Guide
What Is a User-Agent?
A user-agent string is request metadata that describes the software making an HTTP request.
Last reviewed: June 12, 2026
Quick reference
At a glance
- Header value
- Client-supplied text sent in the User-Agent request header.
- Useful for
- Compatibility debugging, coarse analytics, bot hints, and log interpretation.
- Not reliable for
- Security identity, fraud decisions, device proof, or access control by itself.
- Modern context
- Some browsers reduce user-agent detail and expose structured User-Agent Client Hints when a site opts in.
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Guide
What it can contain
User-agent strings often include browser family, browser version, rendering engine, operating system, device type, app name, or bot name.
They are useful for debugging compatibility, rough analytics, and identifying obvious crawlers.
Guide
Why it is imperfect
User-agent strings are sent by the client and can be spoofed, shortened, frozen, or intentionally generic.
Treat parsed user-agent data as a hint. It is not a secure identity signal and should not be used as the only fraud or access-control signal.
Guide
Client hints and user-agent reduction
Modern browsers can reduce passive user-agent detail to limit fingerprinting. Some browser and platform details may instead be available through User-Agent Client Hints when a server explicitly asks for them.
If a parser returns generic or incomplete fields, compare the raw string, request headers, and any available Sec-CH-UA headers before assuming the browser or device is unknown.
Reference
Key terms
- Browser
- Application family such as Chrome, Safari, or Firefox
- Engine
- Rendering engine such as WebKit, Blink, or Gecko
- Bot
- Crawler or automated client signal when detectable
Examples
Examples
Browser string
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 Chrome/148.0.0.0 Safari/537.36
A browser-style user-agent string with platform and engine hints.
Parse this browser string
Bot token
Googlebot/2.1
Example of a crawler-identifying token.
Parse this bot token
Client hint header
Sec-CH-UA
Structured browser brand and version hints can appear when supported and requested.
Next steps
Related guides and tools
Questions
FAQ
Can a user-agent be fake?
Yes. Clients control the user-agent string, so it can be changed or spoofed.
Why do many user agents start with Mozilla?
It is a long-lived compatibility convention. Many modern browsers include Mozilla/5.0 even though they are not the old Mozilla browser.
Sources