Testing Anti-Patterns

Overview

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.

The Iron Laws

1. NEVER test mock behavior
2. NEVER add test-only methods to production classes
3. NEVER mock without understanding dependencies

Anti-Pattern 1: Testing Mock Behavior

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.

Anti-Pattern 2: Test-Only Methods in Production

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.

Anti-Pattern 3: Mocking Without Understanding

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.

Anti-Pattern 4: Incomplete Mocks

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.

Anti-Pattern 5: Integration Tests as Afterthought

BAD: "Implementation complete" without tests. FIX: TDD cycle: write test → implement → refactor → claim complete.

When Mocks Become Too Complex

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 Prevents These Anti-Patterns

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.

Quick Reference

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

Red Flags

• Assertion checks for *-mock test IDs
• Methods only called in test files
• Mock setup is >50% of test
• Test fails when you remove mock
• Can't explain why mock is needed
• Mocking "just to be safe"

The Bottom Line

Mocks 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.