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Principle Test Behavior Not Implementation

Principle Test Behavior Not Implementation is an agent skill (a SKILL.md file) from backnotprop/pstack. Apply when you write, change, or keep a test. It works with Claude Code, Codex, Cursor and OpenCode and has 1,172 GitHub stars across a repository of 2 listed skills.

github.com/backnotprop/pstack/skills/principle-test-behavior-not-implementation (opens in a new tab)

View on GitHub (opens in a new tab)

Add this skill

Claude

Claude Code loads skills from ~/.claude/skills/ (all projects) or .claude/skills/ (one project):

git clone --depth 1 https://github.com/backnotprop/pstack.git
cp -r pstack/skills/principle-test-behavior-not-implementation ~/.claude/skills/principle-test-behavior-not-implementation   # personal, or .claude/skills in a project

In the Claude apps, zip the principle-test-behavior-not-implementation folder and upload it under Customize > Skills > + > Upload a skill (code execution must be on).

ChatGPT / Codex

Codex reads skills from .agents/skills/ in a repo or ~/.agents/skills/ for every project:

git clone --depth 1 https://github.com/backnotprop/pstack.git
cp -r pstack/skills/principle-test-behavior-not-implementation .agents/skills/principle-test-behavior-not-implementation   # repo; ~/.agents/skills for all projects

Standalone skills also load in the ChatGPT desktop app.

Cursor

Cursor loads skills from .cursor/skills/ (or ~/.cursor/skills/) and also reads .claude/skills/:

git clone --depth 1 https://github.com/backnotprop/pstack.git
cp -r pstack/skills/principle-test-behavior-not-implementation .cursor/skills/principle-test-behavior-not-implementation   # project; ~/.cursor/skills for all projects

Source (checked Oct 7, 2026): code.claude.com/docs/en/skills (opens in a new tab), support.claude.com/en/articles/12512180-using-skills-in-claude (opens in a new tab), learn.chatgpt.com/docs/build-skills (opens in a new tab), cursor.com/docs/context/skills (opens in a new tab)

What this skill does

Apply when you write, change, or keep a test. Call the code the way its users do and assert the result they observe against a literal expected value. If the test would still pass when every imported function returns undefined, rewrite the assertion or delete the test. A test calls the code the way its users do and asserts the result they observe against a literal expected value. A test that asserts which calls the code made, or restates a constant the code contains, does neither.

More skills in backnotprop/pstack

2 skills are listed from this repository.

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FAQ

Principle Test Behavior Not Implementation FAQ

Still curious? Email info@appsgit.com.

What is the Principle Test Behavior Not Implementation skill?

Principle Test Behavior Not Implementation is an agent skill (a SKILL.md file) from backnotprop/pstack. Apply when you write, change, or keep a test. It works with Claude Code, Codex, Cursor and OpenCode and has 1,172 GitHub stars across a repository of 2 listed skills. Its SKILL.md lives at github.com/backnotprop/pstack/skills/principle-test-behavior-not-implementation.

How do I install the Principle Test Behavior Not Implementation skill?

Copy the principle-test-behavior-not-implementation folder (the one containing SKILL.md) into ~/.claude/skills/ for Claude Code, .agents/skills/ for Codex or .cursor/skills/ for Cursor. The agent picks it up automatically when a task matches its description.

Is the Principle Test Behavior Not Implementation skill free?

Yes. The repository is open source under the MIT license.

Is Principle Test Behavior Not Implementation maintained?

The repository's most recent commit was on Oct 4, 2026. appsgit only lists skills from repositories with a commit in the last six months.