YAML (.yaml)
A human-readable data format used for configuration and for the front matter at the top of Agent Skills' SKILL.md files.
- Specification
- YAML 1.2
Connected
- Agent SkillsFolders of instructions, scripts and resources that Claude loads on demand when a task matches the skill's description, introduced by Anthropic in October 2025.
- Markdown (.md)A plain-text formatting syntax that renders to HTML; the default format for agent instructions, plans and READMEs because it is cheap to generate, easy to diff and readable unrendered.
- JSON (.json)A data-interchange format; the usual shape for agent tool calls, structured outputs and configuration such as Claude Code settings.
More file formats
- HTML (.html)The markup language of the web. For agent output it gives people a designed, interactive page, at a token cost that depends on whether the agent re-invents the design each run.
- CSS (.css)Style sheets for HTML. In agent-generated pages it is most of the token bill, which is why template kits move it into one shared file.
- SVG (.svg)XML-based vector graphics. Hand-drawing diagrams in SVG forces a model to reason about coordinates, which can cost thousands of thinking tokens per diagram.