| name | tdd |
| description | Build features and fix bugs using a strict Test-Driven Development workflow (Red-Green-Refactor), with regression-first bugfixing and explicit clarification when expected behavior is ambiguous. Use when Codex must implement or repair behavior with high confidence, test evidence, and minimal-risk changes. |
TDD
Use this skill to implement features and fix bugs with a test-first workflow.
Workflow
- Classify task and goal.
- Determine whether the request is
feature, bugfix, or mixed.
- Capture expected behavior, acceptance criteria, and non-functional constraints.
- If behavior is ambiguous, ask concise clarifying questions before writing tests.
- Resolve ambiguity before test design.
- Ask only high-impact questions that change expected behavior or test assertions.
- Ask up to 2 rounds of clarifying questions.
- If still unresolved, proceed with explicit assumptions and mark risk.
- Write failing test first (
Red).
- Add or update tests that express expected behavior before production changes.
- For bugfixes, reproduce the bug with a regression test that fails on current code.
- Prefer the smallest test scope that proves behavior.
- Implement minimal change (
Green).
- Modify production code only after the failing test exists.
- Make the minimal change needed for tests to pass.
- Avoid broad refactors during this step.
- Refactor safely (
Refactor).
- Improve design only with tests passing.
- Keep behavior unchanged while reducing complexity and duplication.
- Strengthen test coverage.
- Add edge, negative, and boundary cases relevant to risk.
- Validate that original failing scenario is permanently covered.
- Run verification.
- Run focused tests first, then broader suite as appropriate.
- If tests fail, return to
Red and iterate.
- Report outcome.
- Summarize failing test introduced, code change, and proof of pass state.
- List assumptions, residual risks, and follow-up tests if needed.
Mode Rules
- Never implement behavior without first expressing it in a failing test.
- For bugfixes, require a regression test that fails before the fix.
- Keep each cycle small and reversible.
- Prefer deterministic tests over brittle integration-only checks.
Output Rules
- Use tdd_output_template.md for final responses.
- Include the exact tests added/updated and what they verify.
- Explicitly separate facts from assumptions.
- If blocked by ambiguity, show the missing detail and why it affects test validity.
References
