## Testing Strategy Expertise

Apply these testing patterns:

### Test Pyramid
- **Unit tests** (70%): Test individual functions/methods in isolation
- **Integration tests** (20%): Test component interactions and boundaries
- **E2E tests** (10%): Test critical user flows end-to-end

### What to Test
- Happy path: the expected successful flow
- Error cases: what happens when things go wrong?
- Edge cases: empty inputs, maximum values, concurrent access
- Boundary conditions: off-by-one, empty collections, null/undefined

### Test Quality
- Each test should verify ONE behavior
- Test names should describe the expected behavior, not the implementation
- Tests should be independent — no shared mutable state between tests
- Tests should be deterministic — same result every run

### Coverage Strategy
- Aim for meaningful coverage, not 100% line coverage
- Focus coverage on business logic and error handling
- Don't test framework code or simple getters/setters
- Cover the branches, not just the lines

### Mocking Guidelines
- Mock external dependencies (APIs, databases, file system)
- Don't mock the code under test
- Use realistic test data — edge cases reveal bugs
- Verify mock interactions when the side effect IS the behavior

### Regression Testing
- Write a failing test FIRST that reproduces the bug
- Then fix the bug and verify the test passes
- Keep regression tests — they prevent the bug from recurring

### Required Output (Mandatory)

Your output MUST include these sections when this skill is active:

1. **Test Pyramid Breakdown**: Explicit count of unit/integration/E2E tests and their coverage targets (e.g., "70 unit tests covering business logic, 12 integration tests for API boundaries, 3 E2E tests for critical paths")
2. **Coverage Targets**: Target coverage percentage per layer and which critical paths MUST be tested
3. **Critical Paths to Test**: Specific test cases for the happy path, 2+ error cases, and 2+ edge cases

If any section is not applicable, explicitly state why it's skipped.