User-Agent
Definition
A User-Agent is an identifier string a client sends to a server to describe the browser, bot, or application making the request.
Full definition
In robots.txt, User-agent: lines select which bot a rule group applies to (for example GPTBot or *). In logs, the User-Agent header helps operators distinguish crawlers from browsers.
Spoofing is possible; User-Agent alone is not authentication. See robots.txt and AI Crawler.