llm.txt
A plain-text file at a site's root describing the site for AI systems, similar in spirit to robots.txt.
An llm.txt file gives AI systems a short, human-written orientation to what a site is and what it publishes.
Worth being blunt: adding an llm.txt file does not make a site visible to AI. It is a convention with uneven adoption, useful as part of a structured setup and meaningless on its own. Any offer built primarily around installing one is selling a file, not a system.