llms.txt
Also known as: llm.txt, LLM manifest
A proposed plain text file at the root of a site describing what the site covers and which pages matter, written for language models rather than crawlers.
Where robots.txt says what may be fetched, llms.txt is meant to say what is worth reading. The proposal is young, support across systems is uneven, and nobody should present it as a ranking factor.
It is cheap insurance: a small file, written once, that costs nothing if it is ignored and helps if it is not.
Both spellings exist in the wild. This thermometer checks llms.txt first and falls back to llm.txt, because the portfolio it was built in standardised on the second one before the first became the common form. That is a historical accident rather than a recommendation.
Keep the caveat attached when you talk about this. A file whose support is genuinely uneven should be described as having uneven support, not sold as a requirement.