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.
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