# Building Your First AI Agent Skill: From Markdown to APIs > Source: > Published: 2026-06-20 07:57:07+00:00 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](https://promptmasterstore.gumroad.com/l/ai-agent-skills-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](https://promptmasterstore.gumroad.com/l/ai-agent-skills-complete-guide) What would your first script-backed skill do? Drop the idea in the comments and I will suggest how to structure it.