Building a Test Writer Skill: TDD Automation

Testing is fundamental to reliable software, yet writing comprehensive tests is time-consuming. A test writer skill can accelerate your testing workflow by automatically generating tests that cover happy paths, edge cases, and error conditions—freeing you to focus on implementation.

In this tutorial, we will build a test writer skill that analyzes code, understands behavior, and generates thorough test suites. Whether you practice TDD or add tests after implementation, this skill will improve your coverage and catch bugs earlier.

What We Are Building

Our test writer skill will:

1. Parse code to understand function signatures and behavior
2. Identify test scenarios (happy path, edge cases, errors)
3. Generate tests using the appropriate testing framework
4. Apply testing best practices
5. Support multiple testing styles (unit, integration)

By the end, your skill will transform code like this:

export function divide(a: number, b: number): number {
  if (b === 0) {
    throw new Error('Division by zero');
  }
  return a / b;
}

Into comprehensive tests like this:

describe('divide', () => {
  describe('happy path', () => {
    it('should divide two positive numbers', () => {
      expect(divide(10, 2)).toBe(5);
    });

    it('should divide negative by positive', () => {
      expect(divide(-10, 2)).toBe(-5);
    });

    it('should return decimal results', () => {
      expect(divide(7, 2)).toBe(3.5);
    });
  });

  describe('edge cases', () => {
    it('should handle zero dividend', () => {
      expect(divide(0, 5)).toBe(0);
    });

    it('should handle very large numbers', () => {
      expect(divide(Number.MAX_SAFE_INTEGER, 1)).toBe(Number.MAX_SAFE_INTEGER);
    });

    it('should handle very small divisor', () => {
      expect(divide(1, 0.0001)).toBeCloseTo(10000);
    });
  });

  describe('error handling', () => {
    it('should throw when dividing by zero', () => {
      expect(() => divide(10, 0)).toThrow('Division by zero');
    });
  });
});

Prerequisites

Before we begin, you should have:

• Understanding of testing concepts and frameworks
• Familiarity with test-driven development
• Basic knowledge of code analysis
• Claude Code installed and configured

Part 1: Architecture Design

Let us design our test writer's approach.

Analysis Pipeline

Code → [Parse] → [Analyze] → [Plan] → [Generate] → Tests

Each stage serves a specific purpose:

## Pipeline Stages

### Stage 1: Parse
- Extract function/class signatures
- Identify parameters and types
- Find return types
- Detect thrown errors

### Stage 2: Analyze
- Understand function purpose
- Identify behavior patterns
- Map input to output relationships
- Find edge cases

### Stage 3: Plan
- Determine test categories
- List specific test cases
- Prioritize by importance
- Plan test structure

### Stage 4: Generate
- Apply framework syntax
- Create test cases
- Add assertions
- Format output

Test Categories

Our skill will generate tests in organized categories:

## Test Categories

### Happy Path Tests
Normal, expected usage:
- Typical inputs
- Common scenarios
- Expected outputs

### Edge Case Tests
Boundary conditions:
- Empty inputs
- Maximum values
- Minimum values
- Boundary transitions

### Error Handling Tests
Failure scenarios:
- Invalid inputs
- Missing data
- Exceptional conditions
- Error messages

### Integration Tests (when applicable)
Component interaction:
- Dependencies
- Side effects
- State changes

Part 2: Building the Parser

The parser extracts structural information from code.

Step 1: Create the Parser Module

---
name: test-parser
description: Parses code to extract testable elements
version: 1.0.0
tools:
  - Read
---

# Test Code Parser

Parse source code to extract information needed for test generation.

## Extraction Targets

### Functions

Extract:
```typescript
{
  name: string;
  parameters: Array<{
    name: string;
    type: string;
    required: boolean;
    defaultValue?: any;
  }>;
  returnType: string;
  async: boolean;
  throws: string[];
  visibility: 'public' | 'private' | 'protected';
}

Classes

Extract:

{
  name: string;
  constructor: FunctionInfo;
  methods: FunctionInfo[];
  properties: PropertyInfo[];
  staticMembers: Array<FunctionInfo | PropertyInfo>;
}

Dependencies

Identify:

• Imported modules
• Injected dependencies
• External calls
• Side effects

Language Support

TypeScript/JavaScript

// Function patterns
function name(params): ReturnType { }
const name = (params): ReturnType => { }
async function name(params): Promise<Type> { }

// Class patterns
class Name {
  constructor(params) { }
  method(params): ReturnType { }
  static staticMethod(): ReturnType { }
}

// Error patterns
throw new Error('message');
throw new CustomError('message');

Python

# Function patterns
def name(params) -> ReturnType:
    """Docstring"""

async def name(params) -> ReturnType:
    """Docstring"""

# Class patterns
class Name:
    def __init__(self, params):
        pass

    def method(self, params) -> ReturnType:
        pass

# Error patterns
raise ValueError('message')
raise CustomError('message')

Output Format

{
  "file": "src/utils/math.ts",
  "elements": [
    {
      "type": "function",
      "name": "divide",
      "parameters": [
        { "name": "a", "type": "number", "required": true },
        { "name": "b", "type": "number", "required": true }
      ],
      "returnType": "number",
      "async": false,
      "throws": ["Error"],
      "visibility": "public",
      "body": "...",
      "location": { "start": 1, "end": 6 }
    }
  ],
  "imports": ["..."],
  "exports": ["divide"]
}

## Part 3: Building the Analyzer

The analyzer understands what the code does.

### Step 2: Create the Analyzer Module

```markdown
---
name: test-analyzer
description: Analyzes code to understand behavior for testing
version: 1.0.0
---

# Test Behavior Analyzer

Analyze parsed code to understand behavior and identify test scenarios.

## Analysis Tasks

### 1. Purpose Analysis

Determine what the function does:
- Read function name and body
- Identify the transformation performed
- Understand success conditions
- Map input to output

### 2. Input Analysis

For each parameter:
```markdown
Identify:
- Valid value ranges
- Special values (null, empty, zero)
- Boundary values
- Invalid values
- Type constraints

3. Output Analysis

For the return value:

Identify:
- Possible return values
- Return conditions
- Side effects
- State changes

4. Error Analysis

For thrown errors:

Identify:
- Trigger conditions
- Error types
- Error messages
- Recovery options

5. Dependency Analysis

For external calls:

Identify:
- Mock requirements
- Stub behavior
- Verification points

Test Case Identification

Happy Path Cases

Based on purpose analysis:

happyPath:
  - scenario: "typical usage"
    inputs: [representative values]
    expected: normal output

  - scenario: "alternative valid path"
    inputs: [different valid values]
    expected: corresponding output

Edge Cases

Based on input analysis:

edgeCases:
  - scenario: "minimum values"
    inputs: [smallest valid inputs]
    expected: boundary output

  - scenario: "maximum values"
    inputs: [largest valid inputs]
    expected: boundary output

  - scenario: "empty/zero values"
    inputs: [empty or zero where valid]
    expected: corresponding output

  - scenario: "boundary transition"
    inputs: [values at boundaries]
    expected: correct behavior

Error Cases

Based on error analysis:

errorCases:
  - scenario: "invalid input"
    inputs: [invalid values]
    expected: specific error

  - scenario: "missing required"
    inputs: [missing required params]
    expected: validation error

  - scenario: "constraint violation"
    inputs: [values violating constraints]
    expected: constraint error

Output Format

{
  "element": "divide",
  "analysis": {
    "purpose": "Divides first number by second",
    "inputs": {
      "a": {
        "type": "number",
        "validRange": "any number",
        "specialValues": [0, "negative", "decimal"]
      },
      "b": {
        "type": "number",
        "validRange": "non-zero",
        "invalidValues": [0],
        "specialValues": ["negative", "decimal", "very small"]
      }
    },
    "output": {
      "type": "number",
      "range": "any number including infinity"
    },
    "errors": [
      {
        "condition": "b === 0",
        "type": "Error",
        "message": "Division by zero"
      }
    ]
  },
  "testCases": {
    "happyPath": [
      { "name": "divides positive numbers", "inputs": [10, 2], "expected": 5 },
      { "name": "handles negative dividend", "inputs": [-10, 2], "expected": -5 },
      { "name": "handles decimal result", "inputs": [7, 2], "expected": 3.5 }
    ],
    "edgeCases": [
      { "name": "zero dividend", "inputs": [0, 5], "expected": 0 },
      { "name": "very large numbers", "inputs": [1e15, 1], "expected": 1e15 },
      { "name": "very small divisor", "inputs": [1, 0.0001], "expected": 10000 }
    ],
    "errorCases": [
      { "name": "division by zero", "inputs": [10, 0], "throws": "Division by zero" }
    ]
  }
}

## Part 4: Building the Planner

The planner organizes test cases into a structured plan.

### Step 3: Create the Planner Module

```markdown
---
name: test-planner
description: Plans test structure and organization
version: 1.0.0
---

# Test Planner

Organize analyzed test cases into a structured test plan.

## Planning Tasks

### 1. Test Structure

Organize tests hierarchically:

describe('functionName', () => { describe('happy path', () => { it('test case 1') it('test case 2') })

describe('edge cases', () => { it('test case 1') it('test case 2') })

describe('error handling', () => { it('test case 1') }) })

### 2. Test Prioritization

Order tests by importance:
1. Critical path tests (most common usage)
2. Error handling (prevent crashes)
3. Edge cases (boundary behavior)
4. Performance considerations

### 3. Setup Requirements

Identify test setup needs:
```yaml
setup:
  beforeAll: [one-time setup]
  beforeEach: [per-test setup]
  afterEach: [per-test cleanup]
  afterAll: [final cleanup]

mocks:
  - module: 'dependency'
    behavior: 'return value'

fixtures:
  - name: 'testData'
    value: { ... }

4. Assertion Strategy

Choose appropriate assertions:

assertions:
  equality:
    - toBe: exact match
    - toEqual: deep equality

  numeric:
    - toBeCloseTo: floating point
    - toBeGreaterThan: comparison
    - toBeLessThan: comparison

  exceptions:
    - toThrow: any error
    - toThrow('message'): specific message
    - toThrowError(Type): specific type

  async:
    - resolves.toBe: successful promise
    - rejects.toThrow: failed promise

Output Format

{
  "element": "divide",
  "testPlan": {
    "structure": {
      "describe": "divide",
      "groups": [
        {
          "describe": "happy path",
          "tests": [
            {
              "it": "should divide two positive numbers",
              "inputs": [10, 2],
              "assertion": { "type": "toBe", "expected": 5 }
            }
          ]
        },
        {
          "describe": "edge cases",
          "tests": [...]
        },
        {
          "describe": "error handling",
          "tests": [
            {
              "it": "should throw when dividing by zero",
              "inputs": [10, 0],
              "assertion": { "type": "toThrow", "expected": "Division by zero" }
            }
          ]
        }
      ]
    },
    "setup": {
      "imports": ["divide"],
      "mocks": [],
      "fixtures": []
    }
  }
}

## Part 5: Building the Generator

The generator produces the actual test code.

### Step 4: Create the Generator Module

```markdown
---
name: test-generator
description: Generates test code from test plans
version: 1.0.0
tools:
  - Write
---

# Test Code Generator

Generate test code from structured test plans.

## Framework Support

### Jest (JavaScript/TypeScript)

```typescript
import { functionName } from './module';

describe('functionName', () => {
  describe('happy path', () => {
    it('should do something', () => {
      expect(functionName(input)).toBe(expected);
    });
  });
});

Pytest (Python)

import pytest
from module import function_name

class TestFunctionName:
    class TestHappyPath:
        def test_should_do_something(self):
            assert function_name(input) == expected

Vitest (Modern JavaScript)

import { describe, it, expect } from 'vitest';
import { functionName } from './module';

describe('functionName', () => {
  it('should do something', () => {
    expect(functionName(input)).toBe(expected);
  });
});

Generation Templates

Basic Test

it('{testName}', () => {
  {setup}
  const result = {functionCall};
  expect(result).{assertion}({expected});
});

Async Test

it('{testName}', async () => {
  {setup}
  const result = await {functionCall};
  expect(result).{assertion}({expected});
});

Error Test

it('{testName}', () => {
  expect(() => {functionCall}).toThrow({expectedError});
});

Async Error Test

it('{testName}', async () => {
  await expect({functionCall}).rejects.toThrow({expectedError});
});

Mock Test

it('{testName}', () => {
  const mockDep = jest.fn().mockReturnValue({mockValue});
  {setup with mock}
  const result = {functionCall};
  expect(mockDep).toHaveBeenCalledWith({expectedArgs});
  expect(result).{assertion}({expected});
});

Formatting Rules

Naming Conventions

Test names should:

• Start with "should"
• Describe the expected behavior
• Be readable as a sentence

Examples:

• "should return the sum of two numbers"
• "should throw when input is null"
• "should call the API with correct parameters"

Code Style

• Consistent indentation (2 spaces)
• Blank lines between test groups
• Comments for complex setups
• Descriptive variable names

Organization

Order within describe blocks:

1. Setup (beforeEach, etc.)
2. Happy path tests
3. Edge case tests
4. Error tests
5. Cleanup (afterEach, etc.)

Output

Write generated tests to appropriate file:

• Input: src/utils/math.ts
• Output: src/utils/math.test.ts or tests/utils/math.test.ts

## Part 6: Complete Skill Integration

### Step 5: Create the Main Skill

```markdown
---
name: test-writer
description: Automatically generates comprehensive tests for code
version: 1.0.0
tools:
  - Read
  - Write
  - Glob
arguments:
  - name: file
    type: string
    required: true
    description: File or pattern to generate tests for
  - name: framework
    type: string
    default: jest
    options: [jest, vitest, pytest, mocha]
    description: Testing framework to use
  - name: style
    type: string
    default: comprehensive
    options: [minimal, standard, comprehensive]
    description: Test coverage level
  - name: output
    type: string
    description: Output file path (default: alongside source)
---

# Test Writer

Automatically generate comprehensive tests for your code.

## Workflow

### Phase 1: Parse

Read and parse the target file(s):
1. Load file contents
2. Parse code structure
3. Extract functions, classes, methods
4. Identify signatures and types

### Phase 2: Analyze

Understand code behavior:
1. Determine function purposes
2. Identify input ranges
3. Find edge cases
4. Detect error conditions

### Phase 3: Plan

Organize test strategy:
1. Create test structure
2. Plan test cases by category
3. Determine setup needs
4. Choose assertions

### Phase 4: Generate

Create test code:
1. Apply framework templates
2. Generate test cases
3. Add setup and teardown
4. Format output

### Phase 5: Write

Output the tests:
1. Determine output path
2. Write test file
3. Report coverage summary

## Test Coverage Levels

### Minimal
- One happy path test per function
- One error test if function throws

### Standard
- Multiple happy path scenarios
- Key edge cases
- All error conditions

### Comprehensive
- Thorough happy path coverage
- Extensive edge cases
- All error conditions
- Boundary testing
- Type checking (if applicable)

## Framework Configurations

### Jest
```typescript
// jest.config.js compatible
// Uses describe/it/expect
// Supports mocking with jest.fn()

Vitest

// vitest.config.ts compatible
// Uses describe/it/expect from vitest
// Supports modern ESM

Pytest

# pytest compatible
# Uses class-based or function-based tests
# Supports fixtures

Example Usage

# Generate tests for a single file
/test-writer --file src/utils/math.ts

# Generate with specific framework
/test-writer --file src/api/users.ts --framework vitest

# Generate minimal tests
/test-writer --file src/services/*.ts --style minimal

# Specify output location
/test-writer --file src/core/auth.ts --output tests/core/auth.test.ts

Output

Complete test file with:

• Imports and setup
• Organized test groups
• Comprehensive test cases
• Appropriate assertions
• Clean formatting

Summary Report

After generation:

Tests Generated for: src/utils/math.ts
Output: src/utils/math.test.ts

Coverage:
- Functions: 3
- Test Cases: 15
  - Happy Path: 6
  - Edge Cases: 6
  - Error Cases: 3

Run tests with: npm test src/utils/math.test.ts

## Part 7: Advanced Features

### Step 6: Add TDD Support

Support test-first development:

```markdown
## TDD Mode

When given a function signature (not implementation):

### Input
```typescript
// Function to implement
function calculateTax(amount: number, rate: number): number;

Output

describe('calculateTax', () => {
  it('should calculate tax for positive amount', () => {
    expect(calculateTax(100, 0.1)).toBe(10);
  });

  it('should handle zero amount', () => {
    expect(calculateTax(0, 0.1)).toBe(0);
  });

  it('should handle zero rate', () => {
    expect(calculateTax(100, 0)).toBe(0);
  });

  // Tests drive implementation requirements
});

Workflow

1. Write tests first (from signature)
2. Tests initially fail
3. Implement to make tests pass
4. Refactor with confidence

### Step 7: Add Mock Generation

Generate mocks for dependencies:

```markdown
## Mock Generation

When function has dependencies:

### Analysis
```typescript
// Function with dependency
async function fetchUserData(userId: string): Promise<User> {
  const response = await api.get(`/users/${userId}`);
  return response.data;
}

Generated Tests

describe('fetchUserData', () => {
  const mockApi = {
    get: jest.fn()
  };

  beforeEach(() => {
    jest.clearAllMocks();
  });

  it('should fetch user by id', async () => {
    const mockUser = { id: '123', name: 'Test User' };
    mockApi.get.mockResolvedValue({ data: mockUser });

    const result = await fetchUserData('123');

    expect(mockApi.get).toHaveBeenCalledWith('/users/123');
    expect(result).toEqual(mockUser);
  });

  it('should handle API error', async () => {
    mockApi.get.mockRejectedValue(new Error('Network error'));

    await expect(fetchUserData('123')).rejects.toThrow('Network error');
  });
});

## Testing Your Skill

Verify the skill works correctly:

```markdown
## Test Cases

### Test 1: Simple Function
Input: Pure function with types
Expected: Complete test coverage

### Test 2: Async Function
Input: Function returning Promise
Expected: Async tests with await

### Test 3: Class with Methods
Input: Class definition
Expected: Tests for constructor and methods

### Test 4: Function with Dependencies
Input: Function calling external services
Expected: Tests with mocks

### Test 5: Error-Throwing Function
Input: Function with error conditions
Expected: Error tests with toThrow

Conclusion

You have built a comprehensive test writer skill that:

1. Parses code to extract testable elements
2. Analyzes behavior to identify test scenarios
3. Plans test structure and organization
4. Generates framework-specific test code
5. Supports multiple frameworks and styles

This skill demonstrates the Chain pattern with tool integration, showing how to build complex functionality through focused stages.

Key takeaways:

• Parse before analyzing for structured data
• Identify test categories systematically
• Generate framework-appropriate code
• Support different coverage levels
• Enable TDD workflows

Your test writer is ready to accelerate your testing workflow!

---

Sources

• Claude Code Documentation — Anthropic
• Claude Prompt Engineering — Anthropic
• GitHub Actions Documentation — GitHub
• AI Skill Market — AI Skill Market