Who runs it
Anthropic. It names itself with the token ClaudeBot. Anthropic doesn't publish a full user agent string for it.
What it does
It collects web content that could potentially contribute to model training. Anthropic runs two other machines, Claude-SearchBot and Claude-User, with their own tokens.
Renders JavaScript
Not yet observed. Anthropic doesn't say whether ClaudeBot runs scripts, and this site hasn't recorded a verified visit yet.
How to verify it
Match the visiting address against Anthropic's published list. It is one list for all three Anthropic machines. Anthropic advises against blocking by IP address, which may not work persistently as an opt-out.
robots.txt
Rules under User-agent: ClaudeBot apply to it. Anthropic says its machines honor industry standard robots.txt directives, and it supports the non-standard Crawl-delay. Put the rule at the top level of every subdomain.
Questions
Do Anthropic's three machines share one robots.txt rule?
No. ClaudeBot, Claude-SearchBot and Claude-User each have their own token.