Agent Skill (SKILL.md)
establishedSettled. Documented by a primary source and not seriously disputed.
An organised folder of instructions, scripts and resources an agent discovers and loads on demand. At minimum a directory containing a SKILL.md with YAML frontmatter; only name and description are required. Introduced by Anthropic in October 2025 and published as an open standard that December. The portability is real, not marketing: a skill using only name, description and plain markdown runs unchanged across Claude Code, Codex, Cursor and others. Note that the skill is portable and the plugin wrapper around it is not.