Unit Test AI Guide — Zero Hallucination, Cross-Stack Standard

Focus: Unit Tests ONLY — no integration, no E2E

Stacks: Node.js (NestJS/Express) · React.js · Python · Angular · Laravel

Goal: AI generates unit tests consistently, deterministically, without hallucination

IDE: Cursor (Primary) + Claude (Secondary)

---

Part 1 — Best Single Library Per Stack (Final Decision)

Do not mix libraries. Pick one per stack, configure it fully, never deviate.

Stack	Library	Why This One
Node.js / NestJS / Express	Jest	Native DI mocking, @nestjs/testing built around it, widest ecosystem
React.js	Vitest + @testing-library/react	Native Vite/ESM support, Jest-compatible API, 3–10x faster
Python	pytest	De facto standard, fixture system eliminates boilerplate, best plugin ecosystem
Angular	Jest (replace Karma)	Karma is deprecated in Angular 17+; Jest is the official migration target
Laravel	Pest	Modern syntax, built on PHPUnit, higher signal-to-noise ratio

Rule: If someone suggests a second library for the same stack, reject it. One library per stack, configured once, followed always.

---

Part 2 — IDE: Cursor (Only Choice for This Goal)

Why Cursor and Not VS Code / WebStorm

Capability	Cursor	VS Code + Copilot	WebStorm
Project-level AI rules	✅ .cursor/rules/	❌	❌
Codebase-aware context	✅ @codebase	Partial	Partial
Run terminal + read output	✅ Composer	❌	❌
Multi-file generation	✅ Agent mode	Limited	❌
Custom instructions per filetype	✅	❌	❌
MCP server integration	✅	❌	❌

Cursor's .cursor/rules/ system is the only IDE-native mechanism that injects persistent, project-scoped instructions into every AI interaction — this is what prevents hallucination at the source.

Cursor Setup for This Project

project-root/
├── .cursor/
│   └── rules/
│       ├── unit-test-global.mdc       ← applies to all files
│       ├── unit-test-nestjs.mdc       ← applies to *.service.ts, *.guard.ts
│       ├── unit-test-react.mdc        ← applies to *.tsx, *.component.tsx
│       ├── unit-test-python.mdc       ← applies to *.py
│       ├── unit-test-angular.mdc      ← applies to *.component.ts, *.service.ts
│       └── unit-test-laravel.mdc      ← applies to *Service.php, *Model.php
├── CLAUDE.md                          ← Claude project memory file
└── ...

---

Part 3 — Cursor Rules Files (Anti-Hallucination Core)

These files are injected into every AI prompt automatically when working on matching files.

3.1 Global Unit Test Rule

File: .cursor/rules/unit-test-global.mdc

---
description: Global unit test rules — applies to all files in this project
globs: ["**/*.spec.ts", "**/*.test.ts", "**/*.spec.tsx", "**/*_test.py", "**/Test*.php"]
alwaysApply: true
---

# Unit Test Contract — MUST FOLLOW, NO EXCEPTIONS

## What is a unit test here
- Tests ONE class or function in complete isolation
- ALL external dependencies are mocked — no real DB, no real HTTP, no real file system
- Each test runs independently — no shared mutable state between tests
- Runs in < 100ms

## Structure: AAA — Always, Without Exception
Every test MUST have these three sections, with comments:

// Arrange — set up inputs, mocks, expected values
// Act — call the single function/method under test
// Assert — verify exactly one outcome

## Naming Convention — Mandatory
- File: `[module-name].spec.ts` or `[module-name].unit.spec.ts`
- Describe block: exact class or function name
- It/test block: "should [expected behavior] when [specific condition]"
  GOOD: "should throw NotFoundException when user does not exist"
  BAD:  "user not found", "test 1", "works correctly"

## What to test per function — minimum coverage
1. Happy path — valid input → expected output
2. Null/undefined input — how does it fail safely
3. Empty input — empty string, empty array, zero
4. Error path — when dependency throws, what happens
5. Boundary — max length, negative numbers, boolean edge

## What NOT to include in unit tests
- Database queries (mock the repository)
- HTTP calls (mock the service/axios/fetch)
- File system operations (mock fs)
- Timer behavior (use jest.useFakeTimers)
- Random values (mock Math.random or faker with fixed seed)

## AI Instruction
When asked to generate a unit test:
1. READ the function signature and types first
2. IDENTIFY all dependencies (constructor args, imported modules)
3. MOCK every dependency — never use real implementations
4. Generate minimum 4 test cases per method (happy, null, error, edge)
5. NEVER import from test doubles or assume what isn't in the source file
6. If you are unsure of a type — ASK, do not assume

---

3.2 NestJS Rule

File: .cursor/rules/unit-test-nestjs.mdc

---
description: NestJS unit test rules
globs: ["src/**/*.service.ts", "src/**/*.guard.ts", "src/**/*.interceptor.ts", "src/**/*.pipe.ts"]
---

# NestJS Unit Test Rules

## Library: Jest + @nestjs/testing + jest-mock-extended

## Always use Test.createTestingModule

typescript
const module = await Test.createTestingModule({
providers: [
SubjectService,
{ provide: DependencyService, useValue: mockDependency }
]
}).compile();

## Mock Pattern — jest-mock-extended

typescript
import { createMock } from '@golevelup/ts-jest';
// OR
import { mock } from 'jest-mock-extended';

const mockUserRepo = mock();

## Never
- Never use `new SubjectService()` directly — always use TestingModule
- Never let `useValue` contain real implementations
- Never test more than one service per describe block

## Repository Mock Template

typescript
const mockRepo = {
findOne: jest.fn(),
save: jest.fn(),
delete: jest.fn(),
findAll: jest.fn(),
update: jest.fn(),
};

## Required imports for every NestJS spec file

typescript
import { Test, TestingModule } from '@nestjs/testing';
import { NotFoundException, BadRequestException } from '@nestjs/common';

markdown

---

3.3 React Rule

File: .cursor/rules/unit-test-react.mdc

---
description: React unit test rules
globs: ["src/**/*.tsx", "src/**/*.component.tsx", "src/hooks/**/*.ts"]
---

# React Unit Test Rules

## Library: Vitest + @testing-library/react + @testing-library/user-event

## Required setup in vitest.config.ts

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

## setup.ts

typescript
import '@testing-library/jest-dom';

## Component test structure

typescript
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { vi } from 'vitest';

## Mocking rules for React
- Mock ALL hooks that call APIs: `vi.mock('../hooks/useUsers')`
- Mock ALL context providers — wrap with test-specific providers
- Mock router: use `MemoryRouter` from react-router-dom
- NEVER mock internal state (useState) — test behavior, not implementation

## Query priority (RTL best practice — mandatory)
1. getByRole — prefer always
2. getByLabelText — for forms
3. getByText — for content
4. getByTestId — LAST RESORT only, requires data-testid attribute

## User event — always use userEvent, never fireEvent

typescript
const user = userEvent.setup();
await user.click(button);
await user.type(input, 'value');

## Custom hook testing

typescript
import { renderHook, act } from '@testing-library/react';
const { result } = renderHook(() => useMyHook());
act(() => result.current.doSomething());
expect(result.current.value).toBe('expected');

markdown

---

3.4 Python Rule

File: .cursor/rules/unit-test-python.mdc

---
description: Python unit test rules
globs: ["**/*.py", "!**/*migrations*"]
---

# Python Unit Test Rules

## Library: pytest + pytest-mock + factory-boy

## File naming
- Source: `app/services/user_service.py`
- Test:   `tests/unit/test_user_service.py`
- ALWAYS mirror the source directory structure under tests/unit/

## Class under test — always use dependency injection

python

GOOD — injectable dependency

class UserService:
def init(self, repo: UserRepository):
self.repo = repo

BAD — untestable

class UserService:
def init(self):
self.repo = UserRepository() # can't mock this

## Mock pattern — pytest-mock (mocker fixture)

python
def test_raises_when_user_not_found(mocker):
mock_repo = mocker.Mock()
mock_repo.find_by_id.return_value = None
service = UserService(mock_repo)
with pytest.raises(UserNotFoundException):
service.get_user("missing-id")

## Async tests — pytest-asyncio

python
import pytest

@pytest.mark.asyncio
async def test_async_service(mocker):
mock_repo = mocker.AsyncMock()
...

## Fixture pattern — for shared setup

python
@pytest.fixture
def user_service(mocker):
repo = mocker.Mock()
return UserService(repo), repo

def test_get_user_happy_path(user_service):
service, repo = user_service
repo.find_by_id.return_value = User(id="1", email="[e-mail]")
result = service.get_user("1")
assert result.email == "[e-mail]"

## Naming
- File: `test_[module_name].py`
- Function: `test_[expected_behavior]_when_[condition]`

shell

---

3.5 Angular Rule

File: .cursor/rules/unit-test-angular.mdc

---
description: Angular unit test rules
globs: ["src/app/**/*.component.ts", "src/app/**/*.service.ts", "src/app/**/*.pipe.ts", "src/app/**/*.guard.ts"]
---

# Angular Unit Test Rules

## Library: Jest (NOT Karma — Karma is deprecated Angular 17+)

## Migration from Karma to Jest (one-time)

bash
ng add @angular-builders/jest

## TestBed configuration — always minimal

typescript
await TestBed.configureTestingModule({
imports: [ComponentUnderTest], // standalone components
providers: [
{ provide: UserService, useValue: mockUserService }
]
}).compileComponents();

## Service mock pattern

typescript
const mockUserService = {
getUser: jest.fn(),
createUser: jest.fn(),
};

## Component test — check DOM behavior, not internal state

typescript
fixture.detectChanges(); // trigger ngOnInit
const button = fixture.debugElement.query(By.css('[data-testid="submit"]'));
button.nativeElement.click();
fixture.detectChanges();
expect(fixture.debugElement.query(By.css('.error-msg'))).toBeTruthy();

## Pipe test — pure function, no TestBed needed

typescript
it('should transform date correctly', () => {
const pipe = new DateFormatPipe();
expect(pipe.transform(new Date('2024-01-01'))).toBe('Jan 1, 2024');
});

## Guard/resolver test — inject and call directly

typescript
const guard = TestBed.inject(AuthGuard);
const result = await guard.canActivate(mockRoute, mockState);
expect(result).toBe(false);

php

---

3.6 Laravel Rule

File: .cursor/rules/unit-test-laravel.mdc

---
description: Laravel unit test rules
globs: ["app/Services/**/*.php", "app/Models/**/*.php", "app/Http/Requests/**/*.php", "app/Actions/**/*.php"]
---

# Laravel Unit Test Rules

## Library: Pest (NOT PHPUnit directly — Pest wraps it with better syntax)

## File location
- Source: `app/Services/UserService.php`
- Test:   `tests/Unit/Services/UserServiceTest.php`

## Class under test — use constructor injection

php
class UserService {
public function __construct(
private readonly UserRepositoryInterface $repo
) {}
}

## Mock pattern — Mockery (included with Pest/PHPUnit)

php
it('throws exception when user not found', function () {
// Arrange
$repo = Mockery::mock(UserRepositoryInterface::class);
$repo->shouldReceive('findById')->once()->andReturn(null);
$service = new UserService($repo);

// Act & Assert
expect(fn () => $service->getUser('abc'))
    ->toThrow(UserNotFoundException::class);

});

## Data provider pattern (Pest datasets)

php
it('validates email format', function (string $email, bool $valid) {
expect(validateEmail($email))->toBe($valid);
})->with([
['[e-mail]', true],
['notanemail', false],
['', false],
['@nodomain.com', false],
]);

## What NOT to do in unit tests
- Never call $this->get() or $this->post() — that is feature testing
- Never use RefreshDatabase — that is feature/integration testing
- Never resolve from service container — inject directly

markdown

---

Part 4 — CLAUDE.md (Claude Project Memory)

Place this at project root. Claude reads it on every session automatically.

File: CLAUDE.md

# Project: [Your Project Name]
# Claude Unit Test Instructions

## Stack
- Backend: NestJS + TypeScript
- Frontend: React + Vite + TypeScript
- Testing: Jest (NestJS) + Vitest (React) + Pest (Laravel) + pytest (Python)

## Unit Test Rules — NON-NEGOTIABLE

### When asked to generate a unit test:
1. NEVER generate integration or E2E tests unless explicitly asked
2. ALWAYS mock every external dependency
3. ALWAYS follow AAA (Arrange-Act-Assert) with comments
4. ALWAYS generate minimum 4 cases: happy path, null/empty, error thrown, edge case
5. NEVER use `any` type in TypeScript tests
6. NEVER leave `TODO` comments in generated tests

### Naming
- NestJS: `[name].service.spec.ts` inside same directory as source
- React: `[ComponentName].test.tsx` inside `__tests__/` or same directory
- Python: `test_[module].py` under `tests/unit/`
- Laravel: `[Name]Test.php` under `tests/Unit/`

### Test should describe BEHAVIOR, not implementation
GOOD: "should return empty array when no users match search"
BAD: "test getUserByFilter"

### If you are unsure of the correct mock structure:
- Ask what the dependency interface looks like
- Do NOT invent method names that don't exist in the source

### Coverage target: 80% lines, 75% branches per module

## Project Structure Reference
src/
  modules/
    users/
      users.service.ts
      users.service.spec.ts   ← unit test lives here
      users.repository.ts
      users.repository.spec.ts

---

Part 5 — MCP Servers for Unit Testing in Cursor + Claude

MCP (Model Context Protocol) servers extend AI capabilities. For unit testing, these are the relevant ones:

5.1 Filesystem MCP (Built-in Cursor — use directly)

Cursor already has filesystem access. No extra MCP needed to read source files and generate tests. The .cursor/rules/ system handles this.

5.2 Available MCP Servers for Testing Workflows

1. @executeautomation/playwright-mcp-server

• Scope: E2E only — not relevant for unit tests
• Skip for this use case

2. @modelcontextprotocol/server-filesystem

• Gives Claude access to project files
• Use in Claude Desktop to read source → generate tests
• Install: configured in claude_desktop_config.json

3. Custom Testing MCP (Build This — Highest Value)

Build a lightweight MCP server that:

• Reads a source file
• Extracts function signatures + types
• Returns structured JSON to Claude
• Claude generates tests from structure, not guesswork

// mcp-test-generator/src/index.ts
import { Server } from '@modelcontextprotocol/sdk/server/index.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
import * as ts from 'typescript';

const server = new Server({ name: 'test-generator', version: '1.0.0' }, {
  capabilities: { tools: {} }
});

server.setRequestHandler('tools/call', async (req) => {
  if (req.params.name === 'extract_signatures') {
    const { filePath } = req.params.arguments;
    const signatures = extractFunctionSignatures(filePath); // parse AST
    return { content: [{ type: 'text', text: JSON.stringify(signatures) }] };
  }
});

function extractFunctionSignatures(filePath: string) {
  // Use TypeScript compiler API to extract:
  // - class name
  // - method names
  // - parameter types
  // - return types
  // - injected dependencies (constructor params)
  const program = ts.createProgram([filePath], {});
  const sourceFile = program.getSourceFile(filePath);
  // ... AST traversal
  return { className, methods, dependencies };
}

Why this matters: When Claude receives typed signatures instead of raw source code, it cannot hallucinate — it generates tests from concrete types, not guesses.

5.3 Claude Desktop MCP Config

// ~/Library/Application Support/Claude/claude_desktop_config.json (macOS)
// %APPDATA%\Claude\claude_desktop_config.json (Windows)
{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "/path/to/your/project"]
    },
    "test-generator": {
      "command": "node",
      "args": ["/path/to/your/mcp-test-generator/dist/index.js"]
    }
  }
}

5.4 Cursor MCP Config

// .cursor/mcp.json (project-level)
{
  "mcpServers": {
    "test-generator": {
      "command": "node",
      "args": ["./tools/mcp-test-generator/dist/index.js"]
    }
  }
}

---

Part 6 — Anti-Hallucination Prompt Templates

The root cause of AI hallucination in tests: AI invents method names, wrong mock structures, non-existent imports. These prompts eliminate that.

6.1 Cursor Cmd+K Prompt (Use This Exactly)

Generate a unit test file for this [NestJS service / React component / Python class / Angular service / Laravel service].

Rules:
- Library: [Jest / Vitest / pytest / Jest / Pest]
- Test only THIS file — no integration
- Mock ALL dependencies listed in the constructor/props
- Follow AAA pattern with comments
- Minimum 4 test cases per public method
- Use ONLY methods/properties that exist in this source file
- Do NOT import anything not shown in the source
- Naming: "should [behavior] when [condition]"

6.2 Claude Prompt (For CLAUDE.md aware sessions)

Read [filename].
Extract:
1. Class/function name
2. All public methods with parameter types and return types
3. All constructor dependencies (for mocking)

Then generate the unit test file following CLAUDE.md rules.
Do NOT assume any method exists unless you can see it in the source.

6.3 Batch Test Generation Prompt

For each file in [directory], generate a corresponding .spec.ts file.
Process one file at a time.
For each file:
  1. Read the source
  2. List the public methods you found (confirm before generating)
  3. Generate the test file
  4. Show the test file path

Do not proceed to the next file until the current one is confirmed.

---

Part 7 — Project Structure Convention

All stacks follow the same proximity principle: test file lives next to source file.

Node.js / NestJS / Angular

src/
  modules/
    users/
      users.service.ts
      users.service.spec.ts        ← unit test
      users.repository.ts
      users.repository.spec.ts     ← unit test
      users.guard.ts
      users.guard.spec.ts          ← unit test

React

src/
  components/
    UserCard/
      UserCard.tsx
      UserCard.test.tsx            ← unit test
      UserCard.stories.tsx         ← storybook (optional)
  hooks/
    useUserData.ts
    useUserData.test.ts            ← unit test

Python

app/
  services/
    user_service.py
tests/
  unit/
    services/
      test_user_service.py        ← mirrors app/ structure

Laravel

app/
  Services/
    UserService.php
tests/
  Unit/
    Services/
      UserServiceTest.php         ← mirrors app/ structure

---

Part 8 — CI Enforcement

All of this means nothing without enforcement. The pipeline is the final gate.

GitHub Actions — Multi-stack parallel

name: Unit Tests

on: [push, pull_request]

jobs:
  nestjs-unit:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: { node-version: '20' }
      - run: npm ci
      - run: npm run test:unit -- --coverage --coverageThreshold='{"global":{"lines":80}}'

  react-unit:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: { node-version: '20' }
      - run: npm ci
      - run: npx vitest run --coverage

  python-unit:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with: { python-version: '3.12' }
      - run: pip install -r requirements-dev.txt
      - run: pytest tests/unit/ --cov=app --cov-fail-under=80

  laravel-unit:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: shivammathur/setup-php@v2
        with: { php-version: '8.3' }
      - run: composer install
      - run: ./vendor/bin/pest --coverage --min=80

Pre-commit Hook (Husky — Node stacks)

// package.json
{
  "lint-staged": {
    "src/**/*.service.ts": ["jest --findRelatedTests --passWithNoTests"],
    "src/**/*.tsx": ["vitest related --run"]
  }
}

---

Part 9 — Quick Start Checklist

New Project

□ Install test library (one per stack — see Part 1)
□ Configure jest.config.ts / vitest.config.ts / pytest.ini / pest.php
□ Create .cursor/rules/ directory with all rule files from Part 3
□ Create CLAUDE.md at project root from Part 4
□ Add coverage thresholds to config
□ Add pre-commit hooks (Husky for Node, pre-commit for Python)
□ Add CI workflow from Part 8
□ Create test factory files (one per major entity)
□ Write first failing test → implement → green → commit

Existing Project

□ Add test library without touching existing code
□ Add .cursor/rules/ + CLAUDE.md
□ Run coverage on existing code → document current baseline
□ Pick 3 critical services → generate tests → set as new baseline
□ Add CI with threshold = current baseline (not ideal — current)
□ Raise threshold by 5% per sprint

---

Appendix — Install Commands Per Stack

NestJS

npm install --save-dev jest ts-jest @types/jest @nestjs/testing jest-mock-extended @golevelup/ts-jest
npx ts-jest config:init

React (Vite)

npm install --save-dev vitest @vitest/coverage-v8 @testing-library/react @testing-library/user-event @testing-library/jest-dom jsdom

Python

pip install pytest pytest-cov pytest-mock pytest-asyncio factory-boy

Angular

ng add @angular-builders/jest
npm install --save-dev jest jest-preset-angular @types/jest

Laravel

composer require pestphp/pest pestphp/pest-plugin-laravel --dev
./vendor/bin/pest --init

---

Last updated: 2025 — verify library major versions before adopting