What Is a User Agent?

In web standards, the user agent is the client program that makes requests for a user: a browser, but also a crawler, a command-line tool such as curl, or an app. The HTTP standard defines the User-Agent header as information "about the user agent originating the request", which servers use to identify interoperability problems, tailor responses and analyze traffic (RFC 9110, section 10.1.5). In everyday use, "user agent" usually means the string itself.

Every request carries it: the page, its images and scripts, and the API calls the page makes. Browsers also expose it to JavaScript as navigator.userAgent.

Take Chrome 156 on Windows (the October 2026 version) and split it into its parts:

Other browsers follow the same pattern with their own tokens: Firefox adds rv:<version> and Gecko/20100101 Firefox/<version>, Edge appends Edg/<version>, and Safari sends Version/<version> Safari/605.1.15. MDN's User-Agent reference documents the formats. Current strings for each browser and device are in our list of user agents for scraping.

In the 1990s, sites sent their best pages only to Netscape, whose user agent began with Mozilla. Other browsers started sending Mozilla too, so they would get the same pages, and each new browser copied the tokens of the one before it to avoid being treated as unknown. That is why Chrome's string still names Mozilla, AppleWebKit, KHTML, Gecko and Safari, and why the string is a poor way to identify a browser exactly.

Because the full string helped sites fingerprint users, Chromium browsers now send a reduced one: only the major version (156.0.0.0), a fixed Windows NT 10.0 or Intel Mac OS X 10_15_7, and on Android a placeholder Android 10; K instead of the real version and device model. The details moved into client hints, separate headers such as Sec-CH-UA, Sec-CH-UA-Mobile and Sec-CH-UA-Platform, and a site can ask for more, such as the device model, with Accept-CH (MDN: Client hints). Safari and Firefox have frozen parts of their strings as well.

For a site, this means the User-Agent header alone says less than it used to. For a scraper, it means a Chrome user agent should come with Chrome's client hints, or it contradicts itself.

Three quick ways:

curl https://httpbin.org/user-agent
# {"user-agent": "curl/<version>"}
curl -x gate.proxyfield.io:7000 -U "<login>-country-us-city-new_york:<password>" -A "MyCrawler/1.0 (+https://example.com/bot)" https://httpbin.org/user-agent

To see the IP and location a site sees through a proxy, use our proxy checker.

• Serving the right page: a mobile layout for phones, a download link for your operating system, a fallback for an old browser.
• Analytics: browser and device shares in traffic reports.
• Fixing bugs: working around a problem in one browser version, the use RFC 9110 names first.
• Security and bot detection: blocking clients that announce themselves as scripts, such as python-requests or curl, and comparing the claimed browser with what the request and the page show.

In web scraping, the user agent is the string your scraper sends to say what it is. HTTP libraries send their own name by default (python-requests/<version>, curl/<version>, Scrapy/<version>), and many sites block those strings or serve them a different page. That is why scrapers replace it with a real browser's string, or, for an open crawler, with an honest name and a contact URL.

A browser string is a claim, though, and sites check it. A Chrome user agent with no client hints, a Python TLS handshake or a Linux font list is a contradiction that anti-bot systems look for. How to send a complete, consistent set is in HTTP headers for web scraping, and what the page itself reveals in browser fingerprinting in web scraping.

The user agent is one value the browser states about itself. A browser fingerprint is the combination of many values the site measures, including the user agent, screen size, fonts, time zone, canvas and WebGL output. You can change the user agent with one line; the fingerprint has to agree with it, or the change makes the client more unusual, not less. The full picture is in what is a browser fingerprint.

If you met the term in CSS, it means something related but different: the user agent stylesheet is the browser's built-in default CSS, which gives headings their size and links their blue color before a site's own styles apply. DevTools labels those rules "user agent stylesheet". It has nothing to do with the User-Agent header except that both belong to the browser.

A browser string tells a site what you are; the IP tells it where you are. ProxyField residential proxies: IPs from real home internet providers with country, region and city targeting, from $1.95/GB with no subscription. Start with the Easy Trial (1 GB); traffic never expires, and all purchases are final.