llms.txt is an emerging proposal for a Markdown file, usually served at /llms.txt, that gives compatible AI tools a curated overview and links to important content. It is not a permissions file, a ranking directive, or a requirement adopted by every major AI or search provider.
In plain terms
The proposal uses a site or project heading, optional context, and grouped Markdown links to canonical resources.
It complements HTML, sitemap.xml, and robots.txt but does not replace them or override crawler access rules.
Google Search states that llms.txt and other special AI files are not required and are not used as special eligibility signals for AI Overviews or AI Mode.
Why it matters
A concise curated index can help compatible agents find canonical reference pages with less navigation.
Because adoption is voluntary and unconfirmed across major providers, llms.txt should be treated as supplementary documentation rather than a visibility requirement.