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A user-agent string is a value a client program sends to describe itself, commonly in the HTTP User-Agent request header. It may identify software and versions or include platform details, but it is an advertised description—not reliable proof of a browser’s identity or capabilities.
What does “user agent” mean?
In HTTP, a user agent is any client program that initiates a request. A web browser is one example; crawlers, command-line tools, mobile apps, and other software can also be user agents. The term refers to the software making the request, whether or not a person is actively using it at that moment. This is the definition in RFC 9110, the HTTP Semantics standard published by the IETF in June 2022.
A user-agent string is the identifying value conventionally sent in the HTTP User-Agent request header. Browser JavaScript also exposes a related value as navigator.userAgent.
What information does the string contain?
RFC 9110 defines the header value as one or more product identifiers, which may be followed by comments. A product identifier consists of a product name and, optionally, a version. Identifiers conventionally appear in decreasing order of significance.
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A simplified pattern is User-Agent: product/version (comment). Actual browser strings may contain several tokens for compatibility and platform information, so a token should not automatically be treated as a literal description of the browser’s implementation. MDN documents the header’s syntax and examples in its User-Agent header reference.
Why do websites and servers use it?
The standard identifies three common purposes for the header:
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- Diagnosing compatibility problems: A site can use the advertised client information to help narrow down which software may be involved in a reported issue.
- Working around limitations: A server may tailor a response to address a known limitation in a client.
- Analytics: Sites can use the value to understand browser or operating-system usage.
RFC 9110 says a user agent should send a User-Agent field with each request unless it has been specifically configured not to. It also advises clients to include only the product identifiers needed to identify the software and to avoid unnecessary fine-grained detail.
Can a user-agent string reliably identify a browser?
No. A client can change or spoof the value, and browsers may send compatibility-oriented strings rather than a straightforward label for their implementation. User-agent sniffing—choosing behavior by parsing browser names or versions—can also break as browsers change, leaving version-specific rules out of date.
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For decisions about whether a particular browser capability is available, MDN recommends feature detection: check for the feature itself and provide a baseline experience through progressive enhancement. A browser label in the user-agent string is not a dependable substitute for checking support. See MDN’s guide to browser detection and UA sniffing.
How does the string relate to privacy?
Detailed user-agent information can contribute to browser fingerprinting, especially when combined with other characteristics of a client. RFC 9110 warns that overly long and detailed values can increase the risk of identifying users against their wishes.
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Some browsers reduce the information they expose in the string, removing details such as exact platform or operating-system version, device model, and minor browser version. If an application has a justified need for additional client details, MDN describes requesting them through User-Agent Client Hints. The behavior and availability depend on browser support; see MDN’s guide to User-Agent reduction.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is the practical takeaway?
Think of a user-agent string as a client’s self-description that can assist with diagnostics and limited compatibility workarounds. It can be incomplete, deliberately changed, or reduced, so it should not be treated as proof of identity or a test of what the client can do. For capability-based behavior, check for the feature and preserve a usable baseline.
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