I figured out my initial issue here. While I was fetching data and converting it to SwiftData, I had forgotten to insert it into SwiftData via context.insert(...). Doing this fixed this issue. Now I have the exact same error in another part of my app after modifying data, so I guess SwiftData/CoreData just throws the same thing regardless of what issue it encounters.

Apple added docs about read-only server side data: https://developer.apple.com/documentation/swiftdata/maintaining-a-local-copy-of-server-data I'm still not sure how one would integrate for read/write.

After I accepted the agreement I had to open Preferences in Xcode and navigate to Accounts to trigger a refresh of my status and make my builds work again. Definitely really frustrating.