Agent skill · software engineering · addyosmani
test-driven-development
Drives development with tests using the red-green-refactor loop. Use when implementing any logic, fixing any bug, or changing any behavior. Use when you need to prove that code works, when a bug report arrives, or when you're about to modify existing functionality.
Why this skill is useful
Provides specific patterns and commands for implementing test-driven development in various programming environments that the AI may not know.
What it needs
About 10k tokens when loaded. Last updated 2026-08-06. 83,053 stars on the source repository.
What this skill does
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 commands — README, CONTRIBUTING, and CI workflows show the commands that actually gate merges Run the repository's focused-test command during the loop and its full-suite command before completion. Never assume a default like npm test — a Gradle, Cargo, or pytest project has its own equivalent. The examples below use TypeScript for illustration; the workflow is identical in any language once you've discovered the project's own tooling. The TDD Cycle Step 1: RED — Write a Failing Test Write the test first. It must fail. A test that passes immediately proves nothing. …
How to use it
Reference it in AdaL, Claude Code, Cursor or any coding agent — nothing to install:
@skills addyosmani/test-driven-development