Sec-CH-UA-Platform and Sec-CH-UA-Mobile
Sec-CH-UA-Platform and Sec-CH-UA-Mobile are User-Agent Client Hint request headers. Platform reports the operating-system family (such as Windows, macOS, or Android) and Mobile is a boolean for whether the client is a mobile device. Both are low-entropy hints sent by default in supporting Chromium browsers, offering coarse context without parsing the legacy user agent.
What this means
User-Agent Client Hints split the old monolithic user-agent string into discrete, requestable headers. Sec-CH-UA-Platform and Sec-CH-UA-Mobile are part of the low-entropy set that supporting Chromium browsers send by default on secure requests, so you usually get them without asking.
Platform carries the operating-system family as a quoted string. Mobile is a boolean expressed as ?1 for mobile and ?0 otherwise. Together they cover the two most common detection needs — which OS family and whether it is a phone-class device.
Default vs requested hints
Low-entropy hints (brand list, Mobile, Platform) are sent by default because they reveal little. Higher-entropy details — full platform version, architecture, device model, full browser version — are not sent by default; a site must request them via Accept-CH, and the browser may decline.
This is the point of the design: get coarse context for free, ask explicitly for fine detail, and degrade gracefully when it is withheld. Do not assume high-entropy hints will be present.
- Sec-CH-UA-Platform: OS family as a quoted string
- Sec-CH-UA-Mobile: boolean, ?1 for mobile and ?0 otherwise
- Low-entropy hints sent by default; high-entropy hints require Accept-CH
Using them well
Prefer these hints over user-agent regex for OS family and form factor: they are structured, less ambiguous, and aligned with where the platform is heading. Keep a legacy user-agent fallback for browsers that do not send Client Hints, such as non-Chromium engines.
Stay coarse. Reading platform family and a mobile boolean is privacy-safe; assembling many high-entropy hints into a stable identifier is fingerprinting and defeats the design's intent.
How it appears in analytics and logs
Sec-CH-UA-Platform names the OS family and Sec-CH-UA-Mobile flags mobile vs not. Their presence indicates a Chromium-based browser that supports Client Hints; their values give coarse, structured context the legacy user agent expresses less cleanly.
Diagnostic use case
Read OS family and mobile form factor from low-entropy Client Hints instead of regex-parsing the user agent, and understand which hints are sent by default versus on request.
What WebmasterID can help detect
WebmasterID can read these low-entropy hints to attribute OS family and form factor without brittle user-agent regex, keeping classification structured and privacy-conscious.
Common mistakes
- Expecting high-entropy hints (platform version, model) without requesting them via Accept-CH.
- Assuming non-Chromium browsers send these hints — keep a user-agent fallback.
- Treating Sec-CH-UA-Mobile as a string rather than a boolean (?1 / ?0).
Privacy and accuracy notes
These are deliberately low-entropy hints — OS family and a boolean — designed to limit fingerprinting surface. WebmasterID uses them only for coarse context and never to single out a user.
Frequently asked questions
- Are Sec-CH-UA-Platform and Sec-CH-UA-Mobile sent automatically?
- Yes, in supporting Chromium browsers on secure requests they are part of the low-entropy set sent by default. Higher-entropy hints like platform version or model must be requested with Accept-CH and can be declined.
Related pages
- User-Agent Client Hints
User-Agent Client Hints are HTTP headers (the Sec-CH-UA family) that let a site request specific browser, platform, and version detail rather than reading it all from one passive string. They underpin UA reduction: the raw user agent is shrinking, and finer detail moves to opt-in hints. This page explains the model.
- Mobile vs desktop UA detection
Distinguishing mobile from desktop clients is one of the few user-agent checks still broadly reliable: mobile browsers include a Mobile token and platform markers. But user-agent reduction trims finer detail, so for form-factor decisions the modern, robust approach is the Sec-CH-UA-Mobile Client Hint and responsive design rather than deep UA parsing.
- User agents vs feature detection
Feature detection checks at runtime whether a specific capability (an API, a CSS feature) is available, rather than inferring it from the user-agent string. It is more reliable than user-agent sniffing because it tests the actual environment instead of a self-reported, spoofable label. The user agent still has narrow, legitimate uses, but capability decisions should rest on feature detection.
- Privacy-first analytics
Coarse OS and form-factor context with no fingerprinting.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.