Anti Patterns

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

Python equivalent:

# ❌ BAD: Testing that the mock was called, not actual behavior
def test_sends_notification(mocker: MockerFixture) -> None:
    mock_sender = mocker.patch("app.notifications.EmailSender")
    notify_user(user_id="123")
    mock_sender.assert_called_once()  # Tests mock, not outcome!

# ✅ GOOD: Test observable behavior
def test_sends_notification(smtp_server: SMTPServer) -> None:
    notify_user(user_id="123")
    assert len(smtp_server.outbox) == 1
    assert smtp_server.outbox[0].to == "user@example.com"

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));

Python equivalent:

# ❌ BAD: _cleanup() pollutes production class
class DatabaseSession:
    def _cleanup(self) -> None:  # Only tests call this!
        self._connection.rollback()
        self._connection.close()

# ✅ GOOD: Fixture handles lifecycle
@pytest.fixture
def db_session(database: Database) -> Iterator[DatabaseSession]:
    session = database.create_session()
    try:
        yield session
    finally:
        session.connection.rollback()
        session.connection.close()

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 ✓
});

Python equivalent:

# ❌ BAD: Over-mocking breaks test invariants
def test_rejects_duplicate_config(mocker: MockerFixture) -> None:
    mocker.patch("app.discovery.scan_plugins")  # Breaks config persistence!

    add_plugin(config)
    add_plugin(config)  # Should raise DuplicateError - but won't!

# ✅ GOOD: Mock only the slow external call
def test_rejects_duplicate_config(
    mocker: MockerFixture,
    tmp_path: Path,
) -> None:
    mocker.patch("app.discovery.fetch_from_network")  # Just the I/O

    add_plugin(config)
    with pytest.raises(DuplicateConfigError):
        add_plugin(config)

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
};

Python equivalent:

# ❌ BAD: Partial mock missing fields downstream code needs
mock_response: dict[str, Any] = {
    "status": "success",
    "data": {"user_id": "123", "name": "Alice"},
    # Missing: metadata.request_id used by logging
}

# ✅ GOOD: Use typed factory for complete mocks
def make_api_response(
    *,
    user_id: str,
    name: str,
    request_id: str | None = None,
) -> APIResponse:
    return {
        "status": "success",
        "data": {"user_id": user_id, "name": name},
        "metadata": {
            "request_id": request_id or str(uuid4()),
            "timestamp": int(time.time()),
        },
    }

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:

1. Write test first → Forces you to think about what you're actually testing
2. Watch it fail → Confirms test tests real behavior, not mocks
3. Minimal implementation → No test-only methods creep in
4. 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

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.