Anubis is designed to stop a certain class of badly behaved bots. It intentionally doesn't run if a bot identifies itself with a UA, such as Googlebot, because then you can rate limit it or block by UA and with other tools.

Anubis is active when a user agent looks like a web browser (e.g. contains the "Mozilla" substring every major browser uses). The reverse proxy serves an interstitial page that does a proof-of-work check, validated server side, setting a cookie if it passes.

This means a legitimate user won't constantly get the proof of work check, because they already passed it. But AI bots rotating through tons of residential IPs to scrape your forum or git forge or whatever will be slowed down.

Overall, I like the idea. It's unobtrusive, privacy preserving, and seems to be working out well for a lot of sites.