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test-driven-development

Use when implementing any feature or bugfix, before writing implementation code

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# Test-Driven Development (TDD)

## Overview

Write the test first. Watch it fail. Write minimal code to pass.

**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing.

**A strong default in service of shipping — not a law that outranks it.** TDD earns its keep where a failing test pins down behavior you'd otherwise get wrong: core logic, contracts, tricky edge cases. Match it to stakes — reach for it where test-first genuinely de-risks the change; throwaway prototypes, one-line copy tweaks, and obvious glue don't need the ceremony. Don't dodge it with synonyms; equally, don't let it block you from shipping the working thing.

## When to Use

**Use test-first for selected behavior where regression risk warrants it:**
- New features