Short answerA skill is a folder with a SKILL.md file: YAML frontmatter with a name and description, then your instructions, plus optional reference files. OpenAI and Anthropic both document skills built on the open Agent Skills standard.
The structure
my-skill/ SKILL.md (frontmatter + instructions) references/ (your knowledge files, optional) assets/ (optional) scripts/ (optional)
SKILL.md frontmatter
name: lowercase letters, numbers and hyphens (the spec says 1 to 64 characters, no leading, trailing or double hyphens, and it must match the folder name).description: what the skill does and when to use it. The model uses it to decide when to apply the skill. Anthropic's docs set a 1,024-character maximum.- Optional fields in the spec include license, compatibility and metadata. The spec advises keeping SKILL.md under 500 lines.
Mapping your GPT
| Custom GPT | Skill |
|---|---|
| Instructions | SKILL.md body |
| Description | description frontmatter (write it as "use when...") |
| Knowledge files | references/ folder |
| Actions | Not part of a skill; use tools or an MCP server: MCP |
OpenAI's own migration does this conversion for you inside ChatGPT: plugin migration. Caveats: Anthropic reserves the words claude and anthropic in skill names, and OpenAI adds an optional agents/openai.yaml file. Whether a skill behaves identically across products is not documented, so test: testing.
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Sources
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