# Tdd > Discipline for applying test-driven development where it adds regression value — preflight the change, reuse usable project test infrastructure, then red-green-refactor. Use before implementing features or fixing bugs to decide whether TDD applies, when practicing TDD, introducing project-appropriate testing for current work, or reviewing whether work was actually test-driven. - Skill: `edram/tdd` (Agent Skill, multi-file: 7 files) - Install (CLI): `npx skillmds@latest add edram/tdd` - Raw SKILL.md: https://api.skillmd.com/api/skills/edram/tdd/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: edram (https://skillmd.com/u/edram) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/edram/tdd --- A discipline an agent applies when adding or changing behavior. Start with a preflight: decide whether TDD adds regression value, then whether the project already has usable test infrastructure for that behavior. Only then enter RED → GREEN → REFACTOR. Synthesized from four public TDD skills (mattpocock, obra/superpowers, NousResearch/hermes-agent, affaan-m/ECC). - **Preflight before RED** — decide whether the change warrants TDD and whether the affected behavior is testable with the project's existing setup. - **Never introduce test infrastructure silently** — ask first; if declined, prioritize the current task with direct verification; if approved, present a project-specific engineering plan and wait for confirmation. - **Write the test first**, watch it fail, write the minimum code to pass, refactor on green. - **No production code without a failing test** — code written before its test gets deleted. - **One behavior per cycle** — vertical tracer bullets, never all-tests-then-all-code. - **Test observable behavior** through the public interface, not implementation details. - **A test only counts** once you've watched it fail for the *right* reason. > Pure markdown, language- and framework-agnostic — preflight selects the path; the cycle stays strict once TDD applies. ## Core | Topic | Description | Reference | |-------|-------------|-----------| | Preflight | Decide TDD value, detect usable project test infrastructure, ask before introducing it | [core-preflight](references/core-preflight.md) | | Cycle | RED → GREEN → REFACTOR mechanics, watch-it-fail mandate, per-cycle checklist | [core-cycle](references/core-cycle.md) | | Discipline | The Iron Law, delete-and-restart, rationalization rebuttals, red flags | [core-discipline](references/core-discipline.md) | | Tracer Bullets | Vertical slices vs horizontal slicing; planning what to test | [core-tracer-bullets](references/core-tracer-bullets.md) | ## Features | Topic | Description | Reference | |-------|-------------|-----------| | Good Tests | Behavior over implementation, the brittle-test smell, anti-patterns, mocking | [features-good-tests](references/features-good-tests.md) | | Coverage | Test types, edge/error coverage, the RED gate, evidence trail | [features-coverage](references/features-coverage.md) |