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.claude-plugin/plugin.json Normal file
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{
"name": "tdd",
"description": "Introduces commands for test-driven development, common anti-patterns and skills for testing using subagents.",
"version": "1.0.0",
"author": {
"name": "Vlad Goncharov",
"email": "vlad.goncharov@neolab.finance"
},
"skills": [
"./skills"
]
}

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# tdd
Introduces commands for test-driven development, common anti-patterns and skills for testing using subagents.

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---
name: test-driven-development
description: Use when implementing any feature or bugfix, before writing implementation code - write the test first, watch it fail, write minimal code to pass; ensures tests actually verify behavior by requiring failure first
---
# Test-Driven Development (TDD)
## Overview
Write the test first. Watch it fail. Write minimal code to pass.
**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing.
**Violating the letter of the rules is violating the spirit of the rules.**
## When to Use
**Always:**
- New features
- Bug fixes
- Refactoring
- Behavior changes
**Exceptions (ask your human partner):**
- Throwaway prototypes
- Generated code
- Configuration files
Thinking "skip TDD just this once"? Stop. That's rationalization.
## The Iron Law
```
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
```
Write code before the test? Delete it. Start over.
**No exceptions:**
- Don't keep it as "reference"
- Don't "adapt" it while writing tests
- Don't look at it
- Delete means delete
Implement fresh from tests. Period.
## Red-Green-Refactor
```dot
digraph tdd_cycle {
rankdir=LR;
red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
verify_red [label="Verify fails\ncorrectly", shape=diamond];
green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
verify_green [label="Verify passes\nAll green", shape=diamond];
refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
next [label="Next", shape=ellipse];
red -> verify_red;
verify_red -> green [label="yes"];
verify_red -> red [label="wrong\nfailure"];
green -> verify_green;
verify_green -> refactor [label="yes"];
verify_green -> green [label="no"];
refactor -> verify_green [label="stay\ngreen"];
verify_green -> next;
next -> red;
}
```
### RED - Write Failing Test
Write one minimal test showing what should happen.
<Good>
```typescript
test('retries failed operations 3 times', async () => {
let attempts = 0;
const operation = () => {
attempts++;
if (attempts < 3) throw new Error('fail');
return 'success';
};
const result = await retryOperation(operation);
expect(result).toBe('success');
expect(attempts).toBe(3);
});
```
Clear name, tests real behavior, one thing
</Good>
<Bad>
```typescript
test('retry works', async () => {
const mock = jest.fn()
.mockRejectedValueOnce(new Error())
.mockRejectedValueOnce(new Error())
.mockResolvedValueOnce('success');
await retryOperation(mock);
expect(mock).toHaveBeenCalledTimes(3);
});
```
Vague name, tests mock not code
</Bad>
**Requirements:**
- One behavior
- Clear name
- Real code (no mocks unless unavoidable)
### Verify RED - Watch It Fail
**MANDATORY. Never skip.**
```bash
npm test path/to/test.test.ts
```
Confirm:
- Test fails (not errors)
- Failure message is expected
- Fails because feature missing (not typos)
**Test passes?** You're testing existing behavior. Fix test.
**Test errors?** Fix error, re-run until it fails correctly.
### GREEN - Minimal Code
Write simplest code to pass the test.
<Good>
```typescript
async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
for (let i = 0; i < 3; i++) {
try {
return await fn();
} catch (e) {
if (i === 2) throw e;
}
}
throw new Error('unreachable');
}
```
Just enough to pass
</Good>
<Bad>
```typescript
async function retryOperation<T>(
fn: () => Promise<T>,
options?: {
maxRetries?: number;
backoff?: 'linear' | 'exponential';
onRetry?: (attempt: number) => void;
}
): Promise<T> {
// YAGNI
}
```
Over-engineered
</Bad>
Don't add features, refactor other code, or "improve" beyond the test.
### Verify GREEN - Watch It Pass
**MANDATORY.**
```bash
npm test path/to/test.test.ts
```
Confirm:
- Test passes
- Other tests still pass
- Output pristine (no errors, warnings)
**Test fails?** Fix code, not test.
**Other tests fail?** Fix now.
### REFACTOR - Clean Up
After green only:
- Remove duplication
- Improve names
- Extract helpers
Keep tests green. Don't add behavior.
### Repeat
Next failing test for next feature.
## Good Tests
| Quality | Good | Bad |
|---------|------|-----|
| **Minimal** | One thing. "and" in name? Split it. | `test('validates email and domain and whitespace')` |
| **Clear** | Name describes behavior | `test('test1')` |
| **Shows intent** | Demonstrates desired API | Obscures what code should do |
## Why Order Matters
**"I'll write tests after to verify it works"**
Tests written after code pass immediately. Passing immediately proves nothing:
- Might test wrong thing
- Might test implementation, not behavior
- Might miss edge cases you forgot
- You never saw it catch the bug
Test-first forces you to see the test fail, proving it actually tests something.
**"I already manually tested all the edge cases"**
Manual testing is ad-hoc. You think you tested everything but:
- No record of what you tested
- Can't re-run when code changes
- Easy to forget cases under pressure
- "It worked when I tried it" ≠ comprehensive
Automated tests are systematic. They run the same way every time.
**"Deleting X hours of work is wasteful"**
Sunk cost fallacy. The time is already gone. Your choice now:
- Delete and rewrite with TDD (X more hours, high confidence)
- Keep it and add tests after (30 min, low confidence, likely bugs)
The "waste" is keeping code you can't trust. Working code without real tests is technical debt.
**"TDD is dogmatic, being pragmatic means adapting"**
TDD IS pragmatic:
- Finds bugs before commit (faster than debugging after)
- Prevents regressions (tests catch breaks immediately)
- Documents behavior (tests show how to use code)
- Enables refactoring (change freely, tests catch breaks)
"Pragmatic" shortcuts = debugging in production = slower.
**"Tests after achieve the same goals - it's spirit not ritual"**
No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"
Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.
Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).
30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.
## Common Rationalizations
| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests passing immediately prove nothing. |
| "Tests after achieve same goals" | Tests-after = "what does this do?" Tests-first = "what should this do?" |
| "Already manually tested" | Ad-hoc ≠ systematic. No record, can't re-run. |
| "Deleting X hours is wasteful" | Sunk cost fallacy. Keeping unverified code is technical debt. |
| "Keep as reference, write tests first" | You'll adapt it. That's testing after. Delete means delete. |
| "Need to explore first" | Fine. Throw away exploration, start with TDD. |
| "Test hard = design unclear" | Listen to test. Hard to test = hard to use. |
| "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. |
| "Manual test faster" | Manual doesn't prove edge cases. You'll re-test every change. |
| "Existing code has no tests" | You're improving it. Add tests for existing code. |
## Red Flags - STOP and Start Over
- Code before test
- Test after implementation
- Test passes immediately
- Can't explain why test failed
- Tests added "later"
- Rationalizing "just this once"
- "I already manually tested it"
- "Tests after achieve the same purpose"
- "It's about spirit not ritual"
- "Keep as reference" or "adapt existing code"
- "Already spent X hours, deleting is wasteful"
- "TDD is dogmatic, I'm being pragmatic"
- "This is different because..."
**All of these mean: Delete code. Start over with TDD.**
## Example: Bug Fix
**Bug:** Empty email accepted
**RED**
```typescript
test('rejects empty email', async () => {
const result = await submitForm({ email: '' });
expect(result.error).toBe('Email required');
});
```
**Verify RED**
```bash
$ npm test
FAIL: expected 'Email required', got undefined
```
**GREEN**
```typescript
function submitForm(data: FormData) {
if (!data.email?.trim()) {
return { error: 'Email required' };
}
// ...
}
```
**Verify GREEN**
```bash
$ npm test
PASS
```
**REFACTOR**
Extract validation for multiple fields if needed.
## Verification Checklist
Before marking work complete:
- [ ] Every new function/method has a test
- [ ] Watched each test fail before implementing
- [ ] Each test failed for expected reason (feature missing, not typo)
- [ ] Wrote minimal code to pass each test
- [ ] All tests pass
- [ ] Output pristine (no errors, warnings)
- [ ] Tests use real code (mocks only if unavoidable)
- [ ] Edge cases and errors covered
Can't check all boxes? You skipped TDD. Start over.
## When Stuck
| Problem | Solution |
|---------|----------|
| Don't know how to test | Write wished-for API. Write assertion first. Ask your human partner. |
| Test too complicated | Design too complicated. Simplify interface. |
| Must mock everything | Code too coupled. Use dependency injection. |
| Test setup huge | Extract helpers. Still complex? Simplify design. |
## Debugging Integration
Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.
Never fix bugs without a test.
## Final Rule
```
Production code → test exists and failed first
Otherwise → not TDD
```
No exceptions without your human partner's permission.
---
# 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.