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User agent
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On the Web, a user agent is a software agent responsible for retrieving and facilitating end-user interaction with Web content.[1] This includes all web browsers, such as Google Chrome and Safari, some email clients, standalone download managers like youtube-dl, and other command-line utilities like cURL.[2]
The user agent is the client in a client–server system. The HTTP User-Agent header is intended to clearly identify the agent to the server.[2] However, this header can be omitted or spoofed,[2] so some websites use other detection methods.
References
[edit]- ^ "W3C Definition of User Agent". www.w3.org. 2011-06-16. Retrieved 2018-10-20.
- ^ a b c "User agent". MDN Web Docs. 8 July 2024.
User agent
View on Grokipediafrom Grokipedia
A user agent is any software that retrieves, renders, and facilitates end-user interaction with web content, acting on behalf of the user in client-server communications such as web browsing.[1]
In the context of the Hypertext Transfer Protocol (HTTP), a user agent refers to the client software—such as a web browser, media player, or automated crawler—that originates requests to servers and processes responses.[2] User agents typically include web browsers (e.g., Firefox or Chrome), browser extensions, plug-ins, and other applications like media players or web-based readers that handle web resources.[1]
A key mechanism for user agents is the User-Agent HTTP request header field, which identifies the originating client software to the server, often including details like product name, version, and operating system to support content negotiation and compatibility.[2] This header enables servers to select appropriate representations of resources based on the agent's capabilities, while also aiding in analytics, debugging, and issue resolution, though it raises privacy concerns due to potential device fingerprinting.[2] Beyond HTTP, the term extends to protocols like the Session Initiation Protocol (SIP), where user agents manage communication sessions, but its primary association remains with web technologies.
This ABNF ensures structured yet flexible construction, with
Overview
Definition
In computing, particularly within the context of web communications and the Hypertext Transfer Protocol (HTTP), a user agent refers to any client program or software entity that initiates requests to a server on behalf of a user or an automated process.[3] This encompasses a wide range of implementations beyond traditional web browsers, including web crawlers (spiders), command-line tools, mobile applications, and even embedded devices such as household appliances or firmware update scripts.[3] The user agent serves as an intermediary, handling the transmission of requests and the reception of responses to facilitate interactions between the client and server.[3] The term "user agent" can denote either the software component itself or, more specifically, the identifying string conveyed in HTTP headers during network communications. In the latter sense, the User-Agent header field provides a characteristic string that reveals details about the originating client, such as its application type, operating system, vendor, and version, enabling servers to recognize the requester.[3] This header is optional but commonly included in requests to offer contextual information without guaranteeing precise capability detection. Representative examples illustrate the diversity of user agents. A desktop web browser might send (as of November 2025):Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/130.0.0.0 Safari/537.36, identifying a recent Chrome browser on Windows. In contrast, a mobile app could use: Mozilla/5.0 (iPhone; CPU iPhone OS 18_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.1 Mobile/15E148 Safari/604.1, signaling a recent iOS Safari instance, while a command-line tool like curl might transmit: curl/8.10.1.[3]
Purpose
The User-Agent header serves primary purposes in enabling server-side content adaptation, protocol negotiation, and compatibility detection within HTTP communications. By including details about the client software, such as product name and version, it allows servers to tailor responses to the requesting user agent's capabilities and limitations, for instance, delivering optimized mobile versions of web pages to devices with constrained screens or processing power rather than full desktop layouts.[4] This adaptation is part of HTTP's proactive content negotiation mechanism, where servers examine request headers like User-Agent to select appropriate representations without needing further client queries.[4] In the HTTP protocol, the User-Agent is transmitted as a request header to inform servers about the originating client, facilitating statistical tracking, debugging, and the identification of potential protocol issues.[5] This informs servers of client characteristics, enabling compatibility checks to avoid serving content that could cause rendering errors or suboptimal experiences based on known browser or device behaviors.[6] Key benefits include enhanced performance through customized responses that reduce unnecessary data transfer and processing on the client side, support for indirect feature detection via inferred capabilities, and aggregation for analytics on user demographics, such as browser usage trends or device types.[5] However, due to privacy concerns, modern web browsers have implemented User-Agent reduction, providing less detailed information in the header to mitigate fingerprinting risks, and instead promote Client Hints for secure capability negotiation.[7] These advantages promote efficient web interactions while minimizing additional round-trips for capability discovery. Beyond web contexts, the User-Agent header appears in protocols like SIP for VoIP applications, where it identifies the user agent client for purposes such as debugging and aiding proxies in routing decisions during session establishment.[8] In SIP, this identification helps network elements trace protocol compliance and select appropriate handling without altering core routing logic.[8]Format and Structure
Syntax
The User-Agent header field contains a characteristic string that enables the recipient to identify the software originating an HTTP request, serving as a product identifier for the user agent.[9] Its syntax is defined in RFC 9110 using Augmented Backus-Naur Form (ABNF) asUser-Agent = product *( RWS ( product / comment ) ), where a product consists of a token optionally followed by a slash and product-version (both tokens), and comment encloses additional details in parentheses.[9] This allows for a sequence of one or more products separated by required whitespace (RWS), forming a free-form string that adheres to informal conventions such as product tokens delimited by slashes (e.g., token/version) and spaces between elements.[9]
User agents are recommended to include this header in requests unless specifically configured otherwise, listing products in order of decreasing significance and limiting details to essential identifiers to avoid unnecessary length or privacy risks.[9] Although RFC 9110 discourages mimicking other agents to declare compatibility, as it circumvents the field's purpose, compatibility tokens such as Mozilla/5.0 are commonly prepended in practice to ensure recognition by legacy servers, even if the actual software differs.[9][10] The string has no strict maximum length in the specification, though practical implementations vary in handling long headers, with some servers imposing limits such as 512 characters or more for total header sizes.
Servers parse the User-Agent string primarily through token matching against known products and versions, as no rigid schema exists beyond the ABNF; common patterns include parenthetical comments for platform and rendering engine details (e.g., (platform; details)), followed by browser or client identifiers.[9] Parsing is tolerant of unrecognized elements, with recipients ignoring extraneous parts while prioritizing the first matching product for identification.[9] The field name itself is case-insensitive per HTTP conventions, though product tokens and versions are treated as case-sensitive to preserve exact identification.[11]
The string must conform to token rules, excluding control characters (CTLs) and separators (tspecials such as parentheses outside comments, slashes, or quotes), ensuring only visible characters from the VCHAR set are used.[12] Senders are prohibited from including empty products or excessive subproduct details, and the overall format avoids mimicking other agents to reduce misidentification risks.[9]
User-Agent = product *( RWS ( product / comment ) )
product = token [ "/" product-version ]
product-version = token
comment = "(" *( ctext / quoted-pair / comment ) ")"
User-Agent = product *( RWS ( product / comment ) )
product = token [ "/" product-version ]
product-version = token
comment = "(" *( ctext / quoted-pair / comment ) ")"
RWS requiring at least one SP or HTAB between elements.[12]
Components
The User-Agent string typically comprises several core components that identify the client software, its version, the underlying platform, and compatibility details, allowing servers to tailor responses accordingly. The primary elements include the product name, version numbers, compatibility tokens, platform information, and optional extensions. These components are assembled according to informal conventions rather than a rigid standard, resulting in variations across different clients.[5][10] A common structure follows the formatProduct/Version (Platform; Details) Engine/Version, where the product identifies the application (e.g., "Mozilla/5.0" as a historical compatibility token used by many browsers), followed by version numbers specifying the software release (e.g., "Chrome/142.0.0.0"). The parenthetical section details the platform, such as the operating system and architecture (e.g., "Windows NT 10.0; Win64; x64"), which indicates the host environment like Windows 10 on a 64-bit system. Compatibility tokens, often appearing as extensions, signal rendering engines or legacy support, such as "Gecko/20100101" for Firefox's engine or "AppleWebKit/537.36" for WebKit-based browsers like Chrome and Safari.[10][13]
For example, a typical Firefox string as of November 2025 might be Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:145.0) Gecko/20100101 [Firefox](/page/Firefox)/145.0, where "rv:145.0" denotes the Gecko version, and "Firefox/145.0" specifies the browser release. In contrast, a Chrome example could be Mozilla/5.0 (Macintosh; [Intel](/page/Intel) Mac OS X 10_15_7) AppleWebKit/537.36 ([KHTML](/page/KHTML), like [Gecko](/page/Gecko)) Chrome/142.0.0.0 [Safari](/page/Safari)/537.36, incorporating "KHTML, like Gecko" as a compatibility token to mimic older browsers and "Safari/537.36" as an extension for rendering hints. These parts enable servers to detect browser capabilities without relying on JavaScript.[10][13][14][15]
Since 2021, major browsers have implemented User-Agent reduction to enhance privacy by omitting or generalizing details like minor version numbers, OS versions, and device models that could aid fingerprinting. For instance, reduced Chrome strings might exclude full OS version (e.g., "Macintosh; Intel Mac OS X 10_15_7" becomes more generic), with additional details provided via Client Hints headers if needed. This initiative, supported by standards like RFC 8941 for Client Hints, results in shorter, less identifying strings while maintaining compatibility.[7][16]
Variability in components arises based on the client type; web browsers often include detailed elements for full identification, while bots or minimal clients like curl may use abbreviated strings such as curl/8.17.0, omitting platform and engine details to reduce overhead.[10][13][17] Optional extensions can include language preferences (e.g., "en-US") or device specifics (e.g., "Mobile" for mobile browsers), but these are not universally present. Common identifiers include "Win64" or "x86_64" for architecture, "Android 12" for OS versions, and engine tokens like "Trident/7.0" for older Internet Explorer versions, each serving to convey precise environmental context.[10][13]
History
Origins
The user agent concept emerged in the early 1990s alongside the invention of the World Wide Web at CERN, where Tim Berners-Lee developed the foundational HTTP protocol and associated software between 1989 and 1991.[18] The initial World Wide Web browser, released in 1990 for NeXT computers, operated under the primitive HTTP/0.9 specification, which lacked formal headers but laid the groundwork for client-server communication.[19] As the web expanded, the need for clients to identify themselves became evident to facilitate server-side logging and basic compatibility checks. Early implementations of user agent identification appeared in CERN's libwww library, a C-based API package released in 1992 to enable portable browser development across platforms like Unix, Windows, and Macintosh.[20] Derived from the initial WorldWideWeb browser, this library facilitated subsequent browsers but did not power the very first ones, such as the graphical WorldWideWeb app (later renamed Nexus) from 1990 or the text-based Line Mode Browser, launched in 1991 and suitable for dumb terminals.[19] These early clients sent simple identification strings via ad hoc HTTP extensions predating formal standards. For instance, CERN browsers identified as "CERN-LineMode/2.15 libwww/2.17b3" to inform servers of their software and library versions.[21] The initial purpose of these strings was basic client identification for statistical tracking and troubleshooting on early web servers, including CERN's httpd and the NCSA HTTPd server released in 1993.[22] NCSA HTTPd, one of the first widely adopted servers, logged such identifiers to monitor usage and debug issues in the nascent web ecosystem. Clients like the NCSA Mosaic browser, released in 1993, adopted similar formats, sending strings such as "NCSA_Mosaic/2.0" to signal their capabilities, like inline image rendering, to servers.[22] The user agent was formally introduced as an HTTP request header in HTTP/1.0, specified in RFC 1945 published in May 1996 by the IETF.[21] This standardization defined it as an optional field containing product tokens and comments for the originating user agent, exemplified by "User-Agent: CERN-LineMode/2.15 libwww/2.17b3," to support statistical purposes and trace proxy involvement without mandating detailed parsing.[21]Evolution and Standards
The evolution of user agent strings was profoundly shaped by the browser wars of the 1990s and early 2000s, where intense competition between Netscape Navigator and Microsoft Internet Explorer drove the need for compatibility with web content optimized for specific browsers.[23] During this period, Netscape's introduction of the "Mozilla" token in its user agent string—intended to identify its rendering engine—prompted competitors to include it for access to Netscape-optimized sites, leading to widespread spoofing and an expansion of string complexity to signal compatibility features.[22] This arms race resulted in increasingly verbose strings, as browsers appended details about versions, platforms, and engines to avoid content restrictions, turning the user agent into a de facto compatibility manifest rather than a simple identifier.[24] Key developments in the late 1990s and beyond further diversified user agent formats. In 1999, Internet Explorer 5.0 introduced more detailed platform information, such as specific Windows versions (e.g., "Windows 95"), while retaining the "Mozilla/4.0 (compatible; MSIE 5.0)" prefix to mimic Netscape for broader access.[23] The early 2000s saw the rise of mobile user agents with the proliferation of WAP-enabled devices, exemplified by Nokia's simple strings like "Nokia7110/1.0 (04.84)," which conveyed device model and firmware to support limited mobile web experiences.[25] Post-2010, the shift to evergreen browsers—such as Google Chrome (launched 2008) and Mozilla Firefox with automatic updates—streamlined updates but perpetuated compatibility hacks, with strings like Firefox's "Mozilla/5.0 (Windows NT 10.0; rv:60.0) Gecko/20100101 Firefox/60.0" balancing legacy support and modernity.[23] Standardization efforts aimed to impose structure amid this chaos. The HTTP/1.1 specification in RFC 2616 (1999) formalized the User-Agent header as an optional field for identifying the client software, recommending product tokens like name and version but allowing free-form comments for additional details.[26] This was refined in RFC 7231 (2014), which deprecated certain ambiguous practices and emphasized that user agents should not be relied upon for precise identification due to spoofing risks.[27] For mobile contexts, the W3C-influenced UAProf (User Agent Profile) standard emerged in the early 2000s via the Open Mobile Alliance, using XML documents linked via HTTP headers to describe device capabilities beyond basic strings; though influential for early smartphones, it has become largely legacy as modern protocols favor dynamic queries.[28] As of 2025, modern trends emphasize reduced disclosure for enhanced privacy, countering the historical verbosity. Google's Chrome User-Agent Reduction initiative, launched in 2021, progressively minimizes platform and version details in strings (e.g., omitting minor versions and OS specifics on desktop), replacing them with opt-in Client Hints for servers needing precise data; by 2024, this reduction had been fully implemented across major browsers including Chrome, Firefox, and Safari.[29][30] This privacy-focused minimalism aligns with broader efforts to limit fingerprinting while maintaining compatibility, though it requires web developers to adapt to less informative defaults.[7]Usage
In Web Browsers
Web browsers generate user agent strings to identify themselves to web servers, incorporating details about the browser, rendering engine, operating system, and version. For Google Chrome, the string is constructed in the Chromium source code, typically following the formatMozilla/5.0 (Platform; features) AppleWebKit/537.36 ([KHTML](/page/KHTML), like [Gecko](/page/Gecko)) Chrome/Version [Safari](/page/Safari)/537.36, where the AppleWebKit and KHTML, like Gecko tokens maintain compatibility with legacy sites while indicating the underlying Blink rendering engine.[31]
Mozilla Firefox builds its user agent string as Mozilla/5.0 (Platform; rv:GeckoVersion) Gecko/20100101 Firefox/BrowserVersion, explicitly including the Gecko token to denote its Gecko rendering engine, with the rv field representing the Gecko revision version for precise engine identification.[32]
Apple's Safari constructs its string in the form Mozilla/5.0 (Platform) AppleWebKit/WebKitVersion (KHTML, like Gecko) Version/BrowserVersion Safari/WebKitVersion, using the AppleWebKit token to signal the WebKit rendering engine, which enables servers to tailor responses based on WebKit-specific capabilities.[31]
Microsoft Edge, built on the Blink engine since version 79, formats its user agent as Mozilla/5.0 (Platform) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/Version Safari/537.36 Edg/EdgeVersion, appending the Edg token after the Chrome version to distinguish it from other Blink-based browsers like Chrome.[33]
Servers parse these user agent strings for content negotiation, delivering optimized CSS and JavaScript tailored to the browser's engine and capabilities—for instance, serving WebKit-optimized stylesheets to Safari or Blink-compatible scripts to Chrome and Edge—while excluding unsupported features to reduce load times.[34][31]
This detection also influences font loading, where servers select formats like WOFF2 for modern Gecko or Blink engines versus older formats for legacy compatibility, ensuring efficient rendering without fallback delays.[31]
For viewport optimization, servers use platform indicators in the string (e.g., mobile-specific tokens in iOS Safari or Android Chrome) to serve responsive layouts, such as narrower viewports or touch-optimized CSS, enhancing mobile user experiences.[34][35]
Evergreen browsers like Chrome undergo frequent automatic updates—major releases every four weeks with minor version increments—resulting in rapidly evolving user agent strings that include full versioning for accurate server-side feature mapping.[29]
However, Chrome's User-Agent Reduction initiative, implemented progressively since 2021, freezes minor version details in the string (e.g., reporting only major versions like Chrome/110 for privacy), which stabilizes strings across updates and reduces fingerprinting risks while maintaining compatibility for content delivery.[29][36]
This versioning approach in evergreen models ensures backward compatibility but requires servers to handle granular changes without over-reliance on exact minor versions.[37]
JavaScript developers access the user agent string via the navigator.userAgent property of the Navigator API, which returns the full string for runtime inspection, such as logging browser details in console outputs.[38]
Despite its availability, industry trends deprecate heavy reliance on navigator.userAgent for browser detection due to spoofing vulnerabilities and inconsistencies across engines, favoring feature detection methods like checking if ('geolocation' in navigator) to verify capabilities directly.[38][34]
As an alternative, the User-Agent Client Hints API (e.g., navigator.userAgentData.getHighEntropyValues()) provides opt-in, reduced data like platform and mobile status, aligning with privacy-focused shifts away from full string parsing.[34][39]