llms.txt Validator

What is llms.txt?

llms.txt is a Markdown file placed at the root of a website (https://example.com/llms.txt) that gives AI agents and LLMs a concise, structured summary of your site and links to its most important pages. Defined at llmstxt.org, the format is plain Markdown — not a key-value config.

The format

Only the H1 site name is required. Everything else is optional but recommended:

# Title

> Optional one-line summary

Optional free-text details.

## Section name

- [Link title](https://example.com/page): optional notes

## Optional

- [Less important link](https://example.com/extra)

What Google's Lighthouse checks

Google's Lighthouse agentic-browsing audit is intentionally minimal: it only fails if the server returns an error (5xx) when fetching /llms.txt. A missing file (404) is treated as "not applicable" — the file is optional. Google defers all format and quality guidance to the llmstxt.org specification, which is what the recommended checks above are based on.

FAQ

Is llms.txt required?

No. It is an optional, advisory file. Google's Lighthouse treats a missing llms.txt as "not applicable", not a failure. Adding one simply helps AI agents understand and cite your site.

What is the only required part?

A single top-level H1 with your site or project name. Everything else — the summary blockquote, sections, and link lists — is optional but improves how agents read your site.

I have an old key-value llms.txt — is it valid?

No. The llmstxt.org standard is Markdown, not the older key-value style (Site:, Contact:, License:). This validator detects legacy files and recommends converting them to the Markdown format.

What is the .md convention?

The spec also suggests publishing a clean Markdown version of important pages at the same URL with .md appended (e.g. /about.md), so agents can read content without HTML noise.