What are agent skills? SKILL.md and the portable-capability standard, explained

Agent skills are portable folders centered on a standardized SKILL.md file containing YAML metadata and markdown instructions, optionally accompanied by scripts, templates, and reference materials that help AI agents perform specific tasks. Originating with Anthropic and now supported by major agent harnesses including Claude Code, Codex CLI, Cursor, OpenCode, OpenClaw, and Hermes, the format uses progressive disclosure so agents load only a skill’s name and description until a relevant task requires its full instructions. Skills provide operational knowledge rather than new permissions, typically teaching agents how to use existing command-line tools, and can be obtained through registries such as ClawHub and Hermes’s hub, first-party repositories, or direct Git links. They differ from MCP servers, which expose typed, often authenticated remote tools and impose ongoing schema context costs, while skills are lightweight, inspectable, and best suited to local workflows. Security concerns became prominent after the ClawHavoc campaign used typosquatted skills to distribute backdoors, prompting registry scanning and reinforcing the need to inspect SKILL.md files, review scan reports, favor established projects, and use skills that wrap documented CLIs.

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