# Test Driven Development

> This skill should be used when implementing any feature or bugfix, before writing implementation code

- **Type:** Skill
- **Install:** `agentstack add skill-etr-groundwork-test-driven-development`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [etr](https://agentstack.voostack.com/s/etr)
- **Installs:** 0
- **Category:** [Agent Skills](https://agentstack.voostack.com/c/agent-skills)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [etr](https://github.com/etr)
- **Source:** https://github.com/etr/groundwork/tree/main/skills/test-driven-development

## Install

```sh
agentstack add skill-etr-groundwork-test-driven-development
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# 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.**

**Test-quality standard:** Write tests to the same bar they'll be judged against. `${CLAUDE_PLUGIN_ROOT}/references/checklists/testing.md` is the canonical test-quality checklist — pyramid, DAMP, mocking gates, what not to test, coverage. This skill is the *workflow*; that file is the *standard* (the `test-quality-reviewer` agent grades against it).

## 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.

```typescript
test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts 

```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

**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.

```typescript
async function retryOperation(fn: () => Promise): Promise {
  for (let i = 0; i 

```typescript
async function retryOperation(
  fn: () => Promise,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise {
  // YAGNI
}
```
Over-engineered

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.

**Lint gate (if project has a linter):**

After refactoring, run the project's lint command on changed files only:

```bash
# Example: ruff check , eslint , cargo clippy
```

Catch lint violations NOW — before the next action item adds more code on top. The specific command comes from the project's CLAUDE.md. If you don't know it yet, run it once on a file you just edited. If it passes, move on. If it fails, fix before proceeding.

**Do NOT defer lint fixes to "after implementation."** Lint errors compound — a single import-style choice (e.g., `from __future__ import annotations`) can cascade through every subsequent file if not caught immediately.

### 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 |

## What NOT to Test

The canonical skip-list lives in `${CLAUDE_PLUGIN_ROOT}/references/checklists/testing.md`. **Rule of thumb:** if a test would only fail when someone *intentionally* changes behavior (static markup, CSS, element counts, presentational-only components), it's preventing change, not testing it — don't write it. DO test conditional rendering, computed values, user interactions, state transitions, edge cases, and business rules.

## 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 function/method with logic (conditionals, computation, side effects) 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. |

## Coverage Requirements

Canonical criteria — test pyramid, the four pillars, coverage completeness — live in `${CLAUDE_PLUGIN_ROOT}/references/checklists/testing.md`. Highlights:

### Target: 80%+ Coverage of Logic Paths
- Coverage means branch/logic coverage, not line coverage of markup
- Pure presentational components (no conditionals, no computed values) do NOT need unit tests — they are validated visually
- Do not write tests solely to increase coverage numbers
- All edge cases covered
- Error scenarios tested
- Boundary conditions verified

### Test Types

| Type | Scope | Examples |
|------|-------|----------|
| **Unit** | Individual functions, utilities, pure functions | Component logic, helpers |
| **Integration** | API endpoints, database operations, service interactions | External API calls |
| **E2E** | Critical user flows, complete workflows | Browser automation, UI interactions |

### E2E Test Pattern (Playwright)

```typescript
import { test, expect } from '@playwright/test'

test('user can search and filter markets', async ({ page }) => {
  await page.goto('/')
  await page.click('a[href="/markets"]')
  await expect(page.locator('h1')).toContainText('Markets')

  await page.fill('input[placeholder="Search"]', 'election')
  await page.waitForTimeout(600) // debounce

  const results = page.locator('[data-testid="market-card"]')
  await expect(results).toHaveCount(5, { timeout: 5000 })
})
```

### Coverage Verification

```bash
npm run test:coverage
# Verify 80%+ coverage achieved
```

### Coverage Thresholds (jest.config.js)

```json
{
  "coverageThresholds": {
    "global": {
      "branches": 80,
      "functions": 80,
      "lines": 80,
      "statements": 80
    }
  }
}
```

## Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

## Testing Anti-Patterns

Mocks are tools to isolate, not things to test. Before every mock, run these gates:

### Don't test mock behavior

```
BEFORE asserting on any mock element:
  Ask: "Am I testing real behavior or mock existence?"
  IF mock existence → delete the assertion or unmock the component
```

```typescript
// ❌ BAD: Testing that the mock exists
test('renders sidebar', () => {
  render();
  expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument();
});

// ✅ GOOD: If sidebar must be mocked, assert on Page's behavior
test('page passes user data to sidebar', () => {
  render();
  expect(Sidebar).toHaveBeenCalledWith(
    expect.objectContaining({ user: testUser }), expect.anything()
  );
});
```

### Don't add test-only methods to production classes

```
BEFORE adding any method to a production class:
  Ask: "Is this only used by tests?"
  IF yes → put it in test utilities instead
```

### Mock at the right level

```
BEFORE mocking any method:
  1. "What side effects does the real method have?"
  2. "Does this test depend on any of those side effects?"
  IF yes → mock lower (the actual slow/external operation), not the method the test depends on
```

```typescript
// ❌ BAD: Mock prevents config write that test depends on
test('detects duplicate server', async () => {
  vi.mock('ToolCatalog', () => ({
    discoverAndCacheTools: vi.fn().mockResolvedValue(undefined)
  }));
  await addServer(config);
  await addServer(config);  // Should throw - but won't!
});

// ✅ GOOD: Mock the slow part, preserve behavior test needs
test('detects duplicate server', async () => {
  vi.mock('MCPServerManager'); // Just mock slow server startup
  await addServer(config);    // Config written
  await addServer(config);    // Duplicate detected ✓
});
```

### Preserve real semantics in mocks

```
BEFORE creating any mock:
  Ask: "What type does the real implementation return?"
  Value vs. stream? Sync vs. async? Array vs. cursor?
  IF mock changes the abstraction type → fix it
```

```typescript
// ❌ BAD: Mock returns array, real implementation returns async cursor
const mockDb = {
  getUsers: vi.fn().mockResolvedValue([
    { id: '1', name: 'Alice' },
    { id: '2', name: 'Bob' },
  ])
};
// Test passes, but production breaks on 1000+ users when
// pagination and backpressure actually matter.

// ✅ GOOD: Mock preserves real cursor semantics
const mockDb = {
  getUsers: vi.fn().mockReturnValue({
    async *[Symbol.asyncIterator]() {
      yield { id: '1', name: 'Alice' };
      yield { id: '2', name: 'Bob' };
    }
  })
};
```

### Red flags

- Assertion checks for `*-mock` test IDs
- Methods only called in test files
- Mock setup is >50% of test code
- `mockResolvedValue()` when real code returns a stream
- Mocking "just to be safe"

## Final Rule

```
Production code → test exists and failed first
Otherwise → not TDD
```

No exceptions without your human partner's permission.

## Source & license

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

- **Author:** [etr](https://github.com/etr)
- **Source:** [etr/groundwork](https://github.com/etr/groundwork)
- **License:** MIT

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

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-etr-groundwork-test-driven-development
- Seller: https://agentstack.voostack.com/s/etr
- Browse the marketplace: https://agentstack.voostack.com/browse

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