Use this skill when implementing any logic, fixing any bug, or changing any behavior. Write a failing test before writing the code that makes it pass — PROVE it works before you trust it. Required for production code changes; skip only for pure configuration, documentation, or static content changes with no behavioral impact.
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# 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.
TDD applies to any language and any framework. The commands change; the cycle does not.
## 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 — see the `browser-testing-with-devtools` skill.
## 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:
…
The line at the top of this page pins the label git:20261019.fbfaf1a, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
GET https://markdownregistry.com/api/v1/artifacts/art_3czr2d3a4nmfkvur
GET https://markdownregistry.com/api/v1/resolve?ref=jpeetz/agent-skills/test-driven-development
GET https://markdownregistry.com/api/v1/blob/8005e4dc2612d3ae7194a01f9b26bc515d2cebc9211094255660db45da7c3d4f
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
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