What is an llms.txt file?

Quick answer

An llms.txt is a plain-text file placed at a website's root that summarises what the site is about and lists its most important pages for AI crawlers — conceptually like robots.txt, but designed to help AI assistants understand and cite your content rather than to control access. A companion llms-full.txt can include expanded content. It's an emerging convention for the AI-first web.