test-driven-development

Test-Driven Development

Mode Selection

Read stack.md and check the tdd: field. If the field is missing, default to recommended.

tdd: value	Behavior
strict	Iron law — no production code without a failing test. Delete and restart on violations.
recommended	TDD is the expected workflow. Warn when skipped, log the skip, but proceed.
off	No TDD enforcement. Tests are encouraged but not gated. Skip the rest of this skill.

If tdd: off — stop reading. Do not enforce any TDD discipline.

Anti-Pattern: Horizontal Slicing

Horizontal slicing — write all tests first, then all implementation: test1, test2, test3, test4, test5 → impl1, impl2, impl3, impl4, impl5 Tests describe imagined behavior. Discovery comes too late. Vertical slicing (tracer bullets) — one test, one impl, repeat: test1 → impl1 → test2 → impl2 → test3 → impl3 Each test responds to what the previous cycle taught you.

The rule: ONE test, ONE implementation, then the next test. Never batch. Each RED-GREEN cycle is a learning step — the next test is informed by what you just discovered.

The Cycle

RED — Write one failing test

• One behavior per test, clear name (e.g., 'rejects empty email with validation error')
• Real code preferred over mocks (mocks only if unavoidable)
• Run the test. Confirm it fails for the expected reason (feature missing, not typo/error)

GREEN — Write minimal code to pass

Write the simplest code to make the test pass. Nothing more.

// Just enough to pass async function retryOperation(fn, maxRetries = 3) { for (let i = 0; i < maxRetries; i++) { try { return await fn(); } catch (e) { if (i === maxRetries - 1) throw e; } } } // YAGNI — over-engineered beyond what the test requires async function retryOperation(fn, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; timeout?: number; }) { ... }

Run the test. Confirm it passes. Confirm all other tests still pass.

REFACTOR — Then repeat

Only after green: remove duplication, improve names, extract helpers. Keep tests green. Don't add behavior. Then write the next failing test for the next behavior.

When TDD Is Skipped (recommended mode only)

In recommended mode, if you write production code before a test:

1. Log the skip — note which function/method was written without test-first
2. Write the test immediately after — cover the code before moving on
3. Flag it in phase verification as a TDD skip with a brief reason

In strict mode, skips are not allowed. Delete the code and start over.

Reference Material

• Test quality: See skills/test-driven-development/references/test-quality.md — load when reviewing test quality
• Rationalization prevention: See skills/test-driven-development/references/rationalizations.md — load in strict mode when tempted to skip TDD

Verification Checklist

Before marking any implementation step complete:

• All tests pass, output clean (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and error paths covered
• Every new function/method has a test (strict + recommended)
• Tests written before implementation, or skip logged (recommended) / code deleted (strict)
• Each test failed for the expected reason before implementation (strict)

Debugging Integration

Bug found? Write a failing test reproducing it first. In recommended mode, strongly encouraged. In strict mode, mandatory.