One Skill Folder That Works in Every Coding Agent

TL;DR Keep a copy of each skill per coding agent and the copies drift; write a skill with no check and nobody knows it is broken until an agent misuses it. agent-skills keeps each skill as one folder in the open Agent Skills format, installs it where Claude Code, Codex, Copilot, Cursor and Gemini CLI look, and fails it in CI when it breaks a rule. Its skills ship checks that can fail.

Do this: run npx skills add https://gitlab.com/agentic-fx/agent-skills in a project, then ask your agent for an email and watch it run the check.

Part 3 ended on a question: can the habits that kept these tools honest, like a check that can fail, reach every coding agent your team uses? This part answers with one folder.

Claude Code looks for skills in .claude/skills. Codex and Gemini CLI look in .agents/skills. Keep a copy in each and you edit two files for every fix. In agent-skills a skill is one folder, written once, and the installer puts it where each agent looks.

1. I Built a Lottery App That Refuses to Predict the Jackpot
2. A Spider Crawls Over My Screen and Remembers What It Read
3. An AI Meeting Assistant That Never Hears the Audio
4. One Skill Folder That Works in Every Coding Agent (you are here)
5. An LLM Proxy That Retries a 429 Before Your Client Sees It

One folder, every agent's path

A skill is a folder with a SKILL.md. The agent reads its short description on every request, loads the body when a task matches, and opens references and scripts only when a step needs them.

The agents disagree only on where to look, so the repo's installer, scripts/install.py, writes every place:

Seven skills are on main today, from drafting emails to diagnosing bugs.

The honesty line: a check that can fail

docs/authoring.md marks the rules a script enforces, and a merge request that breaks one fails CI. Among them:

1. A description of 200 characters at most. Every installed description sits in the context on every request. The spec allows 1,024, but overflow is dropped silently.
2. SKILL.md under 500 lines. The rest moves into references/.
3. No shouting. MUST, NEVER and CRITICAL in capitals make models over-apply a rule.
4. No tool names that only one agent has.
5. At least three evals per skill, in the format of Anthropic's skill-creator.

And one rule for the skill itself: it ends with a check the agent runs, never "review your work".

The email skill fails its own draft

doc-drafting-emails ships check_email.py. It fails a draft for stock filler, a subject too long for a phone, a body over the word limit for its type, or a placeholder nobody listed to fill in. The agent runs it before handing the draft over:

ERROR filler phrase: "hope this email finds you"

It exits 1. The skill tells the agent to fix every issue it prints and run it again.

Try it

In your project, with Node installed:

npx skills add https://gitlab.com/agentic-fx/agent-skills

It finds all seven skills and writes them to .agents/skills, linked into .claude/skills for Claude Code. Then ask your agent to draft an email asking your manager to approve a training course, and watch it run the check before it hands the draft over.

Every tool in this series was written by a coding agent, and a Claude Code session can stop mid-task on 429 rate_limit_error. Part 5, An LLM Proxy That Retries a 429 Before Your Client Sees It, asks what a local proxy can honestly do about a rate limit, and what it can't.

Read this with your AI

Read the post "One Skill Folder That Works in Every Coding Agent".
1. Explain why skill descriptions are capped at 200 characters.
2. Check the skills I use (pasted below) against the five rules in the post.
<paste your SKILL.md files>