Claude Skills Guide

Claude Skills for Writing Unit Tests Automatically

Writing unit tests is one of those tasks every developer knows matters, yet finding time to write comprehensive test coverage feels like a luxury. Claude Code offers several approaches to automate test generation, ranging from built-in skills to custom configurations. This guide covers the practical methods for getting Claude to write unit tests automatically.

The TDD Skill: Your Primary Test Generator

The /tdd skill is the most direct way to generate unit tests in Claude Code. This skill doesn’t require installation—it activates through a simple command and transforms Claude’s behavior toward test-first development.

To use the TDD skill effectively, activate it in your session:

/tdd

Once activated, describe the code you want tested. Claude responds by generating a test file before writing implementation code. This approach ensures your code remains testable from the start.

For example, when working on a JavaScript utility function:

/tdd write Jest tests for a validateEmail function that checks format, domain existence, and disposable email providers

Claude generates tests covering valid emails, common formatting errors, and edge cases like empty strings. The tests use the Arrange-Act-Assert pattern familiar to most developers.

The skill supports multiple testing frameworks:

Specify your preferred framework in the request for the most relevant output.

Generating Tests from Existing Code

Beyond the TDD skill, Claude can analyze existing implementation code and generate corresponding unit tests. This works well for adding test coverage to legacy projects or quickly scaffolding tests for new functions.

The process for generating tests from existing code follows a consistent pattern:

  1. Provide the source file to Claude
  2. Request test generation with specific coverage goals
  3. Review and refine the generated tests
  4. Integrate into your test suite

Provide Claude with your source file and ask for test generation:

Generate unit tests for this Python module using pytest. Include tests for edge cases and error conditions:

[paste your module code]

Claude analyzes function signatures, identifies dependencies, and produces testable assertions. For a typical utility function, expect coverage of:

Additional Skills for Specialized Testing

Beyond the /tdd skill, several other Claude skills enhance test generation for specific use cases:

Custom Skills for Test Automation

For teams requiring consistent test patterns across projects, creating a custom test-generation skill makes sense. Store your skill definition in ~/.claude/skills/test-gen.md:

# Test Generation Skill

When asked to generate unit tests:

1. Analyze the provided code and identify testable functions
2. Generate tests using [YOUR_PREFERRED_FRAMEWORK]
3. Include the following patterns:
   - Basic functionality tests
   - Input validation tests
   - Error handling tests
   - Edge case coverage
4. Use descriptive test names following [YOUR_NAMING_CONVENTION]
5. Add docstrings explaining each test's purpose
6. Include setup/teardown where necessary

Output format:
- Test file with appropriate imports
- Clear test function names
- Minimal but sufficient assertions
- Comments for complex test logic

Activate this custom skill anytime with:

/test-gen

Practical Example: Testing a TypeScript Service

Consider a typical service class you want tested:

export class OrderProcessor {
  constructor(private discountCalculator: DiscountCalculator) {}

  calculateTotal(items: CartItem[]): number {
    const subtotal = items.reduce((sum, item) => sum + item.price * item.quantity, 0);
    const discount = this.discountCalculator.apply(subtotal);
    return subtotal - discount;
  }

  processOrder(order: Order): OrderResult {
    if (!order.items.length) {
      return { success: false, error: 'Empty order' };
    }
    
    const total = this.calculateTotal(order.items);
    
    if (total > order.maxBudget) {
      return { success: false, error: 'Exceeds budget' };
    }
    
    return { success: true, total };
  }
}

Using the TDD skill:

/tdd write Vitest tests for this OrderProcessor class. Mock the DiscountCalculator dependency.

Claude produces:

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

describe('OrderProcessor', () => {
  const createMockDiscountCalculator = () => ({
    apply: vi.fn().mockReturnValue(0),
  });

  describe('calculateTotal', () => {
    it('calculates subtotal minus discount', () => {
      const discountCalc = createMockDiscountCalculator();
      discountCalc.apply.mockReturnValue(10);
      const processor = new OrderProcessor(discountCalc);
      
      const result = processor.calculateTotal([
        { price: 100, quantity: 2 },
      ]);
      
      expect(result).toBe(190);
    });

    it('returns zero for empty items array', () => {
      const processor = new OrderProcessor(createMockDiscountCalculator());
      expect(processor.calculateTotal([])).toBe(0);
    });
  });

  describe('processOrder', () => {
    it('returns error for empty order', () => {
      const processor = new OrderProcessor(createMockDiscountCalculator());
      const result = processor.processOrder({ items: [], maxBudget: 1000 });
      
      expect(result.success).toBe(false);
      expect(result.error).toBe('Empty order');
    });

    it('returns error when exceeding budget', () => {
      const discountCalc = createMockDiscountCalculator();
      const processor = new OrderProcessor(discountCalc);
      
      const result = processor.processOrder({
        items: [{ price: 2000, quantity: 1 }],
        maxBudget: 1000,
      });
      
      expect(result.success).toBe(false);
      expect(result.error).toBe('Exceeds budget');
    });
  });
});

This test suite covers the primary behaviors without requiring manual input. Run these tests immediately to validate the implementation.

Integrating Test Generation into Your Workflow

Beyond ad-hoc test generation, consider these workflow integrations:

Pre-commit hooks: Generate baseline tests before committing new code. This catches missing coverage early.

Code review assistance: During reviews, ask Claude to verify test coverage for new functions:

Review this PR and verify all new functions have corresponding unit tests

Refactoring safety: Before major refactoring, generate tests for existing code to establish a safety net:

Generate tests for these three files so we have coverage before the refactor

Limitations and Best Practices

Automated test generation has boundaries. Generated tests cover happy paths and obvious edge cases, but they cannot discover domain-specific business logic bugs or understand behavioral expectations beyond the code itself.

For optimal results:

Claude skills for writing unit tests automatically handle the mechanical parts of test creation, freeing you to focus on test strategy and meaningful assertions that protect your codebase.

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