llms.txt
A proposed file at the root of a site that hands a model a clean index of the pages worth reading, in Markdown.
The idea is the same as robots.txt but for reading rather than crawling: a short, curated map that saves a model from parsing navigation, banners, and boilerplate to find the documentation.
It is a convention, not a standard, and no crawler is obliged to honour it. Publishing one costs a file; treating it as a guarantee that models will read only what you listed does not follow.