What are AI skills (Agent Skills and SKILL.md)?

What an AI skill is

Anthropic introduced Agent Skills on 16 October 2025 as "folders that include instructions, scripts, and resources that Claude can load when needed", in the Claude apps, Claude Code and the API. On 18 December 2025 it published the format as an open standard for cross-platform portability, and added organization-wide management so admins can manage skills across an organization.

The specification at agentskills.io is short. A skill is a directory with a SKILL.md file. Its front matter needs a name (up to 64 characters, lowercase letters, numbers and hyphens, matching the folder) and a description (up to 1,024 characters) that says what the skill does and when to use it. The body holds the instructions. Optional folders hold scripts, reference documents and templates.

The design idea is progressive disclosure. An AI loads every skill's name and description at the start (about 100 tokens each), loads a skill's full instructions when a task calls for it (the spec recommends under 5,000 tokens), and opens extra files only when needed. That is why a team can keep dozens of skills available without filling the AI's context.

OpenAI's documentation for ChatGPT and Codex says their skills "build on the open agent skills standard", and that a SKILL.md must include a name and a description. Codex reads skills from .agents/skills in a repository and in the user's home folder.

Why a small team should care

• It turns a good prompt into a team habit. The way your best person writes a proposal or triages a ticket becomes steps everyone's AI follows.
• It travels. The same SKILL.md runs in Claude and ChatGPT and in coding agents, so a teammate on another AI doesn't need a rewrite.
• It drifts unless you share it properly. ChatGPT Skills sit on Business, Enterprise, Healthcare and Edu plans, Claude's sharing works inside one organization, and each upload is a separate copy. Fix a step in one, and the others run the old version.

A worked example

Illustration: a made-up business. Meadow & Co, a seven-person marketing agency, has one account manager whose weekly client reports clients love. Her prompt lives in her own ChatGPT. The founder turns it into a skill named weekly-client-report: a description that says "Use when writing the Friday report for a client", six steps (collect the numbers, compare with last week, lead with the one change that matters, and so on), a template and one finished example.

Two account managers use Claude, one uses ChatGPT Pro, and two freelancers use their own Claude Pro accounts. Uploading the file to each account works for a week, until the founder changes step three and four people are still on the old version. The agency moves the skill to one shared place that every AI reads from, so the next change reaches all seven people the same day.

How to turn a prompt into a skill

1. Pick a prompt that already works and that more than one person needs.
2. Name it and write the description: what it does and "Use when ..." in words people actually say.
3. Write the steps as a numbered list, with the output template and one real example.
4. Add the checks a person should run before using the output.
5. Test it on three real inputs, then share it from one place so everyone runs the same version.

Common confusions

• "Skills are custom GPTs." A custom GPT is a configured assistant in ChatGPT. A skill is a task the AI picks up when needed, in any assistant that reads the format.
• "Skills and MCP compete." Skills carry know-how; MCP carries tools and data. A server can also serve skills over MCP.
• "Write once, done." Test a skill with a small eval set and update it when the process changes.

Questions

What is a SKILL.md file?

The file at the heart of an AI skill: a short YAML block with a name (lowercase, hyphens, up to 64 characters) and a description of what the skill does and when to use it (up to 1,024 characters), followed by Markdown instructions. It may sit in a folder with scripts, references and templates.

Which AI tools support Agent Skills?

Claude (the apps, Claude Code and the API), and OpenAI's ChatGPT and Codex, whose documentation says their skills build on the open agent skills standard. Coding agents such as Cursor and OpenCode read SKILL.md folders too. Anthropic published the format as an open standard on 18 December 2025.

How do I share a skill with my team?

Inside one Claude Team or Enterprise organization, or one ChatGPT Business workspace, the product's own sharing works. For a team on different plans or different AIs, keep the skill in one place and serve it to everyone, for example through one MCP link, so an edit reaches everyone at once.