TDD (Test-Driven Development) Skill

Guide Claude to follow strict Test-Driven Development practices when implementing new features or fixing bugs. Ensures tests are written BEFORE implementation code.

When to Use

• Implementing new features
• Adding new methods to existing services/repositories
• Fixing bugs (write failing test first)
• Refactoring existing code

Core TDD Cycle

RED → GREEN → REFACTOR
 ↓      ↓         ↓
Fail   Pass    Improve

1. RED - Write Failing Test

• Write test that describes desired behavior
• Test MUST fail (compilation error or assertion failure)
• Verify test fails for the right reason

2. GREEN - Make Test Pass

• Write minimal code to make test pass
• Don't worry about perfect code yet
• Just make it work

3. REFACTOR - Improve Code

• Clean up implementation
• Remove duplication
• Improve naming
• Tests still pass

Critical Rules

Rule 1: ALWAYS Identify Entity Under Test

Before writing any test, Claude MUST:

1. Ask the user if entity under test is ambiguous
2. State explicitly which entity is being tested
3. Use correct naming in test file and test functions

See examples/dialogue-examples.md for examples of clear vs ambiguous entity identification.

Rule 2: Test File Location

Place test files next to the code being tested:

internal/service/
├── transaction_service.go
└── transaction_service_test.go  ← Test file here

pkg/ofx/
├── parser.go
└── parser_test.go  ← Test file here

Rule 3: Test Naming Convention

Pattern: Test<EntityName>_<MethodName>_<Scenario>

See data/naming-conventions.md for detailed naming guidelines and examples.

// ✅ GOOD - Clear entity, method, and scenario
func TestTransactionService_ImportFromOFX_WithDuplicateFITID(t *testing.T)
func TestOFXParser_Parse_WithInvalidXML(t *testing.T)
func TestBudgetRepository_Create_WithNullAmount(t *testing.T)

// ❌ BAD - Missing entity or scenario
func TestImport(t *testing.T)
func TestValidData(t *testing.T)

Rule 4: Test Main Cases (Not Everything)

Focus on:

• Happy path (1 test)
• Common errors (2-3 tests)
• Edge cases (1-2 tests)
• Business rules (as many as needed)

Don't test:

• Language features (e.g., "does append work?")
• Third-party library internals
• Trivial getters/setters
• Every possible combination

See data/coverage-guidelines.md for what to test and what to skip.

Test Structure (AAA Pattern)

Use templates from templates/test-structure.md:

func TestTransactionService_ImportFromOFX_Success(t *testing.T) {
    // ARRANGE - Setup test data and dependencies
    mockRepo := &mockTransactionRepository{}
    parser := ofx.NewParser()
    service := NewTransactionService(mockRepo, parser)

    ofxData := []byte(`<OFX>...</OFX>`)
    accountID := uuid.New()

    // ACT - Execute the method being tested
    result, err := service.ImportFromOFX(context.Background(), ofxData, accountID)

    // ASSERT - Verify expectations
    require.NoError(t, err)
    assert.Equal(t, 5, result.Inserted)
    assert.Equal(t, 0, result.Skipped)
}

TDD Workflow Examples

See examples/feature-implementation.md for complete examples:

Example 1: New Feature

1. RED - Write failing test for BudgetService.CalculateEffectiveAmount()
2. GREEN - Implement minimal code to pass
3. REFACTOR - Add more test cases and improve

Example 2: Bug Fix

1. RED - Reproduce bug with failing test
2. GREEN - Fix the bug to make test pass
3. REFACTOR - Verify no other bugs introduced

Example 3: Multiple Scenarios

Test 4 main cases for each method:

• Happy path
• Error handling
• Boundary conditions
• Business rules

Test Coverage Guidelines

From data/coverage-guidelines.md:

What to Test ✅

• Repository: CRUD, constraints, complex queries
• Service: Business logic, validation, error handling
• Handler: Request/response, status codes, auth

What NOT to Test ❌

• Language features
• Framework internals
• Database engine behavior
• Third-party libraries
• Trivial code

Mocking Guidelines

When to Mock

• External dependencies (HTTP clients, databases)
• Other services in service layer
• Slow operations
• Non-deterministic behavior (time, random)

How to Mock

See templates/test-structure.md for mock templates:

// Define interface in domain
type TransactionRepository interface {
    BulkInsert(ctx context.Context, txs []*Transaction) error
}

// Create mock in test file
type mockTransactionRepository struct {
    mock.Mock
}

func (m *mockTransactionRepository) BulkInsert(ctx context.Context, txs []*Transaction) error {
    args := m.Called(ctx, txs)
    return args.Error(0)
}

Claude's TDD Checklist

Before writing implementation code, Claude MUST:

• Identify entity under test explicitly
• Create/open correct test file (*_test.go)
• Write test with clear AAA structure
• Run test and verify it FAILS
• State why test fails (compilation or assertion)
• Ask user if entity or scenario is ambiguous

After test fails, Claude can:

• Implement minimal code to pass test
• Run test and verify it PASSES
• Refactor if needed
• Add more test cases for edge cases

Common Mistakes to Avoid

❌ Writing Implementation First

WRONG: "I'll implement ImportFromOFX, then test it"
RIGHT: "I'll write a test for ImportFromOFX first, then implement"

❌ Testing Multiple Things in One Test

One scenario per test function.

❌ Unclear Entity Under Test

Always specify which entity (Service, Repository, Handler) is being tested.

❌ Testing Implementation Details

Test public behavior, not internal variables or private methods.

TDD Dialog Examples

See examples/dialogue-examples.md for:

• How to handle clear entity requests
• How to ask for clarification when ambiguous
• How to propose multiple test scenarios
• How to stop before writing implementation without tests

Summary

TDD in 3 Steps:

1. 🔴 RED - Write failing test (describe what you want)
2. 🟢 GREEN - Make test pass (implement minimal code)
3. 🔵 REFACTOR - Improve code (keep tests passing)

Always remember:

• Identify entity under test explicitly
• Write test BEFORE implementation
• Test main cases (not every possibility)
• Use clear naming conventions
• Ask when unsure

Golden Rule: If you're writing implementation code without a failing test, STOP and write the test first.

Reference Files

• Examples: See examples/feature-implementation.md and examples/dialogue-examples.md
• Templates: See templates/test-structure.md for test patterns and mocks
• Naming: See data/naming-conventions.md for naming guidelines
• Coverage: See data/coverage-guidelines.md for what to test