What a valid llms.txt looks like
Under the llmstxt.org proposal, the file is Markdown at /llms.txt. It starts with a single H1 (# Name), then a blockquote summary (> ...), optional paragraphs, and H2 sections of links written as - [Title](url): note.
A section titled Optional marks links an assistant can skip when it is short on space.
Questions
Is llms.txt required?
No. It is a proposed standard that some AI tools read. It is quick to add and does no harm.
Should it be HTML?
No, plain Markdown text. An HTML page at /llms.txt usually means the server returned your 404 page.
What about llms-full.txt?
Some sites add llms-full.txt with the full content in one file. This validator checks llms.txt.
Do you check that every link works?
It checks that links are well formed and on a real host; it does not fetch every page.