Short answerAgent Skills is an open format, originally developed by Anthropic, for packaging instructions as a SKILL.md folder. OpenAI says its skills build on the same standard, which makes your instructions more portable between tools.
- What it is: an open standard for a skill folder containing SKILL.md with name and description, plus optional files.
- Who supports it: the standard's site lists Claude, Claude Code, ChatGPT and Codex among clients, and OpenAI's documentation says its skills build on the standard.
- Where Claude skills work: Anthropic documents skills in Claude apps, the API (through the skills endpoint with code execution) and Claude Code. Custom skills do not sync between those places.
Why it matters for migration
Instructions packaged as a skill can move between supporting tools more easily than a Custom GPT can. The standard does not promise identical behavior in every product, and Actions are not part of it. How-to: convert a GPT to a skill.
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Sources
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