User-Agent Guide
A collection of short documents summarizing the concept, structure, and use of the User-Agent.
What Is a User-Agent Explains the basic concept of what the User-Agent string a browser sends to a server is and why it exists.
Anatomy of the User-Agent String Structure Breaks down what each piece of the UA string starting with Mozilla/5.0 means.
What Are User-Agent Client Hints Introduces User-Agent Client Hints (UA-CH), the modern standard that replaces and supplements the UA string.
How to Identify Bots and Crawlers Safely A rundown of how to reliably distinguish bots without judging by the User-Agent alone.
Distinguishing Mobile from Desktop UA Explains the signals for telling mobile, tablet, and desktop apart in the User-Agent, and common pitfalls.
Changing the User-Agent and Ethics Examines when changing the UA is legitimate, when to avoid it, and crawling etiquette.