Test quality guard - prevents testing mock behavior, production pollution with test-only methods, and mocking without understanding dependencies.
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name: testing-anti-patterns description: | Test quality guard - prevents testing mock behavior, production pollution with test-only methods, and mocking without understanding dependencies.
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Tests must verify real behavior, not mock behavior. Mocks are a means to isolate, not the thing being tested.
Core principle: Test what the code does, not what the mocks do.
Following strict TDD prevents these anti-patterns.
1. NEVER test mock behavior
2. NEVER add test-only methods to production classes
3. NEVER mock without understanding dependencies
BAD: expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument() - testing mock exists, not real behavior.
GOOD: expect(screen.getByRole('navigation')).toBeInTheDocument() - test real component or don't mock.
Gate: Before asserting on mock element → "Am I testing real behavior or mock existence?" If mock → delete assertion or unmock.
BAD: session.destroy() method only used in tests - pollutes production, dangerous if called.
GOOD: cleanupSession(session) in test-utils/ - keeps production clean.
Gate: "Is this method only used by tests?" → Put in test utilities. "Does this class own this lifecycle?" → If no, wrong class.
BAD: Mocking discoverAndCacheTools breaks config write test depends on - test passes for wrong reason.
GOOD: Mock only the slow part (MCPServerManager), preserve behavior test needs.
Gate: Before mocking → (1) What side effects does real method have? (2) Does test depend on them? If yes → mock at lower level. Red flags: "Mock to be safe", "might be slow", mocking without understanding.
BAD: Partial mock missing metadata field - breaks when downstream code accesses response.metadata.requestId.
GOOD: Complete mock mirroring real API - ALL fields real API returns.
Iron Rule: Mock COMPLETE data structure, not just fields your test uses. Partial mocks fail silently.
Gate: Before mock → Check real API response, include ALL fields. If uncertain → include all documented fields.
BAD: "Implementation complete" without tests. FIX: TDD cycle: write test → implement → refactor → claim complete.
Warning signs: Mock setup longer than test logic, mocking everything, mocks missing methods real components have. Consider: Integration tests with real components often simpler than complex mocks.
TDD forces: (1) Think about what you're testing, (2) Watch fail confirms real behavior not mocks, (3) See what test needs before mocking. If testing mock behavior, you violated TDD.
| Anti-Pattern | Fix |
|---|---|
| Assert on mock elements | Test real component or unmock it |
| Test-only methods in production | Move to test utilities |
| Mock without understanding | Understand dependencies first, mock minimally |
| Incomplete mocks | Mirror real API completely |
| Tests as afterthought | TDD - tests first |
| Over-complex mocks | Consider integration tests |
*-mock test IDsMocks are tools to isolate, not things to test.
If TDD reveals you're testing mock behavior, you've gone wrong.
Fix: Test real behavior or question why you're mocking at all.