SwiftData document-based app crashes on undo/redo with autosaveEnabled

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Overview

I'm developing a document-based app for macOS using SwiftData. When I undo/redo changes using Command-Z/ Command-Shift-Z, the app randomly crashes with the following error:

SwiftData/BackingData.swift:425: Fatal error: Failed to retrieve the identifier for \ChildItem.parentItem from KnownKeysDictionary:KnownKeysMap: ["parentItem": 2, "isModified": 1, "index": 0] values: [Optional(0), Optional(false), Optional(DocumentTest.ParentItem)] SwiftData._KKMDBackingData<DocumentTest.ChildItem>

And sometimes, instead of the app crashing, my created @Model objects simply disappear. They do not reappear in the @Query on undo/redo.

Both of these issues go away when I set

modelContext.autosaveEnabled = false

The issues are occurring with Xcode 26.4 (17E192) and macOS Tahoe 26.4 (25E246).

I have modified the macOS Document App project template to showcase the issue. The project, along with a screen recording of the crash, can be downloaded from here:

https://drive.google.com/drive/folders/1aDO34QleTm_rB9BuvVGjzzAP6jDXOc-o?usp=share_link

Has anyone else experienced this? I'd like to know if this is a bug in the autosave feature of SwiftData and if I should file a bug report via Feedback Assistant.

Steps to Reproduce

To recreate the issue, follow these steps:

Download and extract the "Xcode Project.zip" file linked above.
Open the extracted "DocumentTest" project in Xcode.
Build and run the "DocumentTest" app.
In the document selection window, click "New Document" at the bottom-left.
In the app, click the "+" button at the top-right to add a ParentItem with ChildItems.
Click on the added ParentItem's button to modify one of its ChildItems.
Repeat steps 5–6 until you have 5 ParentItems with a modified ChildItem.
Press Command-Z 10 times to undo all the changes.
Press Command-Shift-Z 10 times to redo all the changes.
Repeat steps 8–9 until either the app crashes or some of the 5 ParentItems go missing in the list (you may have to repeat them 10–20 times before the issue occurs).

If you change line 43 of ContentView.swift to

modelContext.autosaveEnabled = false

and repeat the same steps above, the app will not crash and no ParentItems will go missing.

Code

ParentItem Model

@Model
final class ParentItem {
    var timestamp: Date
    
    @Relationship(
        deleteRule: .cascade,
        inverse: \ChildItem.parentItem
    )
    var childItems: [ChildItem] = []

    init(timestamp: Date) {
        self.timestamp = timestamp
    }
}

ChildItem Model

@Model
final class ChildItem {
    var index: Int
    
    var isModified = false
    
    var parentItem: ParentItem?

    init(index: Int) {
        self.index = index
    }
}

Creating, Inserting, and Linking ParentItem and ChildItem

// Create and insert ParentItem

let newParentItem = ParentItem(
    timestamp: Date()
)
modelContext.insert(newParentItem)

// Create and insert ChildItems

var newChildItems: [ChildItem] = []

for index in 0..<Int.random(in: 2...8) {
    let newChildItem = ChildItem(index: index)
    newChildItems.append(newChildItem)
    modelContext.insert(newChildItem)
}

/* Establish relationship between
 ParentItem and ChildItems */

newParentItem.childItems = newChildItems

Modifying ChildItem

let firstChildItem = parentItem.childItems
    .sorted(by: { $0.index < $1.index }).first

if
    let firstChildItem,
    !firstChildItem.isModified
{
    firstChildItem.isModified = true
}

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