Building Your First AI Agent Skill: From Markdown to APIs

A developer explains how to build AI agent skills, starting from a simple markdown file to scripts and API integrations. The approach scales from a SKILL.md with instructions to adding scripts for execution and external service calls, with safety notes for running commands.

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.