The AI Skills Field Guide
Clear guides to Agent Skills, SKILL.md, Claude Skills, Codex Skills and the tooling emerging around reusable AI capabilities.
What are AI Agent Skills?
Learn what AI Agent Skills are, how SKILL.md works, why agents use skills, and how skills differ from prompts, tools and traditional plugins.
SKILL.md: the file behind Agent Skills
A practical guide to the SKILL.md file: folder structure, frontmatter, name and description, instructions, scripts, references, and the security implications of skill files.
Claude Skills: how reusable skills work in Claude
How reusable Agent Skills work across Claude, Claude Code and the Claude API — skill structure, activation, scripts and resources, with practical guidance.
Codex Skills: a practical guide
How Agent Skills work in OpenAI Codex — the skill directory, SKILL.md, explicit and implicit invocation, descriptions, and how skills differ from project instructions.
ChatGPT Skills: what they are and how they work
"ChatGPT skills" can mean two things. This guide clarifies the difference and focuses on reusable Agent Skills ChatGPT can load — structure, activation and use cases.
Agent Skills vs MCP
Agent Skills package reusable know-how; MCP connects agents to external tools and context. This guide compares the two and explains how they complement each other.
Agent Skills marketplaces, directories and registries
What's the difference between a skills directory, marketplace and registry? This guide covers discovery, versioning, security and what makes a useful registry as the ecosystem grows.
How to create an Agent Skill
A step-by-step, deterministic tutorial for creating an Agent Skill — directory, SKILL.md, an activation-focused description, instructions, optional scripts, testing, and Git versioning.
Before you install an Agent Skill, inspect what it can do.
A calm, practical security checklist for Agent Skills: read SKILL.md, inspect scripts, check shell commands, network calls, credentials, package installs, filesystem writes, provenance and version pinning.
"AI skills" now means more than one thing.
"AI skills" is ambiguous. This guide separates the three meanings — human AI skills, Agent Skills (SKILL.md), and vendor "skills" features — and explains why context matters.
Written by AISkills.guru · Independent of Anthropic, OpenAI and any other vendor. Always check current official documentation for specifics.