Test-Driven Development

Write test first. Commit it red. Write minimal code to pass. Commit green. Refactor.

Core principle: tests verify behavior through public interfaces, not implementation. Code can change entirely; tests shouldn't. A test that breaks when you rename an internal function — with no behavior change — was testing implementation. Delete it.

No watched failure = no proof the test tests the right thing. Commit history is the evidence; hooks check its structure.

See tests.md for examples and mocking.md for mocking guidelines.

Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST COMMITTED FIRST.

Wrote code before the test? Delete it. Implement fresh from tests. Delete means delete. Exceptions (throwaway prototypes, generated code, config) need human sign-off. Thinking "skip TDD just this once"? That is rationalization.

1. Plan (before any code)

Use the project's domain glossary so test names and interface vocabulary match the codebase; respect ADRs in the area you touch.