test-driven-development

# Test-Driven Development ## Overview Write a failing test before writing the code that makes it pass. For bug fixes, reproduce the bug with a test before attempting a fix. Tests are proof — "seems right" is not done. A codebase with good tests is an AI agent's superpower; a codebase without tests is a liability. ## When to Use - Implementing any new logic or behavior - Fixing any bug (the Prove-It Pattern) - Modifying existing functionality - Adding edge case handling - Any change that could break existing behavior **When NOT to use:** Pure configuration changes, documentation updates, or static content changes that have no behavioral impact. **Related:** For browser-based changes, combine TDD with runtime verification using Chrome DevTools MCP — see the Browser Testing section below. ## Discover the Stack First The TDD cycle is universal; the commands are not. Before writing the first test, discover how *this* repository tests, and use its commands for every RED, GREEN, and verification step: - **Language and build system** — `package.json`, `pom.xml`/`build.gradle`, `pyproject.toml`, `go.mod`, `Cargo.toml`, `Gemfile`, a `Makefile` - **Checked-in wrappers** — prefer `./gradlew`, `./mvnw`, `make test`, or a repo script over globally installed tools - **Test framework and configuration** — and how it runs a single focused test vs the full suite - **Existing conventions** — where tests live, how files are named, what patterns neighboring tests follow - **Documented co...