GlossaryUser agent string

What is user agent string?

A user agent string is the text a client sends with every web request to say what it is. Browsers, crawlers and assistants all send one, and it is how a log line reveals which AI agent made a request.

Also called: user agent, UA string, UA.

A crawler’s user agent usually contains a short name (GPTBot, ClaudeBot, PerplexityBot) plus a link to the operator’s documentation. agenttraffic recognises agents by these names, using a maintained registry of the exact strings each operator has published, including older names that still appear in old logs.

The string is self-declared. Anyone can send any user agent, which is why a name in a log proves nothing on its own; see spoofed user agent and bot verification.

A few names people expect to see never appear in a user agent at all. Google-Extended and Applebot-Extended are robots.txt tokens only.

Related terms

  • Spoofed user agent: A spoofed user agent is a request that claims to be a known bot, such as Googlebot or ChatGPT-User, but comes from an IP address outside the ranges that bot’s operator publishes.
  • Bot verification: Bot verification is checking that a request claiming to be a known bot really came from its operator, by comparing the request’s IP address with the addresses the operator publishes, or by reverse DNS when no list exists..
  • robots.txt token: A robots.txt token is the name an operator tells you to use in the User-agent line of robots.txt to address one of its crawlers.

Published 2026-09-21, updated 2026-09-21.

See it in your logs

One command, nothing to install

Point it at CloudFront, nginx or Apache logs. Every AI agent that visited gets its own row, verified against the addresses its operator publishes.

$ npx agenttraffic ./logs --days 7Quick start

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