Format SKILL.md
SKILL.md is the open Agent Skills standard Anthropic shipped at agentskills.io in December 2025, a markdown file whose YAML frontmatter (description, allowed-tools) tells Claude when to invoke the skill, with the description being the primary triggering signal. Markdown file spec describing a Claude Code or agent skill: required YAML frontmatter with description (the triggering signal), optional allowed-tools / disable-model-invocation, then markdown instructions Claude bash-reads at runtime. Compatible across Claude Code, OpenAI Codex, OpenClaw, open standard maintained on github.com/anthropics/skills. Update of October 22, 2026: Anthropic donated the standard (name, logo, GitHub organization and the agentskills.io domain) to the Agentic AI Foundation, under the Linux Foundation, as it had done for MCP. It becomes "open, community-driven and vendor-neutral", maintained as individuals by engineers from Anthropic, Google, OpenAI and others. Anthropic counts more than 70 agent platforms that have integrated it (including ChatGPT, Cursor, GitHub Copilot and VS Code), 1 million skills published and nearly 280 million installs, figures from the announcement. In practice your skill folder is written once and works in every agent that reads the format. Nothing in the announcement requires joining the foundation to use it, and it does not say existing files change.
Strengths
- Open standard (agentskills.io, github.com/anthropics/skills), not locked into Anthropic, works in Codex and OpenClaw
- Atomic brick: one .md file, no build, no mandatory versioning, start in 30 seconds
- Frontmatter's description drives automatic invocation, no slash command needed if the description matches user intent
Limitations
- Quality of the description field controls 100% of routing, a skill with a bad description never gets invoked
- No native versioning at the SKILL.md level, for team distribution you need to move to a plugin
Best for
- Personal workflow or single project, a standalone .skill.md file is enough, no plugin overhead
- Devs who want to understand Claude Code routing mechanics before packaging into a plugin