The convention mirrors robots.txt in location but inverts its purpose: instead of restricting crawlers, it curates for them. A standard llms.txt contains an H1 with the site name, a blockquote one-line summary, and grouped links to key pages with short descriptions. A companion llms-full.txt can carry extended detail.
Its highest-value line is the summary: AI systems frequently quote it verbatim when describing a site, making llms.txt the cheapest message control available. The file must stay accurate and in sync with the sitemap — a stale llms.txt misdescribes you in every answer that uses it.