What Are Agent Skills? The New Standard for AI Coding Tools

When you hear "Agent Skills," you might think "yet another standard to learn." AI tools keep multiplying, each with their own extension methods.

But Agent Skills is different. It's designed as an open standard that works across multiple AI tools—write a skill once, and use it in Claude Code, Codex CLI, GitHub Copilot, and more.

This article explains how Agent Skills work and how to start using them.

The Basics: What's a SKILL.md File?

At the core of Agent Skills is the SKILL.md file—a Markdown file containing instructions and settings that extend AI behavior.

Minimal SKILL.md Example

Copy---
name: Code Review
description: Review pull request code changes
---

## Instructions

1. Check the changed files
2. Review for code quality, security, and performance
3. Point out specific improvements if needed

Write name and description in the frontmatter (between ---), then add instructions in Markdown below. That's it.

Directory Structure

For more complex skills, you can use this structure:

Copymy-skill/
├── SKILL.md          # Required: instructions and metadata
├── scripts/          # Optional: executable code
├── references/       # Optional: documentation
└── assets/           # Optional: templates, resources

That said, starting with just a SKILL.md file is recommended for beginners.

Why Agent Skills Matter

The Problem: Different Tools, Different Formats

Previously, extending AI tools meant learning each tool's proprietary format. Claude Code used CLAUDE.md, Cursor had Rules files, and so on. (See our CLAUDE.md migration guide for transitioning.)

If your team uses multiple tools, you end up rewriting the same instructions in different formats.

The Solution: Write Once, Use Everywhere

Agent Skills is an open standard supported by (as of January 2026):

• Claude Code
• OpenAI Codex CLI
• GitHub Copilot (VS Code)
• Cursor
• And many more

Write a SKILL.md once, and expect similar behavior across these tools. Full compatibility isn't guaranteed, but core functionality works consistently.

Progressive Disclosure: Large Skills, Light Footprint

A key design feature of Agent Skills is "Progressive Disclosure."

How It Works

1. Registration: Only name and description are loaded initially
2. Activation: When a task matches, the full SKILL.md is loaded
3. Execution: Files in scripts/ or references/ load only when needed

This design lets you register hundreds of skills without overwhelming the AI's context window.

Recommended Sizes

• SKILL.md body: Under 5,000 tokens recommended
• Line count: Under 500 lines recommended

For larger skills, split details into the references/ directory and reference them from SKILL.md.

Tool Support and Differences

Each tool builds on the Agent Skills standard with its own extensions.

Not everything works identically across tools, so test important skills in your target environment.

Getting Started: 3 Steps

Step 1: Download a Skill

Find a skill on Skill Gallery and click "Download," or use these commands:

Copy# For Claude Code
claude /install-skill https://github.com/owner/repo

# For Codex CLI
codex /skills  # Use the built-in skill installer

Step 2: Place the Skill

Put downloaded skills in:

• Project-specific: .claude/skills/ (active for that project only)
• User-wide: ~/.claude/skills/ (active for all projects)

Step 3: Use the Skill

In Claude Code, type the skill name or use a slash command. In Codex CLI, use the $skillname format.

Limitations and Caveats

Agent Skills are useful, but keep these points in mind.

Cross-Tool Compatibility

Basic features are shared, but tool-specific extensions (like Claude Code's subagents) won't work elsewhere.

Quality Varies

It's an open ecosystem, so skill quality on GitHub varies. Check star counts and last update dates to find reliable skills.

Evolving Specification

Agent Skills is still developing. The spec may change, so check the official documentation at agentskills.io periodically.

Summary

Agent Skills is an open standard that works across multiple AI tools. Write instructions in a SKILL.md file to extend AI behavior.

Start by browsing the Skills list and trying some out. Once you know what you need, you can create your own.

Full specification available at agentskills.io.

Related Articles

• How to Set Up Skills in Claude Code - Step-by-step setup guide
• MCP vs Agent Skills - Understanding the difference
• AI Coding Tools Comparison 2026 - Claude Code, Codex CLI, Cursor comparison
• Agent Skills for Non-Developers - No programming required