SwiftData 'simple' migration failing

This is a long post, so let me start with a summary: I am attempting to implement what "ought to be" a simple SwiftData migration, and am receiving the following fatal error from the ModelContainer initializer:

NSCocoaErrorDomain Code=134504 "Cannot use staged 
migration with an unknown model version."

The crash occurs both in the Simulator and on a physical device.

Both the original schema and the new schema load and run as expected if loaded from scratch — so I conclude the Models are OK; it is the migration from the original schema to the new schema which is the issue.

I have reported this as FB22652791 and Technical Incident Case # 19893980. I have two model projects available — one contrived, the other using my actual SwiftData models.

Now the Details

I am developing a SwiftUI/SwiftData app. I am (currently) using Xcode 26.5-beta-3.

I set up an alpha-test build using the following approach:

public class DatabaseSchema {
	public let dbSchema: Schema = Schema([
		Model1.self, ... , Model16.self
	], version: Schema.Version(0, 9, 0))
	
	public var modelContainer: ModelContainer {
		let container: ModelContainer
		let modelConfiguration = ModelConfiguration(schema: dbSchema,
				isStoredInMemoryOnly: false)
	
	do {
		container = try ModelContainer(for: dbSchema,
				migrationPlan: nil,
				configurations: [modelConfiguration])
	} catch {
		fatalError("Failed to creae model conainer")
	}
	return container
}

This defines database version 0.9.

For version 1.0, I made three changes to the database:

• added an attribute of type String to Model2.
• added three attributes of type [Struct], where Struct conforms to Codable, Equatable and Hashable to Model3, and
• added a new model (which I'll call Model17)

I define two schemas:

public enum Schema090: VersionedSchema {
	public static var versionIdentifier = Schema.Version(0, 9, 0)
	public static var models: [any PersistentModel.Type] = [
		Model1.self, Schema090.Model2.self, 
		Schema090.Model3.self, ... ]
}

and

public enum Schema100: VersionedSchema {
	public static var versionIdentifier = Schema.Version(1, 0, 0)
	public static var models: [any PersistentModel.Type] = [
		Model1.swift, Schema100.Model2.self,
		Schema100.Model3.self, ..., Model16.self, 
		Schema100.Model17.self ]
}

For models that changed, I use the following approach:

public typealias Model3 = Schema100.Model3

extension Schema090 {
	@Model final class Model3 {
		...
	}
	
	public init() {
		...
	}
}

extension Schema100 {
	@Model final class Model3 {
		...
		<added attributes, initialized>
	}
	
	public init() {
		...
	}
}

The DatabaseSchema class was modified as follows:

public class DatabaseSchema {
	public let dbSchema: Schema = Schema([
		Model1.self, Schema100.Model2.self, Schema100.Model3.self,
		... , Model16.self, Schema100.Model17.self
	], version: Schema.Version(1, 0, 0))
	
	public var modelContainer: ModelContainer {
		let container: ModelContainer
		let modelConfiguration = ModelConfiguration(schema: dbSchema,
				isStoredInMemoryOnly: false)
	
	do {
		container = try ModelContainer(for: dbSchema,
				migrationPlan: MigrationPlan.self,
				configurations: [modelConfiguration])
	} catch {
		fatalError("Failed to creae model conainer")
	}
	return container
}

where the migration plan is the trivial custom migration that makes sure that all added attributes of existing records are properly initialized.

enum MigrationPlan: SchemaMigrationPlan {
	static var schemas: [any VersionedSchema.Type] = [
		Schema090.self, Schema100.self ]
	
	static var stages: [MigrationStage] = [version090ToVersion100]
	
	static let version090ToVersion100 = 
		MigrationStage(fromVersion: Schema090.self,
			toVersion: Schema100.self, willMigrate: { _ in },
			didMigrate: { context in
		let models = try context.fetch(
			FetchDescriptor<Schema100.Model3>())
		for model in models {
			< initial the added attributes >
		{
		try context.save()
	})
}

This is all simple stuff. Nothing particularly fancy here. But running this code always crashes in the ModelContainer initializer. In my two sample projects, I get two different error messages —

• in the contrived example, the error message is

Code=134110 "An error occurred during persistent store migration."
reason=Cannot migrate store in-place: Validation error missing 
attribute values on mandatory destination attribute, ...

and in the sample project that uses my actual data model, I get

NSCocoaErrorDomain Code=134504 "Cannot use staged 
migration with an unknown model version."

My Thoughts

Since obviously most folks are doing SwiftData migrations without the problems I am experiencing, the obvious possibilities are

• I'm doing something stupid that I just don't see.
• There is a problem because the original schema was given a version value of Schema.Version(0, 9, 0). (i.e., major version number was 0)
• There is a problem because I am adding an attribute of type [Struct] where Struct is Codable, Hashable, and Equatable. I.e., migration isn't working properly with attributes which are stored as their codable representations.

Or maybe something else? In any case, any help you can offer would be greatly appreciated.