Added $http_accept and re-ran. None of them use text/markdown. Results:

ChatGPT-User/1.0 text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,/;q=0.8,application/signed-exchange;v=b3;q=0.9 Claude-User/1.0 / Perplexity-User/1.0 (empty, no Accept header) PerplexityBot/1.0 (empty, no Accept header) ChatGPT sends a Chrome-style Accept string. Claude sends a wildcard. Perplexity sends nothing at all. Gemini didn't fetch in my test.

Also worth noting: Claude-User hit /robots.txt before the page.