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

The violation:

// ❌ BAD: Testing that the mock exists test('renders sidebar', () => { render(<Page />); expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument(); });

Why this is wrong:

• You're verifying the mock works, not that the component works

• Test passes when mock is present, fails when it's not

• Tells you nothing about real behavior

your human partner's correction: "Are we testing the behavior of a mock?"

The fix:

// ✅ GOOD: Test real component or don't mock it test('renders sidebar', () => { render(<Page />); // Don't mock sidebar expect(screen.getByRole('navigation')).toBeInTheDocument(); });

// OR if sidebar must be mocked for isolation: // Don't assert on the mock - test Page's behavior with sidebar present

Gate Function

BEFORE asserting on any mock element: Ask: "Am I testing real component behavior or just mock existence?"

IF testing mock existence: STOP - Delete the assertion or unmock the component

Test real behavior instead

Anti-Pattern 2: Test-Only Methods in Production

The violation:

// ❌ BAD: destroy() only used in tests class Session { async destroy() { // Looks like production API! await this._workspaceManager?.destroyWorkspace(this.id); // ... cleanup } }

// In tests afterEach(() => session.destroy());

Why this is wrong:

• Production class polluted with test-only code

• Dangerous if accidentally called in production

• Violates YAGNI and separation of concerns

• Confuses object lifecycle with entity lifecycle

The fix:

// ✅ GOOD: Test utilities handle test cleanup // Session has no destroy() - it's stateless in production

// In test-utils/ export async function cleanupSession(session: Session) { const workspace = session.getWorkspaceInfo(); if (workspace) { await workspaceManager.destroyWorkspace(workspace.id); } }

// In tests afterEach(() => cleanupSession(session));

Gate Function

BEFORE adding any method to production class: Ask: "Is this only used by tests?"

IF yes: STOP - Don't add it Put it in test utilities instead

Ask: "Does this class own this resource's lifecycle?"

IF no: STOP - Wrong class for this method

Anti-Pattern 3: Mocking Without Understanding

The violation:

// ❌ BAD: Mock breaks test logic test('detects duplicate server', () => { // Mock prevents config write that test depends on! vi.mock('ToolCatalog', () => ({ discoverAndCacheTools: vi.fn().mockResolvedValue(undefined) }));

await addServer(config); await addServer(config); // Should throw - but won't! });

Why this is wrong:

• Mocked method had side effect test depended on (writing config)

• Over-mocking to "be safe" breaks actual behavior

• Test passes for wrong reason or fails mysteriously

The fix:

// ✅ GOOD: Mock at correct level test('detects duplicate server', () => { // Mock the slow part, preserve behavior test needs vi.mock('MCPServerManager'); // Just mock slow server startup

await addServer(config); // Config written await addServer(config); // Duplicate detected ✓ });

Gate Function

BEFORE mocking any method: STOP - Don't mock yet

1. Ask: "What side effects does the real method have?"
2. Ask: "Does this test depend on any of those side effects?"
3. Ask: "Do I fully understand what this test needs?"

IF depends on side effects: Mock at lower level (the actual slow/external operation) OR use test doubles that preserve necessary behavior NOT the high-level method the test depends on

IF unsure what test depends on: Run test with real implementation FIRST Observe what actually needs to happen THEN add minimal mocking at the right level

Red flags: - "I'll mock this to be safe" - "This might be slow, better mock it" - Mocking without understanding the dependency chain

Anti-Pattern 4: Incomplete Mocks

The violation:

// ❌ BAD: Partial mock - only fields you think you need const mockResponse = { status: 'success', data: { userId: '123', name: 'Alice' } // Missing: metadata that downstream code uses };

// Later: breaks when code accesses response.metadata.requestId

Why this is wrong:

• Partial mocks hide structural assumptions - You only mocked fields you know about

• Downstream code may depend on fields you didn't include - Silent failures

• Tests pass but integration fails - Mock incomplete, real API complete

• False confidence - Test proves nothing about real behavior

The Iron Rule: Mock the COMPLETE data structure as it exists in reality, not just fields your immediate test uses.

The fix:

// ✅ GOOD: Mirror real API completeness const mockResponse = { status: 'success', data: { userId: '123', name: 'Alice' }, metadata: { requestId: 'req-789', timestamp: 1234567890 } // All fields real API returns };

Gate Function

BEFORE creating mock responses: Check: "What fields does the real API response contain?"

Actions: 1. Examine actual API response from docs/examples 2. Include ALL fields system might consume downstream 3. Verify mock matches real response schema completely

Critical: If you're creating a mock, you must understand the ENTIRE structure Partial mocks fail silently when code depends on omitted fields

If uncertain: Include all documented fields

Anti-Pattern 5: Integration Tests as Afterthought

The violation:

✅ Implementation complete ❌ No tests written "Ready for testing"

Why this is wrong:

• Testing is part of implementation, not optional follow-up

• TDD would have caught this

• Can't claim complete without tests

The fix:

TDD cycle:

1. Write failing test
2. Implement to pass
3. Refactor
4. THEN claim complete

When Mocks Become Too Complex

Warning signs:

• Mock setup longer than test logic

• Mocking everything to make test pass

• Mocks missing methods real components have

• Test breaks when mock changes

your human partner's question: "Do we need to be using a mock here?"

Consider: Integration tests with real components often simpler than complex mocks

TDD Prevents These Anti-Patterns

Why TDD helps:

• Write test first → Forces you to think about what you're actually testing

• Watch it fail → Confirms test tests real behavior, not mocks

• Minimal implementation → No test-only methods creep in

• Real dependencies → You see what the test actually needs before mocking

If you're testing mock behavior, you violated TDD - you added mocks without watching test fail against real code first.

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.