How the User-Agent header works
“User agent” can mean the client software itself. User-Agent is the specific HTTP header that the client sends with a request.
GET /products HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36The string may claim a browser family, operating system and rendering engine. Many tokens exist for historical compatibility, so reading them literally can be misleading. Clients can also change this header, which means it isn’t proof of their real software or device.
What websites use it for
- Compatibility: work around known browser limitations or send supported features.
- Device-specific content: choose a mobile or desktop response, though responsive design is often preferred.
- Analytics: estimate browser, operating system and device usage.
- Traffic checks: flag missing, malformed or unusual strings as one input to bot detection.
A site shouldn’t treat the header as a reliable identity. Many users can send the same string and one client can send different strings.
User-Agent vs browser fingerprinting
| User-Agent | Browser fingerprinting | |
|---|---|---|
| Source | Declared HTTP header | Measured browser signals |
| Shows | Claimed client and platform | Combined properties such as fonts and canvas output |
| Can change | Easy to edit | Requires several signals to remain consistent |
| Identification | Weak by itself | Probabilistic, not certain |
Browser fingerprinting can compare the declared User-Agent with JavaScript results, screen size and other properties. A desktop Chrome string paired with mobile browser behavior may look inconsistent.
Using user agents with proxies
A proxy server changes the source IP that the destination sees, but it usually doesn’t choose your User-Agent. Your browser, HTTP library or crawler supplies that header.
Use a string that matches the client you actually run. For browser automation, letting the browser generate its normal headers keeps the User-Agent aligned with its rendering engine and supported features. Follow the site’s terms and robots.txt, keep request rates reasonable and don’t treat a different User-Agent as permission to bypass access controls.
FAQ
Does a proxy change my user agent?
Usually no. A proxy changes the visible IP address, while your browser or HTTP client continues to send its configured User-Agent header.
How can I check my user agent?
Open your browser’s developer tools, select a request in the Network panel and inspect its request headers. Command-line clients can print or set the header through their request options.
Can changing User-Agent stop bot detection?
No. Websites may also inspect request patterns, cookies, JavaScript properties and TLS behavior. A fabricated User-Agent that conflicts with those signals can make the client easier to flag.