Test Traps

Test Traps

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();
  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();  // 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

Anti-Pattern 6: Timeout-Based Waiting

The violation:

// Wait an arbitrary amount of time, then check
await new Promise(r => setTimeout(r, 50));
const result = getResult();
expect(result).toBeDefined();

time.sleep(0.5)
assert get_result() is not None

Why this is wrong:

• Passes on fast machines, fails in CI under load (or vice versa).
• Either too short (flaky) or too long (slow suite) — every value is wrong somewhere.
• Hides the actual condition the test cares about.
• A real bug in the async machinery looks identical to a "too-short sleep."

The fix — poll for the actual condition:

await waitFor(() => getResult() !== undefined);
const result = getResult();
expect(result).toBeDefined();

| Scenario | Pattern | |----------|---------| | Wait for event | waitFor(() => events.find(e => e.type === 'DONE')) | | Wait for state | waitFor(() => machine.state === 'ready') | | Wait for count | waitFor(() => items.length >= 5) | | Wait for file | waitFor(() => fs.existsSync(path)) | | Complex condition | waitFor(() => obj.ready && obj.value > 10) |

Minimal generic poller:

async function waitFor(
  condition: () => T | undefined | null | false,
  description = 'condition',
  timeoutMs = 5000
): Promise {
  const start = Date.now();
  while (true) {
    const result = condition();
    if (result) return result;
    if (Date.now() - start > timeoutMs) {
      throw new Error(`Timeout waiting for ${description} after ${timeoutMs}ms`);
    }
    await new Promise(r => setTimeout(r, 10));
  }
}

See example.ts for domain-specific helpers (waitForEvent, waitForEventCount, waitForEventMatch) drawn from a real debugging session.

Polling pitfalls:

• Polling too fast (setTimeout(check, 1)) wastes CPU. Fix: poll every ~10ms.
• No timeout means a never-met condition hangs the suite forever. Fix: always include a timeout with a descriptive error.
• Stale data — if you read state once before the loop, you'll never see updates. Fix: call the getter inside the loop.

When an arbitrary timeout IS correct:

// Tool ticks every 100ms; observe 2 ticks of partial output.
await waitForEvent(manager, 'TOOL_STARTED'); // condition first
await new Promise(r => setTimeout(r, 200));  // then a documented, calculated wait
// 200ms = 2 ticks at the documented 100ms cadence

Requirements: (1) wait for the triggering condition first, (2) the delay is based on a known interval (not a guess), (3) a comment explains why a sleep is correct here.

Real-world impact (debugging session, 2025-10-03): 15 flaky tests across 3 files, pass rate 60% → 100%, suite 40% faster, race conditions gone.

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

| 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 | | Timeout-based waiting | Poll for the actual condition with waitFor() | | 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.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: hamr0
• Source: hamr0/liteagents
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.