On the Web, a user agent is a software agent capable of and responsible for retrieving and facilitating end user interaction with Web content.^[1] This includes all common web browsers, such as Google Chrome, Mozilla Firefox, and Safari, some email clients, standalone download managers like youtube-dl, other command-line utilities like cURL, and arguably headless services that power part of a larger application, such as a web crawler.

The user agent plays the role of the client in a client–server system. The HTTP User-Agent header is intended to clearly identify the agent to the server. However, this header can be omitted or spoofed, so some websites use other agent detection methods.