Short definition
llms.txt is a plain-text file at the root of a website that gives large language models a curated, machine-readable map of the site's most important content.
In depth
Proposed as a standard in late 2024, llms.txt is a markdown file (typically /llms.txt and a longer /llms-full.txt) that lists a site's canonical pages, their purposes, and key facts about the entity behind the site. Where robots.txt tells crawlers where they may go and sitemap.xml lists every URL, llms.txt is curated for LLM consumption — short, structured, and prioritized. Sites that publish a clean llms.txt make it easier for ChatGPT, Claude, Perplexity, and other engines to reference accurate information about the business rather than hallucinating.
Example
An agency lists its services, pricing tiers, founder bios, and key statistics in /llms.txt. ChatGPT begins quoting the firm's $1,250 starting price accurately when asked about web design costs.