Apple released the first beta for iOS 12.1 on Tuesday, Sept. 18. The update comes just one day after the official release of iOS 12. For developers, it's the first beta release since the "gold master" version on Sept. 12.
The biggest change to come with iOS 12.1 is the return of Group FaceTime. The feature was promised for iOS 12, but quietly delayed with the release of dev beta 7. You'll only be able to use Group FaceTime with those running 12.1, however, and will only be able to launch a call from a group chat in Messages. Names of members of that group running iOS 12 or lower will appear in your group chat, but won't be able to join your call.
The only other notable change with this first developer beta is the added compatibility with Xcode beta 10.1. While 12.1 dev beta 1 includes no release notes, Xcode 10.1 does. If you're an Xcode user, make sure to check out what's new.
To download and update your iPhone to iOS 12.1 dev beta 1, you first need to be a registered developer with a valid configuration profile. If you have iOS 12's new "Automatic Updates" enabled, you can let the software update overnight. To download the beta right now, however, launch the Settings app, select "General," then "Software Update." Tap "Download and Install," then follow the on-screen prompts to download and install iOS 12.1 dev beta 1.
Check back here for all issues resolved with iOS 12.1 dev beta 1.
Check back here for all bugs still present in iOS 12.1 dev beta 1.
Apple just started the iOS 12.1 beta, which means the new iPhone XS and XS Max will be shipping with iOS 12. However, it is possible the iPhone XR will ship with 12.1, as Apple's 2018 "budget" iPhone has a delayed release. It all depends when Apple decides to release iOS 12.1 to the public as a stable release.
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