The Iron Law: no production code without a failing test first.
Red-Green-Refactor:
- RED — write the smallest test that captures the next bit of behavior and watch it fail for the right reason.
- GREEN — write the minimum code to make it pass.
- REFACTOR — clean up with the test as a safety net.
Rejected rationalizations: "it's too simple to test", "I'll add tests after", "the test is obvious". Each is a trap — write the test first anyway. One behavior per test; descriptive test names; keep tests fast and independent.
Source: github.com/obra/superpowers (MIT). Condensed + attributed for the corlinman marketplace; see ../../ATTRIBUTION.md.