AI agent skills are easier to build than they look, and they scale from a simple markdown file to something that runs scripts and calls APIs. Here is a practical walkthrough of the whole spectrum, starting from the simplest useful skill.
The simplest skill is a folder with a single SKILL.md inside. No code. Just frontmatter and instructions.
code-reviewer/
SKILL.md
The SKILL.md has frontmatter (a name and a description) and a markdown body with the instructions. For a code reviewer, the body might be an ordered checklist: check correctness first, then security, then style, then tests, and report findings grouped by severity with the file and line cited.
That is a complete, useful skill. Most of the value in skills lives at exactly this level — encoding the conventions you would otherwise repeat every session.
When a task needs real execution rather than guidance, add scripts to the folder:
code-reviewer/
SKILL.md
scripts/
check.py
The SKILL.md instructs the agent to run the script as part of the task. Now the skill does not just advise — it acts. This is the step from a skill that tells the agent how to think to one that performs a concrete operation.
The most capable skills reach external services — fetching data, posting updates, integrating a tool into the agent's workflow. Structurally it is still a SKILL.md plus the code to make the calls, but now the skill connects your agent to the outside world.
A useful safety note: skills that run commands or call APIs are exactly where you want to be deliberate about scope. Restrict what the skill can touch, review what it does, and treat third-party skills with the same caution you would any code you run.
Regardless of complexity, building a skill is always the same three steps:
Then test it by phrasing real requests and confirming it triggers. Widen the description wherever a natural request misses.
Your first skill is one capability. A library of them — markdown skills for conventions, script skills for operations, API skills for integrations — quietly upgrades every agent you use. Start with a markdown-only skill today, and add complexity only when a task actually needs it.
Free starter: The format, a working template, and the description technique are all on a free cheat sheet: AI Agent Skills Quick-Start Cheat Sheet
Go deeper: The full guide covers the complete specification, five build walkthroughs, ten production-ready templates, security, and a 30-day plan: AI Agent Skills: The Complete SKILL.md Standard Guide
What would your first script-backed skill do? Drop the idea in the comments and I will suggest how to structure it.