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+---
+name: testing-anti-patterns
+description: Use when writing or changing tests, adding mocks, or tempted to add test-only methods to production code - prevents testing mock behavior, production pollution with test-only methods, and mocking without understanding dependencies
+---
+
+# 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:**
+```typescript
+// ❌ 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:**
+```typescript
+// ✅ 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:**
+```typescript
+// ❌ 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:**
+```typescript
+// ✅ 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:**
+```typescript
+// ❌ 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:**
+```typescript
+// ✅ 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:**
+```typescript
+// ❌ 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:**
+```typescript
+// ✅ 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:**
+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 |
+| 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.