macOS Mojave (Developer Beta) - Safari User Agent change not persistent I’m attempting to set a permanent new User Agent for Safari running on macOS Mojave (Developer Beta 10.14.0). However, it doesn’t work and changes to default each time. I ran the command below but it doesn’t seem to do anything. Is the behaviour possibly changed?

defaults write com.apple.Safari CustomUserAgent "\"Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en) AppleWebKit/419 (KHTML, like Gecko) Safari/419.3\""
If things stop working (or don't work as expected) after an upgrade to a beta version of Mojave the most likely cause is a bug in macOS (that's why it's called Beta after all). Best course of action is to report it to Apple using the usual feedback channels for beta releases, ideally in a way which makes it easy for Apple to reproduce the issue.