Principle Redesign From First Principles
cursor/plugins
Apply when integrating a new requirement into an existing design.
Apply when you write, change, or keep a test. An agent skill from backnotprop/pstack.
MITAuto-check passed
Install Principle Test Behavior Not Implementation
$ npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install backnotprop/pstack principle-test-behavior-not-implementation --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/backnotprop/pstack.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/principle-test-behavior-not-implementation .claude/skills/principle-test-behavior-not-implementation && rm -rf skills-srcUse ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.
Claude Code skills documentation · loads skills from .claude/skills/
Install the "principle-test-behavior-not-implementation" agent skill from https://github.com/backnotprop/pstack/tree/main/skills/principle-test-behavior-not-implementation into .claude/skills/principle-test-behavior-not-implementation/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "principle-test-behavior-not-implementation", then confirm the skill loads.Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$skill-installer install https://github.com/backnotprop/pstack/tree/main/skills/principle-test-behavior-not-implementationType this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.
$ npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install backnotprop/pstack principle-test-behavior-not-implementation --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/backnotprop/pstack.git skills-src && mkdir -p .agents/skills && cp -r skills-src/skills/principle-test-behavior-not-implementation .agents/skills/principle-test-behavior-not-implementation && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "principle-test-behavior-not-implementation" agent skill from https://github.com/backnotprop/pstack/tree/main/skills/principle-test-behavior-not-implementation into .agents/skills/principle-test-behavior-not-implementation/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "principle-test-behavior-not-implementation", then confirm the skill loads.Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install backnotprop/pstack principle-test-behavior-not-implementation --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/backnotprop/pstack.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/skills/principle-test-behavior-not-implementation .cursor/skills/principle-test-behavior-not-implementation && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "principle-test-behavior-not-implementation" agent skill from https://github.com/backnotprop/pstack/tree/main/skills/principle-test-behavior-not-implementation into .cursor/skills/principle-test-behavior-not-implementation/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "principle-test-behavior-not-implementation", then confirm the skill loads.Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gemini skills install https://github.com/backnotprop/pstack.git --path skills/principle-test-behavior-not-implementation--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install backnotprop/pstack principle-test-behavior-not-implementation --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/backnotprop/pstack.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/skills/principle-test-behavior-not-implementation .gemini/skills/principle-test-behavior-not-implementation && rm -rf skills-srcUse ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "principle-test-behavior-not-implementation" agent skill from https://github.com/backnotprop/pstack/tree/main/skills/principle-test-behavior-not-implementation into .gemini/skills/principle-test-behavior-not-implementation/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "principle-test-behavior-not-implementation", then confirm the skill loads.Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gh skill install backnotprop/pstack principle-test-behavior-not-implementationInstalls for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).
$ npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/backnotprop/pstack.git skills-src && mkdir -p .github/skills && cp -r skills-src/skills/principle-test-behavior-not-implementation .github/skills/principle-test-behavior-not-implementation && rm -rf skills-srcUse ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "principle-test-behavior-not-implementation" agent skill from https://github.com/backnotprop/pstack/tree/main/skills/principle-test-behavior-not-implementation into .github/skills/principle-test-behavior-not-implementation/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "principle-test-behavior-not-implementation", then confirm the skill loads.GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install backnotprop/pstack principle-test-behavior-not-implementation --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/backnotprop/pstack.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/skills/principle-test-behavior-not-implementation .opencode/skills/principle-test-behavior-not-implementation && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "principle-test-behavior-not-implementation" agent skill from https://github.com/backnotprop/pstack/tree/main/skills/principle-test-behavior-not-implementation into .opencode/skills/principle-test-behavior-not-implementation/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "principle-test-behavior-not-implementation", then confirm the skill loads.OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
Facts
- Skill name
principle-test-behavior-not-implementation- GitHub stars
- 1.5k
- Used in
- 1 other repo
- Token cost
- ~644 tokens
- SKILL.md length
- 319 words
- Files
- 1
- Repo updated
- First seen
- Licence
- MIT
At a glance
Apply when you write, change, or keep a test. An agent skill from backnotprop/pstack.
- Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
What it does
Principle Test Behavior Not Implementation is an agent skill from backnotprop/pstack. 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.
Its SKILL.md is about 640 tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.
The repository describes itself as: Skills and principles for rigorous AI-assisted engineering. The licence is MIT.
Example prompts
- “/principle-test-behavior-not-implementation”
What it can do on your machine
Read from SKILL.md and the folder at commit 3a60467. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md.
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.
Context cost
Principle Test Behavior Not Implementation loads about 644 tokens when it runs. Until then it costs about 78 tokens; SKILL.md has 319 words of instructions outside code blocks.
Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.
Safety
Auto-check passed
The automated check found no risky patterns in SKILL.md.
Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.
SKILL.md
The full file from backnotprop/pstack at commit 3a60467, republished under its MIT licence (© backnotprop). 319 words, ~644 tokens.
Download SKILL.mdSave it as
.claude/skills/principle-test-behavior-not-implementation/SKILL.md (or your agent's skills folder).- name
- principle-test-behavior-not-implementation
- description
- 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.
- disable-model-invocation
- true
Test Behavior, Not Implementation
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.
The check: before you keep a test, ask whether it would still pass if every function it imports returned undefined. If yes, it observes no behavior and cannot fail for a defect. Rewrite the assertion or delete the test.
Why: A test that cannot fail for a defect costs CI time and review attention and catches nothing. A constant pin also fails when someone edits the constant or the prompt it restates, so it prevents that edit.
Five shapes that still pass when every imported function returns undefined:
- Weak or no assertion. No
expect, or onlytoBeDefined,toBeTruthy,not.toThrow,toBeInstanceOf,toBeGreaterThan(0). - Mock or absence only. Only
toHaveBeenCalled,not.toHaveBeenCalled,toBeUndefined,toEqual([]),toHaveLength(0),not.toBe(wrongValue). - Self-referential. The expected value comes from the code under test:
expect(f(a)).toBe(f(a)),expect(parsed.url).toBe(buildUrl(...)). - Constant pin. The assertion restates a hand-maintained constant, config default, table row, or prompt string:
expect(LIMITS.maxTools).toBe(8),expect(PROMPT).toContain("You are"). - Fixture asserts fixture. The assertion reads data the test built or a value computed in
beforeEach, and the subject never runs inside the body.
The fix: call the subject inside the test body with one concrete input and assert the literal output or the observable effect, expect(slugify("Hello, World!")).toBe("hello-world"). For an absence, assert the presence on the other input in the same test. For a constant, test the mechanism that reads it with one input instead of restating the value. For a mock, assert the payload it received or the state after the call, not that it was called. When no such assertion exists, delete the test.
Keep a test of a relation across a table's rows (a key present in two tables, a parent that exists), and a compile-time check in a *.test-d.ts file.
© backnotprop, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
Just SKILL.md in skills/principle-test-behavior-not-implementation of backnotprop/pstack.
Open the folder on GitHubat commit 3a60467
Used in 1 other repository
We found 1 copy of this SKILL.md (exact, near-identical or edited) in other folders, from 1 other GitHub owner. This page covers the copy in backnotprop/pstack, which our catalogue first saw on October 11, 2026.
Compare with similar skills
Principle Test Behavior Not Implementation next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Principle Test Behavior Not Implementation this skillbacknotprop/pstack | 1.5k | 1 repos | ~644 | Automated safety check: Pass | MIT | |
| Principle Redesign From First Principlescursor/plugins | 11k | 8 repos | ~211 | Automated safety check: Pass | None | |
| Uxui Principlessickn33/agentic-awesome-skills | 47k | 2 repos | ~548 | Automated safety check: Pass | MIT | |
| First Principles Thinkingmindfold-ai/Trellis | 15k | — | ~4.1k | Automated safety check: Pass | MIT | |
| Keep Trackdavidondrej/skills | 4.1k | — | ~132 | Automated safety check: Pass | MIT | |
| Editorial Burgundy Principles Templatenexu-io/open-design | 100k | — | ~461 | Automated safety check: Pass | Apache-2.0 |
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Evaluate interfaces against 168 research-backed UX/UI principles, detect antipatterns, and inject UX context into AI coding sessions.
First Principles Thinking
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Systematic first principles thinking for any problem domain.
Keep Track
davidondrej/skills
Keep track of open topics and decisions. An agent skill from davidondrej/skills.
Editorial Burgundy Principles Template
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Editorial studio deck template in burgundy / blush / muted-gold palette.
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Define actionable principles that resolve trade-offs when the team disagrees.
Questions about Principle Test Behavior Not Implementation
What does Principle Test Behavior Not Implementation do?
Apply when you write, change, or keep a test. An agent skill from backnotprop/pstack. Principle Test Behavior Not Implementation is an agent skill from backnotprop/pstack. Apply when you write, change, or keep a test.
How do I install Principle Test Behavior Not Implementation in Claude Code?
Run `npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a claude-code`. Or copy the skill folder (skills/principle-test-behavior-not-implementation in backnotprop/pstack) into .claude/skills/principle-test-behavior-not-implementation in your project. Claude Code loads it when a task matches its description.
How do I install Principle Test Behavior Not Implementation in Codex?
Run `npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a codex`. Or copy the skill folder (skills/principle-test-behavior-not-implementation in backnotprop/pstack) into .agents/skills/principle-test-behavior-not-implementation in your project. Codex loads it when a task matches its description.
Can I use Principle Test Behavior Not Implementation in Cursor, Gemini CLI or GitHub Copilot?
Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add backnotprop/pstack --skill principle-test-behavior-not-implementation -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/principle-test-behavior-not-implementation, .gemini/skills/principle-test-behavior-not-implementation, .github/skills/principle-test-behavior-not-implementation and .opencode/skills/principle-test-behavior-not-implementation in your project.
What does Principle Test Behavior Not Implementation need to run?
SKILL.md names no scripts, command-line tools or credentials: Principle Test Behavior Not Implementation is instructions for the agent only.
Does Principle Test Behavior Not Implementation access the network?
SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.
Is Principle Test Behavior Not Implementation safe to install?
Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.
What licence does Principle Test Behavior Not Implementation use?
Principle Test Behavior Not Implementation is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Principle Test Behavior Not Implementation use?
About 644 tokens (SKILL.md is roughly 2.6k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.
What are the alternatives to Principle Test Behavior Not Implementation?
Skills that share tags, products or a category with Principle Test Behavior Not Implementation: Principle Redesign From First Principles (cursor/plugins, 11k stars), Uxui Principles (sickn33/agentic-awesome-skills, 47k stars), First Principles Thinking (mindfold-ai/Trellis, 15k stars) and Keep Track (davidondrej/skills, 4.1k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Principle Test Behavior Not Implementation?
backnotprop (a GitHub user) maintains it in backnotprop/pstack, which has 1,542 GitHub stars. The repository was last updated on October 8, 2026.
Source: backnotprop/pstack on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.