Write comprehensive failing tests following TDD red phase principles.

[Extended thinking: Generates failing tests that properly define expected behavior using test-automator agent.]

Use this skill when

• Starting the TDD red phase for new behavior
• You need failing tests that capture expected behavior
• You want edge case coverage before implementation

Do not use this skill when

• You are in the green or refactor phase
• You only need performance benchmarks
• Tests must run against production systems

Instructions

1. Identify behaviors, constraints, and edge cases.
2. Generate failing tests that define expected outcomes.
3. Ensure failures are due to missing behavior, not setup errors.
4. Document how to run tests and verify failures.

Safety

• Keep test data isolated and avoid production environments.
• Avoid flaky external dependencies in the red phase.

Role

Generate failing tests using Task tool with subagent_type="unit-testing::test-automator".

Prompt Template

"Generate comprehensive FAILING tests for: $ARGUMENTS

Core Requirements

1. Test Structure

   • Framework-appropriate setup (Jest/pytest/JUnit/Go/RSpec)
   • Arrange-Act-Assert pattern
   • should_X_when_Y naming convention
   • Isolated fixtures with no interdependencies

2. Behavior Coverage

   • Happy path scenarios
   • Edge cases (empty, null, boundary values)
   • Error handling and exceptions
   • Concurrent access (if applicable)

3. Failure Verification

   • Tests MUST fail when run
   • Failures for RIGHT reasons (not syntax/import errors)
   • Meaningful diagnostic error messages
   • No cascading failures

4. Test Categories

   • Unit: Isolated component behavior
   • Integration: Component interaction
   • Contract: API/interface contracts
   • Property: Mathematical invariants

Framework Patterns

JavaScript/TypeScript (Jest/Vitest)

• Mock dependencies with vi.fn() or jest.fn()
• Use @testing-library for React components
• Property tests with fast-check

Python (pytest)

• Fixtures with appropriate scopes
• Parametrize for multiple test cases
• Hypothesis for property-based tests

Go

• Table-driven tests with subtests
• t.Parallel() for parallel execution
• Use testify/assert for cleaner assertions

Ruby (RSpec)

• let for lazy loading, let! for eager
• Contexts for different scenarios
• Shared examples for common behavior

Quality Checklist

• Readable test names documenting intent
• One behavior per test
• No implementation leakage
• Meaningful test data (not 'foo'/'bar')
• Tests serve as living documentation

Anti-Patterns to Avoid

• Tests passing immediately
• Testing implementation vs behavior
• Complex setup code
• Multiple responsibilities per test
• Brittle tests tied to specifics

Edge Case Categories

• Null/Empty: undefined, null, empty string/array/object
• Boundaries: min/max values, single element, capacity limits
• Special Cases: Unicode, whitespace, special characters
• State: Invalid transitions, concurrent modifications
• Errors: Network failures, timeouts, permissions

Output Requirements

• Complete test files with imports
• Documentation of test purpose
• Commands to run and verify failures
• Metrics: test count, coverage areas
• Next steps for green phase"

Validation

After generation:

1. Run tests - confirm they fail
2. Verify helpful failure messages
3. Check test independence
4. Ensure comprehensive coverage

Example (Minimal)

// auth.service.test.ts
describe('AuthService', () => {
  let authService: AuthService;
  let mockUserRepo: jest.Mocked<UserRepository>;

  beforeEach(() => {
    mockUserRepo = { findByEmail: jest.fn() } as any;
    authService = new AuthService(mockUserRepo);
  });

  it('should_return_token_when_valid_credentials', async () => {
    const user = { id: '1', email: 'test@example.com', passwordHash: 'hashed' };
    mockUserRepo.findByEmail.mockResolvedValue(user);

    const result = await authService.authenticate('test@example.com', 'pass');

    expect(result.success).toBe(true);
    expect(result.token).toBeDefined();
  });

  it('should_fail_when_user_not_found', async () => {
    mockUserRepo.findByEmail.mockResolvedValue(null);

    const result = await authService.authenticate('none@example.com', 'pass');

    expect(result.success).toBe(false);
    expect(result.error).toBe('INVALID_CREDENTIALS');
  });
});

Test requirements: $ARGUMENTS

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.