History of User Agent Header in Http Requests

The user-agent is a line of text for identifying the browser you are using, its version number and some system details such as operating system, its version by the hosting web server. When a browser connects to a website, it will add the user-agent field in HTTP header for all the requests by default.

You can imagine the user-agent is the way of introducing himself to the hosting web server. Even though the concept sounds very simple, the logic around has grown increasingly complicated over the years.