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.
- Specification
- CommonMark
- Media type
- text/markdown
In a 72-run Claude Code benchmark published October 2026, a Markdown status report cost 883 output tokens versus 7,204 for freeform HTML; Markdown plus a renderer cost 569.
Connected
- CLAUDE.mdA Markdown file of standing instructions that Claude Code loads at the start of every session, at user, project and folder level.
- AGENTS.mdAn open, tool-neutral Markdown convention for giving coding agents project instructions, introduced by OpenAI in August 2025 and now stewarded by the Linux Foundation's Agentic AI Foundation.
- llms.txtA proposed convention, published by Jeremy Howard in September 2024, for a Markdown file at a site's root that tells language models what the site is and where to read more.
- 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.
- agent-html-kitAn open-source template kit and Claude Code skill (kit.js) that lets agents emit content-only HTML components while design lives in one shared file, published with a 72-run HTML vs Markdown benchmark.
Linked from
Field notes that cover Markdown (.md)
More file formats
- 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.
- JSON (.json)A data-interchange format; the usual shape for agent tool calls, structured outputs and configuration such as Claude Code settings.