version: "1.0.2" name: authoring-portable-skills description: "Use when writing or editing a SKILL.md that more than one agent will read (Claude Code, Codex, Antigravity, Cursor, Copilot, Gemini CLI, OpenCode). Keeps the shared file to what every agent honours, sizes the body, checks it with validate-skills.py and the agents themselves, and requires evidence before calling it hardened. Not for choosing what a skill should teach." license: MIT compatibility: "Tested with Claude Code, Codex, and Aider skill layouts" allowed-tools: "Read Glob Grep Write Edit Bash" metadata: agtmls-owner: "Sebastien Rousseau" agtmls-maturity: "draft" agtmls-risk-level: "low" agtmls-network-access: "none" agtmls-writes-files: "true" agtmls-executes-commands: "true" agtmls-handles-secrets: "false" agtmls-requires-human-review: "true"

Authoring portable skills

Trigger. You are creating a skill, editing a SKILL.md, adding frontmatter, or porting a skill written for one agent so others can use it.

The same SKILL.md is read by many agents, each honouring a different set of extras. A skill that works in the agent it was written in and silently degrades in the others is the common failure, and nobody sees it, because each author tests in one agent.

The shared file

Frontmatter. Only what every agent reads:

Agent-specific fields (Claude Code allowed-tools, context, paths; Codex agents/openai.yaml) belong to that agent's install, not to the shared text. In this registry allowed-tools is generated from metadata.json by sync-skill-frontmatter.py; never write it by hand.

Leave out a model field unless the skill cannot work on the session's model. In Claude Code a skill that names another model makes that turn a model switch, so the next request re-reads the whole conversation with no prompt-cache hits. If the skill needs a different model, run it as a forked subagent instead (context: fork), which leaves the main conversation's cache intact.

Body.

• Under about 5,000 tokens and 500 lines. The body loads whole on

activation; detail goes in a reference file the body names.

• References one level deep: SKILL.md may send the agent to

reference.md, but reference.md must not send it on to a third file. Agents follow chains unevenly.

• Nothing only Claude Code understands: no ! placed before a code span

to run a command, no code fence opened with a !, and no path built from the CLAUDE_SKILL_DIR variable. Other agents show them as literal text or a path that does not exist. Refer to bundled scripts by relative path and say what to run.

Write it from a real procedure

A skill earns its place when it carries steps the agent would not otherwise take, with outputs someone can check. Skills a model writes for itself, with no procedure behind them, measure worse than no skill. Generic advice ("write clean code") changes nothing.

• One job per skill. If the description needs "and" twice, split it.
• Every mandatory step must pay for itself on every use. Heavy verification

rituals and long pipelines are the most common ways a skill makes an agent worse; make expensive steps conditional on the risk.

• Say when not to use it, naming the skill that does.

Check it

python3 scripts/validate-skills.py            # spec, trigger cue and portability
agtmls audit skills/<name>                    # security rules on every file
agtmls audit --foreign /path/to/repo          # portability notes for a repo that is not this registry

Then add its evals, which this registry requires: routing cases (evals/cases/<name>.json, prompts that should and should not load it) and a behavioural case (evals/behavioral/cases/<name>.json, text the skill must and must not contain).

Then check the agents actually load it:

agtmls install <language> claude --target /tmp/try && agtmls verify claude --target /tmp/try --live
agtmls install <language> codex --target /tmp/try && agtmls verify codex --target /tmp/try --live

--live asks the agent which skills it loads (Claude Code through one headless session, Codex through its rendered prompt, no model call) and fails naming any installed skill it does not list.

Draft until measured

A new skill ships as draft. It becomes hardened when a run of the same tasks with and without it, on at least two agents, shows it helps (scripts/run-uplift-evals.py), and a skill that shows no gain is retired rather than kept. Helping includes cost: on each agent the skill may use at most 1.5 times the tokens of the task without it, unless it gains 20 points or more. Passing the validators proves the file is well-formed, not that it helps.

When not to use

• Deciding what a skill should teach, or whether it should exist: that is the

domain expert's call. This skill is the form, not the content.

• Installing someone else's skill: vetting-a-skill-before-install.