A user-agent string is a value a client program sends to identify itself in a request, most often in the HTTP User-Agent header. It commonly contains a product name, version, and comments about the platform. Servers can use it for troubleshooting, limited compatibility workarounds, and analytics—but it can be changed or reduced, so it is not reliable proof of a browser’s identity or capabilities.
What does “user agent” mean?
A user agent is any client program that initiates a request. A web browser is the familiar example, but crawlers, command-line tools, mobile apps, and other software can also be user agents. The term refers to the requesting program, not necessarily a person actively using it. The HTTP standard defines the term in RFC 9110.
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A user-agent string is the identifying value conventionally sent in the HTTP User-Agent request header. Browser JavaScript exposes a related value as navigator.userAgent. These are ways to read the advertised value; neither makes it a verified identity.
What information is in the string?
RFC 9110 describes the field value as one or more product identifiers, optionally accompanied by comments. A product identifier consists of a product name and, optionally, a version. Identifiers are conventionally ordered from more to less significant, but real browser strings may contain several compatibility and platform tokens.
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A simplified pattern is:
User-Agent: <product>/<product-version> <comment>This is a general format, not a guarantee that every token literally names the browser’s underlying implementation. Some tokens exist for compatibility, so interpreting a real value can be less straightforward than reading its parts. MDN documents the header and browser examples in its User-Agent header reference.
Why do websites and servers use it?
RFC 9110 identifies several common uses for the header:
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- Analytics: estimating browser or operating-system usage from requests.
The standard says a user agent should send a User-Agent field with each request unless it has been specifically configured not to. It also advises including only the product identifiers needed to identify the software and avoiding unnecessary fine-grained detail.
Can a user-agent string reliably identify a browser?
No. The value is an advertisement supplied by the client, not proof. It can be changed or spoofed, and browser vendors may use compatibility-oriented tokens. Browser versions also change, so rules built around particular names or version strings can become outdated. MDN explains these limitations in its guide to browser detection using the user-agent string.
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That makes user-agent sniffing a poor way to decide whether a browser supports a feature. If a site needs to adapt to browser capabilities, MDN recommends feature detection: check whether the feature is actually available, and use progressive enhancement so the basic experience still works when it is not. A browser label describes what the client says it is; a feature check tests what it can do.
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A detailed user-agent string can contribute to browser fingerprinting when combined with other characteristics of a client. RFC 9110 warns that long, detailed values increase the risk of identifying users against their wishes, which is one reason it discourages unnecessary detail.
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In browsers that support User-Agent reduction, some information previously exposed in the string—such as exact platform or operating-system version, device model, and minor browser version—is reduced. When a website has a justified need for additional client information, MDN describes User-Agent Client Hints as an alternative mechanism. Availability and the information provided depend on browser support and configuration; see MDN’s guide to User-Agent reduction.
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