Sec-CH-UA-PlatformSec-CH-UA-Platform is a low-entropy User-Agent Client Hint conveying the client's operating system (platform) name as a quoted string. With values like Windows, macOS, Linux, Android, or iOS, servers use it to branch OS-specific downloads, guidance, and features.
The value is a Structured Fields string, always sent wrapped in quotes (e.g. "Windows"). It carries only the platform name; the detailed version (e.g. Windows 10 vs 11, macOS 14.x) is split into the high-entropy Sec-CH-UA-Platform-Version header, received only with an Accept-CH opt-in.
Being low-entropy it is sent by default, so the OS can be identified from the first request. Typical uses include auto-selecting the 'download the installer for this OS' button, OS-specific shortcut guidance (Ctrl vs Cmd), and handling font/emoji rendering differences. As a browser-computed value it is more stable than parsing the UA string.
As a Sec-prefixed hint it cannot be forged or set by script. However, the value set may grow over time (including "Unknown") and may be reduced or hidden in privacy modes, so provide a fallback for unknown values. If the response varies by platform, send Vary: Sec-CH-UA-Platform to keep caches consistent.
Sec-CH-UA-Platform: "<platform>"e.g. Sec-CH-UA-Platform: "Windows"
"Windows" / "macOS" / "Linux" | Desktop OS name (a quoted string). |
"Android" / "iOS" / "Chrome OS" | Mobile or other platform names. |
"Unknown" | Value when the platform is unknown or withheld. |