# Standards Testing

> Prevent testing anti-patterns by ensuring tests verify real behavior, keeping production code free from test-only pollution, and enforcing thoughtful mocking strategies. Apply when writing test files, adding mocks or test doubles, or reviewing test implementation patterns.

- Skill: `majiayu000/standards-testing-3` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add majiayu000/standards-testing-3`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/standards-testing-3/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/majiayu000/standards-testing-3

---


# Testing Standards

**Rule:** Test real behavior, not mock behavior. Never pollute production code with test-only methods.

## When to use this skill

- When writing new test files or test cases in any testing framework (Jest, Vitest, pytest, RSpec, Go testing, JUnit)
- When modifying existing tests that use mocks, stubs, spies, or test doubles
- When considering whether to add a method or property to a production class that would only be used in tests
- When test setup involves creating mock objects or configuring mock behavior
- When deciding between testing with real dependencies versus mocked dependencies
- When tests are failing and you need to determine if the issue is in the code or in how mocks are configured
- When implementing fixtures, test data builders, or test utilities
- During test code reviews to ensure testing best practices are followed
- When test files contain assertions that check for mock existence (e.g., `expect(screen.getByTestId('component-mock'))`)
- When refactoring production code and tests need to be updated accordingly
- When choosing isolation strategies for unit tests versus integration tests

## Core Principles

1. **Mocks are isolation tools, not test subjects** - Assert on real behavior, not mock existence
2. **Production code stays pure** - Test utilities handle test-specific needs
3. **Understand before mocking** - Know what side effects you're removing
4. **Complete mocks or none** - Partial mocks create silent failures
5. **TDD prevents anti-patterns** - Write failing tests first to avoid testing mocks

## Anti-Pattern 1: Testing Mock Behavior

**Violation:**
```typescript
// ❌ BAD: Asserting on mock existence
test('renders sidebar', () => {
  render(<Page />);
  expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument();
});
```

**Why wrong:**
- Verifies mock works, not component behavior
- Test passes with mock present regardless of real functionality
- Provides zero confidence about production behavior

**Fix:**
```typescript
// ✅ GOOD: Test real component
test('renders sidebar', () => {
  render(<Page />);  // Use real sidebar
  expect(screen.getByRole('navigation')).toBeInTheDocument();
});

// OR if isolation required: Test Page's behavior, not mock presence
test('renders page with sidebar slot', () => {
  render(<Page />);
  expect(screen.getByTestId('sidebar-container')).toBeInTheDocument();
});
```

**Detection rule:**
```
IF assertion contains '*-mock' OR checks mock.toHaveBeenCalled():
  Ask: "Am I testing real behavior or mock existence?"
  IF testing mock existence → STOP, delete assertion or unmock
```

## Anti-Pattern 2: Test-Only Methods in Production Code

**Violation:**
```typescript
// ❌ BAD: Method only called from tests
class Session {
  async destroy() {
    await this._workspaceManager?.destroyWorkspace(this.id);
  }
}

afterEach(() => session.destroy());
```

**Why wrong:**
- Pollutes production API with test-specific code
- Risk of accidental production calls
- Violates YAGNI (You Aren't Gonna Need It)
- Confuses class responsibilities

**Fix:**
```typescript
// ✅ GOOD: Test utilities handle cleanup
// Session class has no destroy() method

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

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

**Detection rule:**
```
BEFORE adding method to production class:
  1. Search codebase: Is this method only called from test files?
  2. Ask: Does this class own this resource's lifecycle?

  IF only used in tests OR class doesn't own lifecycle:
    STOP - Create test utility function instead
```

## Anti-Pattern 3: Mocking Without Understanding Dependencies

**Violation:**
```typescript
// ❌ BAD: Mock removes side effect test depends on
test('detects duplicate server', () => {
  vi.mock('ToolCatalog', () => ({
    discoverAndCacheTools: vi.fn().mockResolvedValue(undefined)
  }));

  await addServer(config);
  await addServer(config);  // Should throw but won't - config never written!
});
```

**Why wrong:**
- Mocked method writes config that duplicate detection needs
- Over-mocking "to be safe" breaks test logic
- Test passes/fails for wrong reasons

**Fix:**
```typescript
// ✅ GOOD: Mock only external/slow operations
test('detects duplicate server', () => {
  vi.mock('MCPServerManager'); // Mock slow server startup only

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

**Decision process:**
```
BEFORE mocking:
  1. List method's side effects (DB writes, file I/O, API calls, state changes)
  2. Identify what test actually needs (duplicate detection needs config write)
  3. Mock ONLY external/slow operations, preserve test dependencies

  IF unsure what test needs:
    Run with real implementation FIRST
    Observe required behavior
    THEN mock minimally at lowest level

  Red flags indicating wrong approach:
    - "Mock this to be safe"
    - "Might be slow, better mock"
    - Can't explain why mocking
    - Mock setup longer than test
```

## Anti-Pattern 4: Incomplete Mock Data Structures

**Violation:**
```typescript
// ❌ BAD: Only fields you think you need
const mockResponse = {
  status: 'success',
  data: { userId: '123', name: 'Alice' }
  // Missing: metadata field
};

// Later: Silent failure when code accesses response.metadata.requestId
```

**Why wrong:**
- Partial mocks hide structural assumptions
- Downstream code may depend on omitted fields
- Tests pass, production fails
- False confidence in test coverage

**Fix:**
```typescript
// ✅ GOOD: Complete structure matching real API
const mockResponse = {
  status: 'success',
  data: { userId: '123', name: 'Alice' },
  metadata: { requestId: 'req-789', timestamp: 1234567890 }
};
```

**Mandatory process:**
```
BEFORE creating mock data:
  1. Check API documentation or real response examples
  2. Include ALL fields from actual structure
  3. Use realistic values (not null/undefined unless API returns them)
  4. Verify mock matches real schema completely

  IF uncertain about structure:
    - Examine real API response
    - Include all documented fields
    - Add comment linking to API docs
```

## Anti-Pattern 5: Tests as Afterthought

**Violation:**
```
Implementation complete → No tests → "Ready for review"
```

**Why wrong:**
- Testing is part of implementation, not optional
- Violates TDD workflow
- Cannot claim completion without tests

**Fix - TDD cycle:**
```
1. Write failing test (RED)
2. Implement minimal code (GREEN)
3. Refactor
4. THEN claim complete
```

## When Mocks Signal Deeper Issues

**Warning signs:**
- Mock setup > 50% of test code
- Mocking everything to make test pass
- Mocks missing methods real components have
- Test breaks when mock implementation changes
- Can't explain why each mock is needed

**Question to ask:** "Should this be an integration test with real components?"

Complex mocks often indicate integration tests would be simpler and more valuable.

## How TDD Prevents These Anti-Patterns

**TDD workflow naturally avoids anti-patterns:**

1. **Write test first** → Forces clarity on what you're testing
2. **Watch it fail** → Confirms test verifies real behavior, not mocks
3. **Minimal implementation** → Prevents test-only methods
4. **Real dependencies first** → See actual needs before mocking

**Key insight:** If you're testing mock behavior, you violated TDD by adding mocks before seeing test fail against real code.

## Detection Checklist

Before finalizing any test, verify:

- [ ] No assertions on mock existence (`*-mock` test IDs, `toHaveBeenCalled` without behavior verification)
- [ ] No methods in production classes only called from test files
- [ ] Understand what each mock removes (side effects, dependencies, behavior)
- [ ] Mock data structures complete (all fields from real API/response)
- [ ] Tests written before or during implementation (not after)
- [ ] Mock setup < 50% of test code (if more, consider integration test)
- [ ] Can explain necessity of each mock

## Quick Reference

| Anti-Pattern          | Detection Signal                              | Fix                                     |
| --------------------- | --------------------------------------------- | --------------------------------------- |
| Testing mock behavior | Assertions on `*-mock` elements or mock calls | Test real component or remove mock      |
| Test-only methods     | Method only in test file searches             | Move to test utilities                  |
| Blind mocking         | Can't explain mock purpose                    | Understand dependencies, mock minimally |
| Incomplete mocks      | Missing fields from real structure            | Include all documented fields           |
| Tests afterthought    | Implementation before tests                   | Follow TDD: test first                  |
| Over-complex mocks    | Setup > 50% of test                           | Use integration test                    |

## Red Flags - Stop and Reconsider

When you encounter these, stop and reassess your approach:

- Assertions check for `*-mock` test IDs
- Methods only called in test files
- Mock setup > 50% of test code
- Test fails when you remove mock
- Can't explain why mock is needed
- Mocking "just to be safe"
- Mock missing methods real component has

## Summary

**Mocks isolate code under test from external dependencies. They are not the subject of tests.**

**When uncertain:**
1. Write test with real dependencies first
2. Identify what's slow or external (API calls, DB queries, file I/O)
3. Mock only that specific dependency at lowest level
4. Verify test still validates real behavior, not mock presence