---
title: "TDD Guide — Agent Skill & Codex Plugin"
description: "Test-driven development skill for writing unit tests, generating test fixtures and mocks, analyzing coverage gaps, and guiding red-green-refactor. Agent skill for Claude Code, Codex CLI, Gemini CLI, OpenClaw."
---

# TDD Guide

<div class="page-meta" markdown>
<span class="meta-badge">:material-code-braces: Engineering - Core</span>
<span class="meta-badge">:material-identifier: `tdd-guide`</span>
<span class="meta-badge">:material-github: <a href="https://github.com/alirezarezvani/claude-skills/tree/main/engineering-team/skills/tdd-guide/SKILL.md">Source</a></span>
</div>

<div class="install-banner" markdown>
<span class="install-label">Install:</span> <code>claude /plugin install engineering-skills</code>
</div>


Test-driven development skill for generating tests, analyzing coverage, and guiding red-green-refactor workflows across Jest, Pytest, JUnit, and Vitest.

---

## Workflows

### Generate Tests from Code

1. Provide source code (TypeScript, JavaScript, Python, Java)
2. Specify target framework (Jest, Pytest, JUnit, Vitest)
3. Run `test_generator.py` with requirements
4. Review generated test stubs
5. **Validation:** Tests compile and cover happy path, error cases, edge cases

### Analyze Coverage Gaps

1. Generate coverage report from test runner (`npm test -- --coverage`)
2. Run `coverage_analyzer.py` on LCOV/JSON/XML report
3. Review prioritized gaps (P0/P1/P2)
4. Generate missing tests for uncovered paths
5. **Validation:** Coverage meets target threshold (typically 80%+)

### TDD New Feature

1. Write failing test first (RED)
2. Run `tdd_workflow.py --phase red` to validate
3. Implement minimal code to pass (GREEN)
4. Run `tdd_workflow.py --phase green` to validate
5. Refactor while keeping tests green (REFACTOR)
6. **Validation:** All tests pass after each cycle

---

## Examples

### Test Generation — Input → Output (Pytest)

**Input source function (`math_utils.py`):**
```python
def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b
```

**Command:**
```bash
python scripts/test_generator.py --input math_utils.py --framework pytest
```

**Generated test output (`test_math_utils.py`):**
```python
import pytest
from math_utils import divide

class TestDivide:
    def test_divide_positive_numbers(self):
        assert divide(10, 2) == 5.0

    def test_divide_negative_numerator(self):
        assert divide(-10, 2) == -5.0

    def test_divide_float_result(self):
        assert divide(1, 3) == pytest.approx(0.333, rel=1e-3)

    def test_divide_by_zero_raises_value_error(self):
        with pytest.raises(ValueError, match="Cannot divide by zero"):
            divide(10, 0)

    def test_divide_zero_numerator(self):
        assert divide(0, 5) == 0.0
```

---

### Coverage Analysis — Sample P0/P1/P2 Output

**Command:**
```bash
python scripts/coverage_analyzer.py --report lcov.info --threshold 80
```

**Sample output:**
```
Coverage Report — Overall: 63% (threshold: 80%)

P0 — Critical gaps (uncovered error paths):
  auth/login.py:42-58   handle_expired_token()       0% covered
  payments/process.py:91-110  handle_payment_failure()   0% covered

P1 — High-value gaps (core logic branches):
  users/service.py:77   update_profile() — else branch  0% covered
  orders/cart.py:134    apply_discount() — zero-qty guard  0% covered

P2 — Low-risk gaps (utility / helper functions):
  utils/formatting.py:12  format_currency()            0% covered

Recommended: Generate tests for P0 items first to reach 80% threshold.
```

---

## Key Tools

| Tool | Purpose | Usage |
|------|---------|-------|
| `test_generator.py` | Generate test cases from code/requirements | `python scripts/test_generator.py --input source.py --framework pytest` |
| `coverage_analyzer.py` | Parse and analyze coverage reports | `python scripts/coverage_analyzer.py --report lcov.info --threshold 80` |
| `tdd_workflow.py` | Guide red-green-refactor cycles | `python scripts/tdd_workflow.py --phase red --test test_auth.py` |
| `fixture_generator.py` | Generate test data and mocks | `python scripts/fixture_generator.py --entity User --count 5` |

Additional scripts: `framework_adapter.py` (convert between frameworks), `metrics_calculator.py` (quality metrics), `format_detector.py` (detect language/framework), `output_formatter.py` (CLI/desktop/CI output).

---

## Input Requirements

**For Test Generation:**
- Source code (file path or pasted content)
- Target framework (Jest, Pytest, JUnit, Vitest)
- Coverage scope (unit, integration, edge cases)

**For Coverage Analysis:**
- Coverage report file (LCOV, JSON, or XML format)
- Optional: Source code for context
- Optional: Target threshold percentage

**For TDD Workflow:**
- Feature requirements or user story
- Current phase (RED, GREEN, REFACTOR)
- Test code and implementation status

---

## Spec-First Workflow

TDD is most effective when driven by a written spec. The flow:

1. **Write or receive a spec** — stored in `specs/<feature>.md`
2. **Extract acceptance criteria** — each criterion becomes one or more test cases
3. **Write failing tests (RED)** — one test per acceptance criterion
4. **Implement minimal code (GREEN)** — satisfy each test in order
5. **Refactor** — clean up while all tests stay green

### Spec Directory Convention

```
project/
├── specs/
│   ├── user-auth.md          # Feature spec with acceptance criteria
│   ├── payment-processing.md
│   └── notification-system.md
├── tests/
│   ├── test_user_auth.py     # Tests derived from specs/user-auth.md
│   ├── test_payments.py
│   └── test_notifications.py
└── src/
```

### Extracting Tests from Specs

Each acceptance criterion in a spec maps to at least one test:

| Spec Criterion | Test Case |
|---------------|-----------|
| "User can log in with valid credentials" | `test_login_valid_credentials_returns_token` |
| "Invalid password returns 401" | `test_login_invalid_password_returns_401` |
| "Account locks after 5 failed attempts" | `test_login_locks_after_five_failures` |

**Tip:** Number your acceptance criteria in the spec. Reference the number in the test docstring for traceability (`# AC-3: Account locks after 5 failed attempts`).

> **Cross-reference:** See `engineering/spec-driven-workflow` for the full spec methodology, including spec templates and review checklists.

---

## Red-Green-Refactor Examples Per Language

### TypeScript / Jest

```typescript
// test/cart.test.ts
describe("Cart", () => {
  describe("addItem", () => {
    it("should add a new item to an empty cart", () => {
      const cart = new Cart();
      cart.addItem({ id: "sku-1", name: "Widget", price: 9.99, qty: 1 });

      expect(cart.items).toHaveLength(1);
      expect(cart.items[0].id).toBe("sku-1");
    });

    it("should increment quantity when adding an existing item", () => {
      const cart = new Cart();
      cart.addItem({ id: "sku-1", name: "Widget", price: 9.99, qty: 1 });
      cart.addItem({ id: "sku-1", name: "Widget", price: 9.99, qty: 2 });

      expect(cart.items).toHaveLength(1);
      expect(cart.items[0].qty).toBe(3);
    });

    it("should throw when quantity is zero or negative", () => {
      const cart = new Cart();
      expect(() =>
        cart.addItem({ id: "sku-1", name: "Widget", price: 9.99, qty: 0 })
      ).toThrow("Quantity must be positive");
    });
  });
});
```

### Python / Pytest (Advanced Patterns)

```python
# tests/conftest.py — shared fixtures
import pytest
from app.db import create_engine, Session

@pytest.fixture(scope="session")
def db_engine():
    engine = create_engine("sqlite:///:memory:")
    yield engine
    engine.dispose()

@pytest.fixture
def db_session(db_engine):
    session = Session(bind=db_engine)
    yield session
    session.rollback()
    session.close()

# tests/test_pricing.py — parametrize for multiple cases
import pytest
from app.pricing import calculate_discount

@pytest.mark.parametrize("subtotal, expected_discount", [
    (50.0, 0.0),       # Below threshold — no discount
    (100.0, 5.0),      # 5% tier
    (250.0, 25.0),     # 10% tier
    (500.0, 75.0),     # 15% tier
])
def test_calculate_discount(subtotal, expected_discount):
    assert calculate_discount(subtotal) == pytest.approx(expected_discount)
```

### Go — Table-Driven Tests

```go
// cart_test.go
package cart

import "testing"

func TestApplyDiscount(t *testing.T) {
    tests := []struct {
        name     string
        subtotal float64
        want     float64
    }{
        {"no discount below threshold", 50.0, 0.0},
        {"5 percent tier", 100.0, 5.0},
        {"10 percent tier", 250.0, 25.0},
        {"15 percent tier", 500.0, 75.0},
        {"zero subtotal", 0.0, 0.0},
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got := ApplyDiscount(tt.subtotal)
            if got != tt.want {
                t.Errorf("ApplyDiscount(%v) = %v, want %v", tt.subtotal, got, tt.want)
            }
        })
    }
}
```

---

## Bounded Autonomy Rules

When generating tests autonomously, follow these rules to decide when to stop and ask the user:

### Stop and Ask When

- **Ambiguous requirements** — the spec or user story has conflicting or unclear acceptance criteria
- **Missing edge cases** — you cannot determine boundary values without domain knowledge (e.g., max allowed transaction amount)
- **Test count exceeds 50** — large test suites need human review before committing; present a summary and ask which areas to prioritize
- **External dependencies unclear** — the feature relies on third-party APIs or services with undocumented behavior
- **Security-sensitive logic** — authentication, authorization, encryption, or payment flows require human sign-off on test scenarios

### Continue Autonomously When

- **Clear spec with numbered acceptance criteria** — each criterion maps directly to tests
- **Straightforward CRUD operations** — create, read, update, delete with well-defined models
- **Well-defined API contracts** — OpenAPI spec or typed interfaces available
- **Pure functions** — deterministic input/output with no side effects
- **Existing test patterns** — the codebase already has similar tests to follow

---

## Property-Based Testing

Property-based testing generates random inputs to verify invariants instead of relying on hand-picked examples. Use it when the input space is large and the expected behavior can be described as a property.

### Python — Hypothesis

```python
from hypothesis import given, strategies as st
from app.serializers import serialize, deserialize

@given(st.text())
def test_roundtrip_serialization(data):
    """Serialization followed by deserialization returns the original."""
    assert deserialize(serialize(data)) == data

@given(st.integers(), st.integers())
def test_addition_is_commutative(a, b):
    assert a + b == b + a
```

### TypeScript — fast-check

```typescript
import fc from "fast-check";
import { encode, decode } from "./codec";

test("encode/decode roundtrip", () => {
  fc.assert(
    fc.property(fc.string(), (input) => {
      expect(decode(encode(input))).toBe(input);
    })
  );
});
```

### When to Use Property-Based Over Example-Based

| Use Property-Based | Example |
|-------------------|---------|
| Data transformations | Serialize/deserialize roundtrips |
| Mathematical properties | Commutativity, associativity, idempotency |
| Encoding/decoding | Base64, URL encoding, compression |
| Sorting and filtering | Output is sorted, length preserved |
| Parser correctness | Valid input always parses without error |

---

## Mutation Testing

Mutation testing modifies your production code (creates "mutants") and checks whether your tests catch the changes. If a mutant survives (tests still pass), your tests have a gap that coverage alone cannot reveal.

### Tools

| Language | Tool | Command |
|----------|------|---------|
| TypeScript/JavaScript | **Stryker** | `npx stryker run` |
| Python | **mutmut** | `mutmut run --paths-to-mutate=src/` |
| Java | **PIT** | `mvn org.pitest:pitest-maven:mutationCoverage` |

### Why Mutation Testing Matters

- **100% line coverage != good tests** — coverage tells you code was executed, not that it was verified
- **Catches weak assertions** — tests that run code but assert nothing meaningful
- **Finds missing boundary tests** — mutants that change `<` to `<=` expose off-by-one gaps
- **Quantifiable quality metric** — mutation score (% mutants killed) is a stronger signal than coverage %

**Recommendation:** Run mutation testing on critical paths (auth, payments, data processing) even if overall coverage is high. Target 85%+ mutation score on P0 modules.

---

## Cross-References

| Skill | Relationship |
|-------|-------------|
| `engineering/spec-driven-workflow` | Spec → acceptance criteria → test extraction pipeline |
| `engineering-team/focused-fix` | Phase 5 (Verify) uses TDD to confirm the fix with a regression test |
| `engineering-team/senior-qa` | Broader QA strategy; TDD is one layer in the test pyramid |
| `engineering-team/code-reviewer` | Review generated tests for assertion quality and coverage completeness |
| `engineering-team/senior-fullstack` | Project scaffolders include testing infrastructure compatible with TDD workflows |

---

## Limitations

| Scope | Details |
|-------|---------|
| Unit test focus | Integration and E2E tests require different patterns |
| Static analysis | Cannot execute tests or measure runtime behavior |
| Language support | Best for TypeScript, JavaScript, Python, Java |
| Report formats | LCOV, JSON, XML only; other formats need conversion |
| Generated tests | Provide scaffolding; require human review for complex logic |

**When to use other tools:**
- E2E testing: Playwright, Cypress, Selenium
- Performance testing: k6, JMeter, Locust
- Security testing: OWASP ZAP, Burp Suite
