The Same Small File Now Teaches Claude, ChatGPT, Copilot and Gemini a Job
A skill turns your best process into a folder you own, share and move between assistants. People sell them now too. Here is how to build one, and what to check before you install anyone else's.
By John Martines · October 31, 2026 · 6 min
A skill is a folder with one plain text file that teaches an AI assistant how your business does a job. The big assistants now read the same format, and small stores have started selling them. Here is how to write your own, keep it private, and check a stranger's before it touches your files.
The office manager at a twelve-person heating and cooling company handles warranty claims better than anyone. She knows which photos to ask for, which manufacturer wants the serial number in which box, and the one polite sentence that gets a homeowner to send the receipt. It all lives in her head and on a sticky note.
On Tuesday she describes the job to Claude, answers a few of its questions, and gets back a small folder with one text file inside. Now anyone in the office can ask for help with a warranty claim and get her way of doing it. The owner, meanwhile, keeps seeing ads for ready-made "skill packs" that promise the same thing for sales and hiring.
That folder is the new way work gets packaged for AI. You can build one, share one, buy one or sell one. The last two need care.
A skill is a folder with one required file called SKILL.md. The .md stands for Markdown: plain text with a few symbols for headings and lists, readable in Notepad. The top of the file gives the skill a name and a one-line description. The rest is instructions in ordinary sentences: how this job is done here. The folder can also hold reference files, like a price list or a template, and sometimes small programs called scripts.
The assistant reads only the names and descriptions until a request matches one. Then it reads the full instructions and follows them.
Two related terms:
- Instruction files such as CLAUDE.md and AGENTS.md sit in a project folder and tell an assistant how that project works. Many coding tools read AGENTS.md.
- Plugins bundle several skills, and sometimes connections to other software, into one install. A marketplace is a list of plugins. In Claude Code, anyone can host one from a code repository.
How it got here:
- October 16, 2025: Anthropic introduced Agent Skills for Claude: folders of instructions, scripts and other files that Claude loads when a task calls for them.
- December 9, 2025: OpenAI gave AGENTS.md to the new Agentic AI Foundation at the Linux Foundation.
- December 18, 2025: Anthropic published skills as an open standard and opened a directory of partner skills, saying there was no revenue sharing. GitHub Copilot added support the same day.
- January 30, 2026: Anthropic released 11 open-source plugins for Cowork, its desktop assistant. The legal one was about a dozen Markdown files and two short settings files. On February 3, shares of legal-information companies, including Thomson Reuters and RELX, fell sharply in a selloff that news reports tied to it.
- February 2026: Koi Security found 341 malicious skills on ClawHub, the public skill library for the OpenClaw assistant. Snyk scanned 3,984 public skills, found critical security problems in 534, and confirmed 76 that carried deliberately harmful instructions.
- July 9, 2026: ChatGPT's app directory became a plugin directory, shared with OpenAI's Codex. A plugin can hold skills.
- September 30, 2026: Google announced Skills for Gemini, built on the same open SKILL.md standard, to replace Gems. They are rolling out in the Gemini app. The launch in Gmail, Docs and the other Workspace apps is delayed.
- October 22, 2026: Anthropic handed the standard to the Agentic AI Foundation. Anthropic counts more than 70 platforms, 1 million published skills and nearly 280 million installs.
Your process becomes a file you own. The same format works in Claude, in ChatGPT and Codex, and in the Gemini app. Copilot in Excel reads skills from a folder in your OneDrive. OpenAI is retiring custom GPTs on December 11, and its migration tool turns each GPT's instructions into a skill. Features come and go; a text file moves.
A market is forming. The directories built into Claude and ChatGPT list skills and plugins from the AI companies and from software makers such as Atlassian, Canva, Figma and Notion. Small independent stores sell skills from anyone. One, Agensi, shows example listings at $6 to $9 and says sellers keep 70% of each sale.
A skill is closer to software than to a document. It tells your assistant what to do, and your assistant can reach your files, your email and sometimes your computer. The bad ClawHub skills posed as crypto tools, YouTube helpers and Google Workspace add-ons. Most told the user to install a "prerequisite" first, and that step installed malware that steals passwords and keys. Anthropic's own guidance says a malicious skill "can direct Claude to run arbitrary code, access sensitive files, or transmit data externally." A trusted name is no guarantee either: on October 29, a fake copy of Anthropic's Claude Code add-on sat in the Cursor code editor's store for about 11 hours.
1. Turn your best process into a skill. Pick one job that one person does well and others do differently: a warranty claim, a new-patient phone call, the way you write an estimate. In Claude, turn on the built-in skill-creator under Settings, then Capabilities. Describe the job, answer its questions, and it writes the SKILL.md. Copilot in Excel has a similar skill-builder rolling out. Test the result on three real requests in a fresh chat. Our free course [Save Your Best Instructions](/courses/reusable-instructions) walks through writing and testing one.
2. Keep the master copy. Download the folder and store it with your other company documents, with one owner and a review date. Skills don't sync between tools, or even between parts of the same tool, so that copy is your source of truth.
3. Share inside, not outside. In Claude, a skill you upload stays private until you share it, and the account owner decides whether sharing is on at all. Google Workspace has a separate admin setting for sharing skills outside your organization. Keep customer names, passwords and account numbers out of skill files. Anyone who gets a Markdown file can read and copy every word, so don't post or sell the skill that holds your edge.
4. Treat every outside skill like software. Before anyone installs a skill from a stranger, a store or a free "100 skills" pack, run the five checks below. Paying for a skill doesn't mean anyone reviewed it.
5. Read a setup file before your assistant runs it. Software makers now publish files for an assistant to follow, so "point your assistant at this file" replaces a page of install steps. Mintlify, a documentation company, publishes a skill.md on its customers' public documentation sites for this. It's handy, but someone else wrote it, and a file at a web address can change after you read it. Read it first, and have the assistant ask before each step.
These follow Anthropic's own review checklist, in plain words.
- Who made it? A company you already do business with, or a person with a track record. Watch for names a letter off from a real product.
- Did you read every file? SKILL.md is plain English. If it says to install something first, paste a command or download a zip file, stop.
- Does it carry code? Files ending in .py, .sh or .js run with your access. A skill for writing emails has no reason to include them.
- Does it reach out? Look for web addresses and for words like upload, send, post or sync. Each one needs a reason you understand.
- Does it want secrets or silence? A skill that asks for passwords or keys, or tells the assistant to hide what it does or ignore its rules, gets deleted.
The standard now sits with a neutral foundation, maintained by engineers from Anthropic, Google, OpenAI and others. Expect more tools to read the same folders, more vendors to ship their own, and more stores selling them. Expect more checking too: Claude's Enterprise plan can already scan uploaded skills and block the ones that fail.
Your own skills are the valuable ones, because they hold how you work. Write them down, keep the master copy, share them inside, and install anyone else's the way you would install software.
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Sources (checked October 31, 2026). What a skill is, where it works, plans and the trusted-sources warning: "Introducing Agent Skills" (Claude blog, Oct 16, 2025, updated Dec 18, 2025) and the Agent Skills overview and specification (agentskills.io). Partner directory, partners named and "no revenue-sharing arrangements": "Anthropic launches enterprise 'Agent Skills' and opens the standard" (VentureBeat, Dec 18, 2025). GitHub support: "GitHub Copilot now supports Agent Skills" (GitHub Changelog, Dec 18, 2025). AGENTS.md and the foundation: "Linux Foundation forms Agentic AI Foundation to be new home for MCP, goose and AGENTS.md" (SD Times, Dec 9, 2025) and the AGENTS.md site (agents.md), which lists the tools that read it. Plugins and self-hosted marketplaces: "Customize Claude Code with plugins" (Claude blog, Oct 9, 2025). Cowork plugins: "Anthropic's Cowork gets plugins that turn Claude into a specialized assistant for knowledge workers" (The Decoder, Jan 30, 2026). The legal plugin's files: Anthropic's public knowledge-work-plugins repository on GitHub, launch version of Jan 29, 2026. The share drop: "Anthropic unveils Claude legal plugin and causes market meltdown" (Legal IT Insider, Feb 3, 2026) and "Thomson Reuters, RELX, and Wolters Stocks Crushed After Anthropic Debuts Claude Legal Plug-In" (Morningstar, Feb 2026). Malicious ClawHub skills and the fake prerequisite step: "Researchers Find 341 Malicious ClawHub Skills Stealing Data from OpenClaw Users" (The Hacker News, Feb 2, 2026). Snyk's scan: "ToxicSkills" study (Snyk blog, Feb 5, 2026). ChatGPT plugin directory and skills in plugins: OpenAI Help Center, "Plugins in ChatGPT and Codex" (2026; read from search excerpts, the page blocks direct reading) and "Build skills" (ChatGPT developer docs, undated). Custom GPTs becoming skills: OpenAI Help Center, "Custom GPT retirement and migration FAQ" (2026). Gemini Skills, the SKILL.md standard and the Workspace delay: "Introducing skills in the Gemini app and Workspace, plus what's next for Gems" (Google Workspace Updates, Sept 30, 2026, updated Oct 13, 2026). Google's sharing setting: "Allow people to share skills externally" (Google Workspace Admin Help). Copilot in Excel: "Use skills with Copilot in Excel to complete repeatable tasks" (Microsoft Support). The donation and adoption figures: "Donating Agent Skills to the Agentic AI Foundation" (Anthropic, Oct 22, 2026) and "Agentic AI Foundation Announces Four New Projects" (Linux Foundation, Oct 22, 2026). Paid skill prices and the 70% share: "Monetize AI Skills 2026" (Agensi, July 14, 2026; the store's own figures). Review checklist, the malicious-skill warning and Enterprise scanning: "Skills for enterprise" (Claude Developer Platform docs). Private-by-default skills, sharing controls and the skill-creator: "Using skills in Claude" and "How to create a skill with Claude through conversation" (Claude Help Center). The fake Claude Code add-on: "Malicious Claude Code extension on Cursor planted SSH backdoors" (BleepingComputer, Oct 30, 2026). Setup files: "install.md: A Standard for LLM-Executable Installation" (Mintlify, Jan 15, 2026, retired Jan 21, 2026 in favor of skill.md) and "skill.md: An open standard for agent skills" (Mintlify, 2026).