PerplexityBot is Perplexity AI's web crawler that builds the engine's own search index. Unlike ChatGPT Search (which falls back to Bing), Perplexity has no fallback backend — if PerplexityBot cannot reach your site, you do not exist for any Perplexity query.
The user-agent string is: Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; PerplexityBot/1.0; +https://perplexity.ai/perplexitybot). Allow it explicitly with "User-agent: PerplexityBot" + "Allow: /" in robots.txt. A wildcard "User-agent: *" allow is technically enough but explicit posture is what AEO analyzers reward.
Perplexity ships a second bot, Perplexity-User, that fires when a user asks the engine to read a specific URL in real time. Blocking it is the most embarrassing failure mode — the user actively chose your URL and you returned a 403. Allow both bots side by side.
Perplexity has a tighter response-time budget than Bingbot or Googlebot. Pages with slow TTFB get sampled less frequently and dropped from the index faster when the crawler is rate-budgeting. A 2-second TTFB ceiling is a practical threshold for Perplexity indexability.