Is your llms.txt valid?
Enter a website. We fetch /llms.txt and check it against the format: a single H1 first, a blockquote summary, then sections of link items.
What this check is not
A server that answers every URL with a web page will answer /llms.txt with HTML too. We report that as missing, not valid. And a valid file has no proven effect on whether AI tools cite your site; the check tells you whether the file is well formed, nothing more.