What llms.txt is
llms.txt is a proposed standard (from llmstxt.org) for a Markdown file at yourwebsite.com/llms.txt that gives AI tools a short, clean map of your site: what it is and where the important pages are. It's popular for documentation and developer products.
It's not a ranking factor. Google has said Search doesn't use it, and no AI search company has confirmed using it to choose sources. Some AI tools and coding assistants do read it when it's there.
The format
The file starts with an H1 title (# Site name), which is the only required part. Next comes a one-line summary as a quote (> ...), optional details, and then ## sections with lists of links in the form - [Page name](https://...): short note. A section called Optional holds links an AI tool can skip when it's short on space.
What this checker looks at
The title, summary and sections, list lines that aren't proper links, the content type the server sends, the file size, whether an llms-full.txt exists, and the status of up to 15 linked pages.