Test Design

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name: test-design description: > Test design and strategy skill for comprehensive testing. Use when: designing test cases, planning testing strategy, writing unit/integration/e2e tests, or improving test coverage. Triggers: test design, test strategy, unit test, integration test, e2e test, TDD, test coverage, test plan, test case design. Provides: testing pyramid guidance, test case templates, test categorization, mocking strategies, and test execution frameworks.

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Test Design Skill

A structured approach to designing tests that give confidence without excessive maintenance burden.

Core Philosophy

Test behavior, not implementation. Tests that couple to internal implementation details become fragile and refactor-resistant. Aim to test what the code does, not how it does it.

The best test is the one that fails when something breaks, and passes when everything works. Perfect coverage means nothing if tests don't catch real bugs.

Testing Pyramid

        ┌─────────┐
        │   E2E   │  ← Few, slow, high confidence
       ┌┴─────────┴┐
       │ Integration│ ← Some, medium speed
      ┌┴───────────┴┐
      │    Unit     │ ← Many, fast, granular
      └─────────────┘

Unit tests form the base — fast, isolated, cover individual functions. Integration tests verify components work together. E2E tests validate complete user journeys.

Test Case Design

Arrange-Act-Assert (AAA) Pattern

def test_withdraw_amount_exceeds_balance():
    # Arrange
    account = Account(balance=Decimal("100.00"))
    
    # Act
    result = account.withdraw(Decimal("150.00"))
    
    # Assert
    assert result.is_failed()
    assert account.balance == Decimal("100.00")  # unchanged

Given-When-Then (GWT) Pattern

Feature: Account Withdrawal
  Scenario: Withdrawal amount exceeds balance
    Given an account with balance $100
    When I withdraw $150
    Then the withdrawal is rejected
    And the balance remains $100

Test Categories

🔴 Happy Path Tests

Verify the main success scenario works

At least one per feature/function

🟡 Edge Case Tests

Boundary values (0, -1, max integer)

Empty/null inputs

Rare but possible conditions

🟡 Error Case Tests

Invalid inputs

Missing dependencies

Permission denied scenarios

🟢 Edge/Extreme Cases

Very large inputs

Very small inputs

Unicode/special characters

Concurrent access

Test Doubles (Mocks/Stubs/Fakes)

| Type | Purpose | Use When | |------|---------|----------| | Dummy | Fill parameter lists | Never actually used | | Stub | Provide predetermined responses | Test requires specific data | | Spy | Record how something was called | Verify interactions | | Mock | Pre-programmed expectations | Verify behavior + interactions | | Fake | Working implementation (simplified) | Real impl too slow/complex |

Rule: Prefer real objects over mocks when practical. Mocks hide integration problems.

Test Naming Convention

test___

Examples:

test_user_create_with_valid_input_succeeds

test_user_create_with_duplicate_email_fails

test_order_cancel_after_shipment_refunds_full_amount

Coverage Targets

| Layer | Target | Purpose | |-------|--------|---------| | Unit | 70-80% | Core business logic | | Integration | 40-60% | Key flows work together | | E2E | Critical paths only | User journeys |

Note: Coverage is a guide, not a goal. 100% coverage with shallow tests is worse than 70% with meaningful assertions.

Test Execution

Unit Tests

Run frequently during development (every save).

# Go
go test ./...
Python
pytest tests/unit/ -v
TypeScript
npm test -- --coverage

Integration Tests

Run before PR, after unit tests pass.

# Start dependencies
docker-compose up -d
Run integration suite
pytest tests/integration/ -v

E2E Tests

Run on CI against deployed environment.

# CI/CD pipeline
npm run test:e2e -- --specs=critical-paths

Test Data Management

Strategies

1. Factories/Fixtures — Create test data on demand 2. Seeded Data — Deterministic test datasets 3. Randomized Data — Fuzz testing with random inputs 4. Snapshot Testing — Verify serializable outputs

Best Practices

Tests should be independent (no shared state)

Each test should clean up after itself

Use meaningful data, not magic values

Avoid test interdependencies

File Structure

test-design/
├── SKILL.md
└── references/
    ├── test-case-templates.md
    ├── testing-pyramid.md
    ├── mocking-strategies.md
    ├── test-naming-conventions.md
    └── per-type/
        ├── unit-test-guide.md
        ├── integration-test-guide.md
        └── e2e-test-guide.md

Tests should be independent (no shared state)

Each test should clean up after itself

Use meaningful data, not magic values

Avoid test interdependencies