@sroy I just faced exactly same error this morning, with a trivial migration like yours. I just needed to add a new field with default value to a model, actually it can be migrated automatically but I needed to do some additional work so I created a migration plan. And now it turned out there is a system bug with CloudKit that was supposed to be fixed! I wonder so no one can do SwiftData migration now? let assume most people use iCloud sync.

Is there anyone see sroy feedback report above? I clicked and it said Feedback not found!!! Thanks Ziqiao. I filed the report. It's FB16712454.

Safari is as slow as Chrome, and as I said I use Chrome for everything and it also works well with this forum too so it's surely DNS, or Apple network issue or something else which I don't know. I suspect it's DNS issue since I experienced same issue in the past with a few other websites (web pages reference to a wrong JS, CSS file). Of course the root cause is a developer carelessly added an incorrect CSS/JS link. Thank you for your response.

Sadly there is not much change in SwiftData in WWDC25 while fatal issues like this one must be fixed as soon as possible. Just to allow SwiftData works normally.