Agent Skills — Specification (agentskills.io)
by Agent Skills (open standard)
The formal open-standard spec for the SKILL.md format, adopted beyond Claude.
Overview
The Specification page at agentskills.io is the complete, formal format spec for Agent Skills — the open standard for the SKILL.md format that originated with Anthropic and is adopted across multiple agent platforms. It defines the directory structure (a skill is a directory with at minimum a SKILL.md, plus optional scripts/, references/, and assets/ folders) and the full YAML frontmatter schema: required name (1-64 chars, lowercase alphanumeric and hyphens, no leading/trailing or consecutive hyphens, must match the parent directory name) and description (1-1024 chars), plus optional license, compatibility (max 500 chars, environment requirements), metadata (arbitrary key-value map), and the experimental allowed-tools (space-separated pre-approved tools). It specifies the Markdown body conventions, the three-tier progressive disclosure model (metadata ~100 tokens loaded at startup, SKILL.md body under 5000 tokens recommended loaded on activation, resources loaded as needed), file-reference rules (relative paths, one level deep), and validation via the skills-ref reference library ('skills-ref validate ./my-skill').
At a Glance
- Topic
- Skills
- Level
- Intermediate
- Format
- Documentation
- Cost
- Free
- Duration
- ~10 min read
- Provider
- Agent Skills (open standard)
- Hands-on
- No
- Certificate
- None
What You’ll Learn
- ✓The complete Agent Skills directory structure and required files
- ✓Every frontmatter field including optional license, compatibility, metadata, and allowed-tools
- ✓The exact naming rules a valid skill name must satisfy
- ✓How progressive disclosure is formalized in the standard
- ✓How to validate a skill with the skills-ref reference tool
Highlights
- •The authoritative, vendor-neutral open standard
- •Documents optional fields not covered in product docs
- •Points to official validation tooling
Who It’s For
Best For
- ✓Tool builders implementing skill support
- ✓Authors writing portable, standards-compliant Skills
Prerequisites
- •Familiarity with YAML and Markdown
- •Basic understanding of what a Skill is
FAQ
What is Agent Skills — Specification (agentskills.io)?
The complete format specification for Agent Skills as an open standard: directory structure, the full frontmatter schema, progressive disclosure, and validation tooling. Best for anyone building tooling or authoring portable, standards-compliant Skills.
Is Agent Skills — Specification (agentskills.io) free?
Agent Skills — Specification (agentskills.io) is free to access.
What level is Agent Skills — Specification (agentskills.io) for?
Agent Skills — Specification (agentskills.io) is aimed at a intermediate audience. Recommended background: Familiarity with YAML and Markdown, Basic understanding of what a Skill is.
How long does Agent Skills — Specification (agentskills.io) take?
Expect roughly ~10 min read. Most learners work through it at their own pace.
What will I learn from Agent Skills — Specification (agentskills.io)?
You'll learn: The complete Agent Skills directory structure and required files; Every frontmatter field including optional license, compatibility, metadata, and allowed-tools; The exact naming rules a valid skill name must satisfy; How progressive disclosure is formalized in the standard; How to validate a skill with the skills-ref reference tool.