What is a SKILL.md file?

SKILL.md is the plain-markdown file that defines an agent skill: a short description of when the skill applies plus the instructions the agent should follow, in the format Claude Code and other skills-compatible agents read.

Every agent skill is, at its core, a SKILL.md file. The top of the file describes what the skill is for and when to trigger it; the body holds the actual instructions — the steps, rules, examples and constraints the agent should follow when the task comes up. Some skills ship supporting files alongside it (scripts, templates, reference docs), but SKILL.md is the entry point the agent loads.

The format matters because it's what makes skills portable. Any skills-aware client — Claude Code, Cursor and others — knows how to read a SKILL.md, so the same file improves every compatible agent without rewriting it. That's why the ecosystem converged on one simple markdown convention rather than a per-tool config.

Writing one is straightforward: describe the trigger clearly so the agent knows when to reach for it, then give focused, opinionated instructions. Narrow skills that do one job well beat sprawling ones. Every skill in this directory is built on a SKILL.md you can inspect from its source repo.