diff --git a/Examples/UndoForMacOS/UndoForMacOSApp.swift b/Examples/UndoForMacOS/UndoForMacOSApp.swift
index 6e4bb2c..75a0644 100644
--- a/Examples/UndoForMacOS/UndoForMacOSApp.swift
+++ b/Examples/UndoForMacOS/UndoForMacOSApp.swift
@@ -37,6 +37,7 @@ struct DemoFeature {
   enum Action: UndoManageableAction {
     case undoManager(UndoManagingAction)
     case addItem
+    case addItemInBackground
     case addUntrackedItem
     case incrementCount(Int)
     case incrementAll
@@ -64,6 +65,16 @@ struct DemoFeature {
         }
         return .none
 
+      case .addItemInBackground:
+        return .run { _ in
+          try await undoable("Add Item (Background)") {
+            try await database.write { db in
+              let nextID = (try DemoItem.all.fetchAll(db).map(\.id).max() ?? 0) + 1
+              try DemoItem.insert { DemoItem(id: nextID, name: "Item \(nextID)") }.execute(db)
+            }
+          }
+        }
+
       case .addUntrackedItem:
         withErrorReporting {
           try undoable("Add Untracked Item") {
@@ -113,6 +124,7 @@ struct DemoFeature {
 struct DemoView: View {
   @Bindable var store: StoreOf<DemoFeature>
   @Environment(\.undoManager) var undoManager
+  @State private var observableUndo = ObservableUndoManager()
 
   var body: some View {
     VStack(spacing: 16) {
@@ -124,8 +136,8 @@ struct DemoView: View {
         .foregroundStyle(.secondary)
 
       HStack {
-        Button("Undo") { undoManager?.undo() }.disabled(!(undoManager?.canUndo ?? false))
-        Button("Redo") { undoManager?.redo() }.disabled(!(undoManager?.canRedo ?? false))
+        Button("Undo") { observableUndo.undo() }.disabled(!observableUndo.canUndo)
+        Button("Redo") { observableUndo.redo() }.disabled(!observableUndo.canRedo)
       }
 
       List {
@@ -154,6 +166,10 @@ struct DemoView: View {
           store.send(.addItem)
         }
         .buttonStyle(.borderedProminent)
+        Button("Add Item (Background)") {
+          store.send(.addItemInBackground)
+        }
+        .buttonStyle(.bordered)
         Button("Increment All") {
           store.send(.incrementAll)
         }
@@ -169,6 +185,62 @@ struct DemoView: View {
     .padding()
     .frame(width: 400)
     .setUndoManager(store: store)
+    .onChange(of: undoManager, initial: true) { _, newValue in observableUndo.set(newValue) }
+  }
+}
+
+/// Makes UndoManager's canUndo/canRedo state observable by SwiftUI.
+///
+/// UndoManager doesn't participate in SwiftUI's observation system, so
+/// changes to canUndo/canRedo don't trigger view updates. This wrapper
+/// listens to NSUndoManager notifications and exposes observable properties.
+@Observable
+final class ObservableUndoManager {
+  private(set) var canUndo = false
+  private(set) var canRedo = false
+
+  private var undoManager: UndoManager?
+  private var observations: [Any] = []
+
+  func set(_ undoManager: UndoManager?) {
+    self.undoManager = undoManager
+    observations.removeAll()
+    guard let undoManager else {
+      canUndo = false
+      canRedo = false
+      return
+    }
+    update()
+    let nc = NotificationCenter.default
+    let handler: (Notification) -> Void = { [weak self] _ in self?.update() }
+    observations = [
+      nc.addObserver(
+        forName: .NSUndoManagerDidCloseUndoGroup,
+        object: undoManager,
+        queue: .main,
+        using: handler
+      ),
+      nc.addObserver(
+        forName: .NSUndoManagerDidUndoChange,
+        object: undoManager,
+        queue: .main,
+        using: handler
+      ),
+      nc.addObserver(
+        forName: .NSUndoManagerDidRedoChange,
+        object: undoManager,
+        queue: .main,
+        using: handler
+      ),
+    ]
+  }
+
+  func undo() { undoManager?.undo() }
+  func redo() { undoManager?.redo() }
+
+  private func update() {
+    canUndo = undoManager?.canUndo ?? false
+    canRedo = undoManager?.canRedo ?? false
   }
 }
 
diff --git a/Sources/SQLiteUndo/UndoEngine.swift b/Sources/SQLiteUndo/UndoEngine.swift
index 1d025c2..6dfea8a 100644
--- a/Sources/SQLiteUndo/UndoEngine.swift
+++ b/Sources/SQLiteUndo/UndoEngine.swift
@@ -183,13 +183,11 @@ extension UndoEngine: DependencyKey {
         guard let barrier = try coordinator.endBarrier(id) else {
           return
         }
-        MainActor.assumeIsolated {
-          undoStack.registerBarrier(
-            barrier,
-            { try coordinator.performUndo(barrier: barrier) },
-            { try coordinator.performRedo(barrier: barrier) }
-          )
-        }
+        undoStack.registerBarrier(
+          barrier,
+          { try coordinator.performUndo(barrier: barrier) },
+          { try coordinator.performRedo(barrier: barrier) }
+        )
       },
       cancelBarrier: { id in
         try coordinator.cancelBarrier(id)
diff --git a/Sources/SQLiteUndo/UndoStack.swift b/Sources/SQLiteUndo/UndoStack.swift
index 13b05ba..91def7d 100644
--- a/Sources/SQLiteUndo/UndoStack.swift
+++ b/Sources/SQLiteUndo/UndoStack.swift
@@ -27,7 +27,7 @@ public struct UndoStack: Sendable {
   ///
   /// Called by UndoEngine when a barrier completes with changes.
   public var registerBarrier:
-    @MainActor @Sendable (
+    @Sendable (
       _ barrier: UndoBarrier,
       _ onUndo: @escaping @Sendable () throws -> Void,
       _ onRedo: @escaping @Sendable () throws -> Void
@@ -144,7 +144,8 @@ extension UndoStack: DependencyKey {
               }
               if let self {
                 logger.info(
-                  "\(self.currentState.logDescription(after: "undo \"\(barrier.name)\""))")
+                  "\(self.currentState.logDescription(after: "undo \"\(barrier.name)\""))"
+                )
               }
               target.registerRedo(barrier: barrier, onUndo: onUndo, onRedo: onRedo)
             } catch {
@@ -185,7 +186,8 @@ extension UndoStack: DependencyKey {
               }
               if let self {
                 logger.info(
-                  "\(self.currentState.logDescription(after: "redo \"\(barrier.name)\""))")
+                  "\(self.currentState.logDescription(after: "redo \"\(barrier.name)\""))"
+                )
               }
               target.registerUndo(barrier: barrier, onUndo: onUndo, onRedo: onRedo)
             } catch {
@@ -204,7 +206,18 @@ extension UndoStack: DependencyKey {
           $0.undo.append(barrier.name)
           $0.redo = []
         }
-        target.registerUndo(barrier: barrier, onUndo: onUndo, onRedo: onRedo)
+        // NSUndoManager requires main thread
+        if Thread.isMainThread {
+          MainActor.assumeIsolated {
+            target.registerUndo(barrier: barrier, onUndo: onUndo, onRedo: onRedo)
+          }
+        } else {
+          DispatchQueue.main.sync {
+            MainActor.assumeIsolated {
+              target.registerUndo(barrier: barrier, onUndo: onUndo, onRedo: onRedo)
+            }
+          }
+        }
       },
       currentState: {
         UndoStackState(
diff --git a/Tests/SQLiteUndoTests/UndoEngineTests.swift b/Tests/SQLiteUndoTests/UndoEngineTests.swift
index 5bab8ba..c650639 100644
--- a/Tests/SQLiteUndoTests/UndoEngineTests.swift
+++ b/Tests/SQLiteUndoTests/UndoEngineTests.swift
@@ -426,6 +426,38 @@ enum UndoEngineTests {
       }
     }
 
+    @Test
+    func endBarrierFromBackgroundThread() throws {
+      let testUndoManager = UndoManager()
+      try withDependencies {
+        let database = try! makeTestDatabase()
+        $0.defaultDatabase = database
+        $0.defaultUndoStack = .live(testUndoManager)
+        $0.defaultUndoEngine = try! UndoEngine(for: database, tables: TestRecord.self)
+      } operation: {
+        @Dependency(\.defaultDatabase) var database
+        @Dependency(\.defaultUndoEngine) var undoEngine
+
+        let barrierId = try undoEngine.beginBarrier("Background Insert")
+        try database.write { db in
+          try TestRecord.insert { TestRecord(id: 1, name: "Test") }.execute(db)
+        }
+
+        // End the barrier from a background thread
+        DispatchQueue.global().sync {
+          try! undoEngine.endBarrier(barrierId)
+        }
+
+        #expect(testUndoManager.canUndo == true)
+        #expect(testUndoManager.undoActionName == "Background Insert")
+
+        testUndoManager.undo()
+
+        let count = try database.read { db in try TestRecord.all.fetchCount(db) }
+        #expect(count == 0)
+      }
+    }
+
     @Test
     func undoRedoStackStateTransitions() throws {
       let testUndoManager = UndoManager()