A user agent is the software that acts on behalf of a user when interacting with web servers. Most commonly, it refers to the web browser that sends requests, renders pages, and executes scripts. The user agent identifies itself to servers using a user agent string, which includes details about the browser, operating system, and device. Servers may use this information to tailor content or troubleshoot compatibility issues. User agents play a central role in how clients interpret HTML, CSS, and JavaScript. Mobile apps, bots, and scripts can also act as user agents when making HTTP requests.
key characteristics
User agents construct HTTP requests, manage cookies, execute frontend code, and enforce security boundaries such as the same origin policy. They interpret responses, render visual output, and handle redirects. User agents affect how pages behave because different browsers implement standards with subtle differences. Developers often inspect user agent strings during debugging or analytics. User agents may also expose capabilities like viewport size, network conditions, and platform features to applications. Understanding user agents helps ensure consistent behavior across devices.