Turns out it was a mismatch between the Xcode version and the Beta version, probably something to do with SwiftUi headers? Anyway updating solved it.

Me too. I messed around with profiles to no use.

Ive logged a bug on this on the Feedback app.

Makes no sense. Regardless of whether vision PRO is beta or not it should be in Xcode 15. I just downloaded the RC on a new machine with a slow connection in the utterly sane assumption that the RC would have everything in the beta version, are we expected to always be on a beta version from now on if we want to continue to develop for this platform.

Is SWIFT_DEFAULT_ACTOR_ISOLATION available by default, does it have to be set , or unset, and is it in Xcode 27

Clearly Xcode 27 was a typo. My question is whether this behaviour was default or not for new Apps, If Apple expects developers to turn it on then its unlikely that it "will end up using the second approach for the main app and any libraries that are overwhelmingly UI-focused".