';

expect.extend(matchers);

afterEach(() => {
  cleanup();
});

React Component Testing

import { describe, it, expect } from 'vitest';
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { Counter } from './Counter';

describe('Counter Component', () => {
  it('renders initial count', () => {
    render(<Counter initialCount={0} />);
    expect(screen.getByText('Count: 0')).toBeInTheDocument();
  });

  it('increments counter on button click', async () => {
    const user = userEvent.setup();
    render(<Counter initialCount={0} />);

    const button = screen.getByRole('button', { name: /increment/i });
    await user.click(button);

    expect(screen.getByText('Count: 1')).toBeInTheDocument();
  });

  it('calls onChange callback', async () => {
    const onChange = vi.fn();
    const user = userEvent.setup();

    render(<Counter initialCount={0} onChange={onChange} />);

    await user.click(screen.getByRole('button', { name: /increment/i }));

    expect(onChange).toHaveBeenCalledWith(1);
  });
});

Testing Hooks

import { describe, it, expect } from 'vitest';
import { renderHook, act } from '@testing-library/react';
import { useCounter } from './useCounter';

describe('useCounter Hook', () => {
  it('initializes with default value', () => {
    const { result } = renderHook(() => useCounter(0));
    expect(result.current.count).toBe(0);
  });

  it('increments counter', () => {
    const { result } = renderHook(() => useCounter(0));

    act(() => {
      result.current.increment();
    });

    expect(result.current.count).toBe(1);
  });

  it('resets counter', () => {
    const { result } = renderHook(() => useCounter(10));

    act(() => {
      result.current.reset();
    });

    expect(result.current.count).toBe(10);
  });
});

Vue Testing Integration

Setup Vue Test Utils

npm install -D @vue/test-utils @vitejs/plugin-vue
npm install -D happy-dom  # Faster alternative to jsdom

vitest.config.ts (Vue):

import { defineConfig } from 'vitest/config';
import vue from '@vitejs/plugin-vue';

export default defineConfig({
  plugins: [vue()],
  test: {
    globals: true,
    environment: 'happy-dom',
    setupFiles: './src/test/setup.ts',
  },
});

Vue Component Testing

import { describe, it, expect } from 'vitest';
import { mount } from '@vue/test-utils';
import Counter from './Counter.vue';

describe('Counter.vue', () => {
  it('renders initial count', () => {
    const wrapper = mount(Counter, {
      props: { initialCount: 5 },
    });

    expect(wrapper.text()).toContain('Count: 5');
  });

  it('increments on button click', async () => {
    const wrapper = mount(Counter, {
      props: { initialCount: 0 },
    });

    await wrapper.find('button').trigger('click');

    expect(wrapper.text()).toContain('Count: 1');
  });

  it('emits update event', async () => {
    const wrapper = mount(Counter, {
      props: { initialCount: 0 },
    });

    await wrapper.find('button').trigger('click');

    expect(wrapper.emitted('update')).toBeTruthy();
    expect(wrapper.emitted('update')?.[0]).toEqual([1]);
  });
});

Async Testing

Testing Promises

import { describe, it, expect } from 'vitest';

describe('Async Operations', () => {
  it('resolves promises', async () => {
    const result = await Promise.resolve(42);
    expect(result).toBe(42);
  });

  it('rejects promises', async () => {
    await expect(Promise.reject(new Error('Failed'))).rejects.toThrow('Failed');
  });

  it('uses resolves matcher', async () => {
    await expect(Promise.resolve(42)).resolves.toBe(42);
  });
});

Testing Async Functions

import { describe, it, expect, vi } from 'vitest';

async function fetchData(id: number): Promise<string> {
  const response = await fetch(`/api/data/${id}`);
  return response.json();
}

describe('Async Functions', () => {
  it('fetches data successfully', async () => {
    global.fetch = vi.fn(() =>
      Promise.resolve({
        json: () => Promise.resolve('data'),
      } as Response)
    );

    const data = await fetchData(1);
    expect(data).toBe('data');
    expect(fetch).toHaveBeenCalledWith('/api/data/1');
  });

  it('handles fetch errors', async () => {
    global.fetch = vi.fn(() => Promise.reject(new Error('Network error')));

    await expect(fetchData(1)).rejects.toThrow('Network error');
  });
});

Snapshot Testing

Basic Snapshots

import { describe, it, expect } from 'vitest';
import { render } from '@testing-library/react';
import { UserCard } from './UserCard';

describe('UserCard Snapshots', () => {
  it('matches snapshot', () => {
    const { container } = render(
      <UserCard name="Alice" email="alice@example.com" />
    );

    expect(container.firstChild).toMatchSnapshot();
  });

  it('matches inline snapshot', () => {
    const user = { id: 1, name: 'Bob' };
    expect(user).toMatchInlineSnapshot(`
      {
        "id": 1,
        "name": "Bob",
      }
    `);
  });
});

Snapshot Serializers

import { describe, it, expect } from 'vitest';

expect.addSnapshotSerializer({
  test: (val) => val && typeof val.toISOString === 'function',
  print: (val) => `Date(${(val as Date).toISOString()})`,
});

describe('Custom Serializers', () => {
  it('serializes dates consistently', () => {
    const data = {
      timestamp: new Date('2024-01-01T00:00:00.000Z'),
      user: 'Alice',
    };

    expect(data).toMatchSnapshot();
  });
});

Coverage Configuration

Advanced Coverage Setup

vitest.config.ts:

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    coverage: {
      provider: 'v8',
      reporter: ['text', 'json', 'html', 'lcov'],
      reportsDirectory: './coverage',
      exclude: [
        'node_modules/',
        'dist/',
        '**/*.test.ts',
        '**/*.spec.ts',
        '**/*.config.ts',
        '**/types/',
      ],
      thresholds: {
        lines: 80,
        functions: 80,
        branches: 75,
        statements: 80,
      },
      all: true,  // Include untested files in coverage report
    },
  },
});

Running Coverage

# Generate coverage
npx vitest run --coverage

# Coverage with UI
npx vitest --coverage --ui

# Specific threshold enforcement
npx vitest run --coverage --coverage.lines=90

Migration from Jest

API Compatibility

Vitest provides Jest-compatible API:

// Jest syntax works in Vitest
import { describe, it, expect, jest } from 'vitest';

// Note: Use 'vi' instead of 'jest' for new code
import { describe, it, expect, vi } from 'vitest';

// Both work, but vi is preferred
const mockFn = vi.fn();  // Preferred
const mockFn2 = jest.fn();  // Also works

Migration Checklist

1. Update Dependencies:

npm uninstall jest @types/jest ts-jest
npm install -D vitest @vitest/ui

2. Update package.json:

{
  "scripts": {
    "test": "vitest run",  // Was: jest
    "test:watch": "vitest"  // Was: jest --watch
  }
}

3. Replace jest.config.js with vitest.config.ts:

// Old: jest.config.js
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};

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

export default defineConfig({
  test: {
    globals: true,
    environment: 'node',
  },
});

4. Update Test Files:

// Change imports
- import { jest } from '@jest/globals';
+ import { vi } from 'vitest';

// Update mocks
- jest.fn()
+ vi.fn()

- jest.spyOn()
+ vi.spyOn()

- jest.mock()
+ vi.mock()

Advanced Patterns

Concurrent Testing

import { describe, it, expect } from 'vitest';

describe.concurrent('Parallel Tests', () => {
  it('test 1', async () => {
    await slowOperation();
    expect(true).toBe(true);
  });

  it('test 2', async () => {
    await slowOperation();
    expect(true).toBe(true);
  });

  // Both tests run in parallel
});

Test Context

import { describe, it, expect, beforeEach } from 'vitest';

interface TestContext {
  user: { id: number; name: string };
  api: ApiClient;
}

describe<TestContext>('With Context', () => {
  beforeEach((context) => {
    context.user = { id: 1, name: 'Alice' };
    context.api = new ApiClient();
  });

  it<TestContext>('uses context', ({ user, api }) => {
    expect(user.name).toBe('Alice');
    expect(api).toBeDefined();
  });
});

Custom Matchers

import { expect } from 'vitest';

expect.extend({
  toBeWithinRange(received: number, floor: number, ceiling: number) {
    const pass = received >= floor && received <= ceiling;
    return {
      pass,
      message: () =>
        pass
          ? `expected ${received} not to be within range ${floor} - ${ceiling}`
          : `expected ${received} to be within range ${floor} - ${ceiling}`,
    };
  },
});

// Usage
expect(100).toBeWithinRange(90, 110);

Best Practices

  1. Use globals: true - Simpler imports, Jest-compatible
  2. Prefer vi over jest - Use Vitest-native API for new code
  3. Use v8 coverage - Faster than Istanbul, works with native ESM
  4. Test in isolation - Each test should be independent
  5. Mock external dependencies - Network, file system, timers
  6. Use TypeScript - Full type safety in tests
  7. Run tests in CI mode - Use vitest run for CI, not watch mode
  8. Leverage UI mode - Debug failing tests visually
  9. Use describe.concurrent - Parallelize independent tests
  10. Keep tests focused - One assertion per test when possible

Common Pitfalls

❌ Not using CI mode in CI/CD:

// WRONG - watch mode hangs in CI
"test": "vitest"

// CORRECT - single run
"test": "vitest run"

✅ Correct approach:

{
  "scripts": {
    "test": "vitest run",       // CI-safe
    "test:watch": "vitest",     // Development
    "test:ui": "vitest --ui"    // Debugging
  }
}

❌ Forgetting to await async tests:

// WRONG - test passes before assertion
it('fetches data', () => {
  fetchData().then(data => {
    expect(data).toBeDefined();  // Never runs!
  });
});

// CORRECT
it('fetches data', async () => {
  const data = await fetchData();
  expect(data).toBeDefined();
});

❌ Not cleaning up mocks:

// WRONG - mocks leak between tests
it('test 1', () => {
  vi.spyOn(console, 'log');
  // No cleanup!
});

// CORRECT
import { afterEach } from 'vitest';

afterEach(() => {
  vi.restoreAllMocks();
});

❌ Using wrong environment:

// WRONG - testing DOM in node environment
test: {
  environment: 'node',  // Can't test React components!
}

// CORRECT
test: {
  environment: 'jsdom',  // For React/Vue components
}

Resources

Related Skills

When using Vitest, consider these complementary skills:

  • typescript-core: Advanced TypeScript type patterns, tsconfig, and runtime validation
  • react: React component testing with Testing Library integration
  • test-driven-development: Complete TDD workflow (RED/GREEN/REFACTOR cycle)

Quick TypeScript Type Patterns (Inlined for Standalone Use)

// Type-safe test factories with generics
function createMockData<T extends Record<string, unknown>>(
  defaults: T,
  overrides?: Partial<T>
): T {
  return { ...defaults, ...overrides };
}

const mockUser = createMockData(
  { id: 1, name: 'Test', email: 'test@example.com' },
  { name: 'Alice' }
);

// Runtime validation with Zod in tests
import { z } from 'zod';

const UserSchema = z.object({
  id: z.number(),
  name: z.string(),
  email: z.string().email(),
});

test('API returns valid user', async () => {
  const response = await fetch('/api/user/1');
  const data = await response.json();

  // Runtime validation + type inference
  const user = UserSchema.parse(data);
  expect(user.email).toContain('@');
});

// Const type parameters for literal inference
const createTestConfig = <const T extends Record<string, unknown>>(config: T): T => config;
const testEnv = createTestConfig({ mode: 'test', debug: false });
// Type: { mode: "test"; debug: false } (literals preserved)

Quick React Testing Patterns (Inlined for Standalone Use)

// React Testing Library with Vitest
import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import { userEvent } from '@testing-library/user-event';
import { describe, test, expect, vi } from 'vitest';

// Component testing
describe('UserProfile', () => {
  test('renders user information', () => {
    const user = { id: 1, name: 'Alice', email: 'alice@example.com' };
    render(<UserProfile user={user} />);

    expect(screen.getByText('Alice')).toBeInTheDocument();
    expect(screen.getByText('alice@example.com')).toBeInTheDocument();
  });

  test('handles form submission', async () => {
    const onSubmit = vi.fn();
    render(<UserForm onSubmit={onSubmit} />);

    const user = userEvent.setup();
    await user.type(screen.getByLabelText('Name'), 'Bob');
    await user.click(screen.getByRole('button', { name: 'Submit' }));

    await waitFor(() => {
      expect(onSubmit).toHaveBeenCalledWith({ name: 'Bob' });
    });
  });
});

// Hook testing
import { renderHook, act } from '@testing-library/react';

test('useCounter hook increments', () => {
  const { result } = renderHook(() => useCounter(0));

  expect(result.current.count).toBe(0);

  act(() => {
    result.current.increment();
  });

  expect(result.current.count).toBe(1);
});

Quick TDD Workflow Reference (Inlined for Standalone Use)

RED → GREEN → REFACTOR Cycle:

  1. RED Phase: Write Failing Test

    test('should authenticate user with valid credentials', () => {
  const user = { username: 'alice', password: 'secret123' };
  const result = authenticate(user);
  expect(result.isAuthenticated).toBe(true);
  // This fails because authenticate() doesn't exist yet
});

  2. GREEN Phase: Make It Pass

    function authenticate(user: User): AuthResult {
  // Minimum code to pass the test
  if (user.username === 'alice' && user.password === 'secret123') {
    return { isAuthenticated: true };
  }
  return { isAuthenticated: false };
}

  3. REFACTOR Phase: Improve Code

    function authenticate(user: User): AuthResult {
  // Clean up while keeping tests green
  const hashed = hashPassword(user.password);
  const storedUser = database.getUser(user.username);
  return {
    isAuthenticated: storedUser?.passwordHash === hashed
  };
}

Test Structure: Arrange-Act-Assert (AAA)

test('creates user successfully', async () => {
  // Arrange: Set up test data
  const userData = { username: 'alice', email: 'alice@example.com' };

  // Act: Perform the action
  const user = await createUser(userData);

  // Assert: Verify outcome
  expect(user.username).toBe('alice');
  expect(user.email).toBe('alice@example.com');
});

Vitest-Specific TDD Features:

// Watch mode with HMR (instant feedback)
// vitest --watch

// UI mode for visual debugging
// vitest --ui

// Run only changed tests
// vitest --changed

// Benchmark mode for performance testing
import { bench } from 'vitest';

bench('authenticate performance', () => {
  authenticate({ username: 'alice', password: 'secret' });
});

[Full TypeScript, React, and TDD workflows available in respective skills if deployed together]

Summary

  • Vitest is the modern standard for TypeScript testing
  • 10-100x faster than Jest through Vite-native HMR
  • ESM-first with native module support
  • Jest-compatible API for easy migration
  • TypeScript-first with built-in type support
  • Component testing for React and Vue
  • v8 coverage faster than Istanbul
  • UI mode for visual test debugging
  • Perfect for: Modern TypeScript projects, Vite-based apps, React/Vue components

FAQ

What does the vitest skill do?

Vitest - Modern TypeScript testing framework with Vite-native performance, ESM support, and TypeScript-first design

How do I install vitest?

Run npx skills add https://github.com/bobmatnyc/claude-mpm-skills --skill vitest in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/vitest/ (personal) or .claude/skills/vitest/ (project).

Which AI agents can use vitest?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is vitest safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains vitest?

It is published by bobmatnyc in bobmatnyc/claude-mpm-skills. It has 932 recorded installs.

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Tailwind CSS utility-first framework for rapid UI development with responsive design and dark mode

Frontend & Design1.2K installs

Popular testing & qa skills

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tdd

mattpocock/skills

Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests.

Testing & QA1.2M installs

grilling

mattpocock/skills

Grill the user relentlessly about a plan, decision, or idea. Use when the user wants to stress-test their thinking, or uses any 'grill' trigger phrases.

Testing & QA996.2K installs

migrate-to-shoehorn

mattpocock/skills

Migrate test files from as type assertions to @total-typescript/shoehorn. Use when user mentions shoehorn, wants to replace as in tests, or needs partial test data.

Testing & QA484.1K installs

run-train

lllllllama/rigorpilot-skills

Rigor Train skill for deep learning research repositories. Use when a documented or selected training command should be run conservatively for startup verification, short-run verification, full kickoff, or resume, with command, config, seed, log, checkpoint, status, and metric evidence written to standardized trainoutputs/. Do not use for environment setup, exploratory sweeps, speculative idea implementation, or end-to-end orchestration.

Testing & QA337.8K installs

test-driven-development

obra/superpowers

Use when implementing any feature or bugfix, before writing implementation code

Testing & QA261.8K installs

finishing-a-development-branch

obra/superpowers

Use when implementation is complete, all tests pass, and you need to decide how to integrate the work

Testing & QA216.8K installs