experiment: ground-up two-principle rewrite of writing-good-tests

Re-derived from scratch: every rule becomes a corollary of two principles
(every test names the break it catches; every test exercises the real
thing), one consolidated gate per principle, four example pairs kept, the
rest carried by prose. Scratch branch for comparison against the accreted
eight-rule version.

skills/test-driven-development/SKILL.md

@@ -203,6 +203,12 @@ Next failing test for next feature.
 | **Clear** | Name describes behavior | `test('test1')` |
 | **Shows intent** | Demonstrates desired API | Obscures what code should do |
 
+When writing or changing any test, read [writing-good-tests.md](writing-good-tests.md) for the rules that keep tests honest:
+- Name the production change that would make the test fail — before writing it
+- Assert on real behavior, never on mock behavior
+- Keep test-only code in test utilities, out of production classes
+- Understand a dependency's side effects before mocking it
+
 ## Why Order Matters
 
 **"I'll write tests after to verify it works"**
@@ -354,13 +360,6 @@ Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix
 
 Never fix bugs without a test.
 
-## Testing Anti-Patterns
-
-When adding mocks or test utilities, read [testing-anti-patterns.md](testing-anti-patterns.md) to avoid common pitfalls:
-- Testing mock behavior instead of real behavior
-- Adding test-only methods to production classes
-- Mocking without understanding dependencies
-
 ## Final Rule
 
 ```

skills/test-driven-development/testing-anti-patterns.md

@@ -1,299 +0,0 @@
-# Testing Anti-Patterns
-
-**Load this reference when:** writing or changing tests, adding mocks, or tempted to add test-only methods to production code.
-
-## 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.

skills/test-driven-development/writing-good-tests.md

@@ -0,0 +1,198 @@
+# Writing Good Tests
+
+**Load this reference when:** writing or changing tests, adding mocks, or
+adding cleanup/helper methods for tests.
+
+## Overview
+
+A test exists to catch a specific break. Two principles govern everything
+here:
+
+```
+1. Every test names the break it catches
+2. Every test exercises the real thing
+```
+
+Strict TDD produces both naturally: a test written first and watched
+failing against real code has already proven it can fail, and only earns
+a mock when the real dependency proves slow or external.
+
+## Principle 1: Name the Break
+
+Before writing the test body, answer: **what production change should
+make this test fail — and is that change a bug or a decision?** A test
+earns its place by catching a wrong branch, missing side effect, wrong
+argument, boundary case, or broken contract.
+
+**Derive expectations independently.** Use literals and hand-checked
+fixtures; table-driven tests with literal `want` values are the preferred
+shape. An expectation computed by the code under test — or its helpers —
+passes no matter what that code does:
+
+```typescript
+// ❌ Mirror assertion: the same builder computes both sides — always true
+const expected = buildSearchQuery({ tag: 'urgent' });
+expect(buildSearchQuery({ tag: 'urgent' })).toBe(expected);
+
+// ✅ Hand-derived literal
+expect(buildSearchQuery({ tag: 'urgent' })).toBe('tag:"urgent"');
+```
+
+**No change detectors.** If only intentional decisions can fail a test —
+a constant's value, exact message wording, private structure — it fires
+on redesign and sleeps through bugs. Test the behavior that depends on
+the decision: not `expect(MAX_RETRIES).toBe(5)` but "a failing call is
+retried 5 times and the 6th attempt never happens."
+
+**Behavior, not text.** Asserting that a script, skill, or config
+contains an exact line proves only that the source is the source. Run
+scripts against controlled inputs and assert outputs, side effects, or
+exit codes. Documents that instruct agents are tested by the consuming
+agent's behavior (superpowers:writing-skills); prose for humans earns no
+test at all.
+
+**Your code, not the framework.** Test the contract your code makes at
+its boundaries — the route you register, the query you emit, the payload
+you produce. Upstream mechanics are their maintainers' tests to write
+(the classic: asserting your router invokes a registered handler — that
+is the framework's test, not yours). When upstream behavior genuinely
+surprised you, write one narrow characterization test naming the
+assumption. The same boundary applies inside your code: constructors,
+getters, constants, and trivial forwarding earn tests only when they
+validate, normalize, default, derive, enforce, or cause side effects —
+otherwise assert the first consumer-visible result that depends on them.
+
+### Gate Function
+
+```
+BEFORE writing the test body:
+  Name the production change that would make this test fail.
+
+  Cannot name one            → redesign around an observable behavior
+  "The source text changed"  → run the artifact and assert its effects
+  Only intentional decisions → change detector; test the behavior
+                               that depends on the decision
+
+  Confirm the expected value is derived without the code under test.
+  IF it reuses the code's logic or helpers:
+    Replace it with a literal or hand-checked fixture
+```
+
+## Principle 2: Exercise the Real Thing
+
+**The mock earns no assertions.** A mock assertion passes when the mock
+is present and fails when it is absent — it says nothing about the
+component. Assert the real component's behavior; if the mock is what you
+are checking, unmock it or delete the assertion.
+
+```typescript
+// ✅ Real behavior
+expect(screen.getByRole('navigation')).toBeInTheDocument();
+
+// ❌ Mock existence
+expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument();
+```
+
+**your human partner's correction:** "Are we testing the behavior of a
+mock?"
+
+**Mock at the right level.** Learn every side effect of the real method
+before replacing it; mock the slow or external operation and keep what
+the test depends on real. When unsure, run the test against the real
+implementation first and observe what actually needs to happen.
+
+```typescript
+// ❌ The mock swallows the config write that duplicate detection reads
+vi.mock('ToolCatalog', () => ({
+  discoverAndCacheTools: vi.fn().mockResolvedValue(undefined)
+}));
+
+// ✅ Mock only the slow server startup; the config write stays real
+vi.mock('MCPServerManager');
+```
+
+**Make doubles specific.** When arguments, call counts, or ordering are
+part of the contract, assert them — a fake that accepts anything verifies
+nothing. Give each branch (success, error, malformed) its own fixture or
+spy, so the wrong branch cannot satisfy the expectation.
+
+**Mirror real data completely.** Mock the complete structure as it exists
+in reality — all documented fields — not just the ones your test reads.
+Partial mocks fail silently when downstream code reads an omitted field:
+the test passes while integration breaks.
+
+**Production classes carry production methods only.** Cleanup that only
+tests need lives in test utilities, never as a `destroy()` on the
+production class. Ask: is this method called only from tests? Does this
+class own this resource's lifecycle? Wrong answers → test utility.
+
+**Prefer real components over complex mocks.** When mock setup outgrows
+the test logic, mocks miss methods the real components have, or tests
+break when the mock changes, switch to an integration test with real
+components. **your human partner's question:** "Do we need to be using a
+mock here?"
+
+### Gate Function
+
+```
+BEFORE adding a mock or test helper:
+  List the real method's side effects; keep the ones the test
+  depends on real — mock the slow/external level below them.
+
+  Mock responses mirror the complete real structure.
+
+  A method only tests call lives in test utilities, not production.
+
+  About to assert on the mock itself?
+    Unmock it or delete the assertion.
+```
+
+## Tests Ship With the Implementation
+
+The TDD cycle — failing test, minimal implementation, refactor — is what
+"complete" means. Ship the tests the behavior needs and only those:
+trivial code and human prose earn none, and a test written to satisfy
+process costs maintenance forever.
+
+## The Mutation Check
+
+Before finishing, mentally mutate the production code; at least one test
+should fail for each realistic mutation:
+
+- Wrong constant or argument
+- Wrong branch handler
+- Missing state change or side effect
+- Empty or default return
+- Missing validation for zero, empty, nil, unauthorized, or malformed input
+
+A mutation nothing catches marks the behavior as unprotected — or the
+test as tautological.
+
+## Quick Reference
+
+| When you... | Do |
+|-------------|-----|
+| Write any test | Name the break it catches — a bug, not a decision |
+| Build an expected value | Derive it by hand; never with the code under test |
+| Test a script or document | Run it / pressure-test its consumer; never grep its text |
+| Reach for a dependency test | Test your boundary contract, not their documented mechanics |
+| Want to assert on a mocked element | Test the real component, or unmock it |
+| Are about to mock a method | Learn its side effects; mock the slow/external level |
+| Build a mock response | Mirror the real structure completely |
+| Need cleanup only tests use | Put it in test utilities |
+| Watch mock setup balloon | Switch to an integration test with real components |
+| Finish a test file | Run the mutation check |
+
+## Warning Signs
+
+- Setup and assertion share the same object, guaranteeing equality
+- The test can fail only through a panic, crash, or missing selector
+- The test fails on every intentional change, never on accidental breakage
+- Expected values are hidden behind loops, builders, or helpers
+- The test greps source text, or asserts a removed symbol stays removed
+- The test would still matter if only the framework remained
+- The test exists for coverage, checking no side effect or outcome
+- An assertion checks a `*-mock` test ID, or fails if you remove the mock
+- A method is called only from test files
+- Mock setup is more than half the test, or you can't explain why the mock is needed
+- Mocking "just to be safe"