Choosing a profile

A browser session has a profile and a platform. The profile sets the TLS ClientHello, the HTTP/2 settings, and the request headers. The platform sets the operating system that the session claims. This page picks both.

Start with the default

Browser::default() is Browser::latest(Family::Chrome), the newest bundled Chrome. Without .platform(), a Chrome session claims Windows.

#![allow(unused)]
fn main() {
use leyline::{Browser, Platform, Session};

fn run() -> leyline::Result<()> {
let windows = Session::browser(Browser::default());
let mac = Session::builder()
    .browser(Browser::default())
    .platform(Platform::MacOS)
    .build()?;
let _ = (windows, mac);
Ok(())
}
}

With the http3 feature, a Chrome session races HTTP/3 against HTTP/2 for an origin that advertised h3. See HTTP/3.

Pick another browser

.browser(Browser::Safari26).platform(Platform::IOS) maps to the Safari on iPhone sibling. Sessions gives the platform defaults and the call order, and Set the languages sets accept-language.

Match the platform

The platform sets Sec-CH-UA-Platform, Sec-CH-UA-Mobile, the User-Agent, and the TCP fingerprint. A site can compare the claimed OS with the TCP fingerprint, so a claim that differs from the real host is visible. Use Platform::Host to claim the OS you build for.

Keep a device stable

Browser::latest moves to newer captures in patch releases, and a release can correct the data behind a pinned variant. For an account that must keep one device, pin the variant and store session.identity().profile_id(). Pass it to SessionBuilder::expect_profile_id on start: build() then fails with Kind::Config when the profile changed, and err.is_profile_changed() is true. To keep the data itself, freeze it with Device::pin_profile. See Profile data stability and Accounts.

Read the JA4 labels

The profile reference labels the JA4 of each profile gated or estimated. A Chrome or Brave profile is estimated because Chrome shuffles the extension order on every connection. JA4 sorts the extensions before it hashes them, so the JA4 stays the same; JA3 does not. To check a live session, see Fingerprints.

Next

Read Requests to build and send a request.