# Principle Test Behavior Not Implementation (agent 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.

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.

## Key facts

| Fact | Value |
|---|---|
| Repository | https://github.com/backnotprop/pstack |
| Skill path | skills/principle-test-behavior-not-implementation/SKILL.md |
| Skill name | principle-test-behavior-not-implementation |
| GitHub stars | 1,172 |
| Installs on skills.sh | not listed |
| License | MIT |
| Skills in repo | 2 |
| Plugin marketplace | no |
| Official | no |
| Works with | Claude Code, Codex, Cursor, OpenCode |
| Category | Testing & QA |
| Last commit | Oct 4, 2026 |

## Add this skill

### Claude Code

```sh
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 skill folder and upload it under Customize > Skills > + > Upload a skill (code execution must be on).

### ChatGPT / Codex

```sh
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
```

### Cursor

```sh
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
```

Sources (checked 2026-10-07): https://code.claude.com/docs/en/skills, https://support.claude.com/en/articles/12512180-using-skills-in-claude, https://learn.chatgpt.com/docs/build-skills, https://cursor.com/docs/context/skills

---

Canonical page: https://appsgit.com/skills/backnotprop-principle-test-behavior-not-implementation
Source: appsgit (https://appsgit.com), the app store for github. Data from the GitHub API, refreshed nightly.
Machine access: JSON API https://appsgit.com/api/v1/apps (OpenAPI: https://appsgit.com/openapi.json), MCP server https://mcp.appsgit.com/mcp, full index https://appsgit.com/llms-full.txt.