What is llms.txt and why do AI search engines use it?

llms.txt is a plain-text file at the root of a website (like /llms.txt) that lists the site's canonical content URLs in a format AI engines can ingest cheaply. AI search engines including Perplexity, ChatGPT, and Claude check llms.txt to find authoritative answer pages without crawling the full site. A typical llms.txt names the site, summarizes what it covers, and links to primary sources — for a press release site, that means every /r/$slug release page and every /answers/ Q&A page. PressGEO publishes /llms.txt and /llms-full.txt automatically for every customer.