diff --git a/apps/builder/app/shared/sync-client.ts b/apps/builder/app/shared/sync-client.ts
index 3abc3199783..ee7a7846126 100644
--- a/apps/builder/app/shared/sync-client.ts
+++ b/apps/builder/app/shared/sync-client.ts
@@ -42,8 +42,9 @@ export class ImmerhinSyncObject implements SyncObject {
   }
   setState(state: Map<string, unknown>) {
     for (const [namespace, $store] of this.store.containers) {
-      // immer is not able to work with Map instances from another realm
-      // use clone to recreate data with current realm classes
+      // Immer cannot handle Map instances from another realm.
+      // Use `clone` to recreate the data with the current realm's classes.
+      // This works because the structured clone algorithm skips prototype chains; classes must be defined in both realms.
       $store.set(structuredClone(state.get(namespace)));
     }
   }
@@ -86,7 +87,10 @@ export class NanostoresSyncObject implements SyncObject {
   }
   addTransaction(transaction: Transaction) {
     this.operation = "add";
-    this.store.set(transaction.payload);
+    // `instanceof` checks do not work with instances like Map, File, etc., from another realm.
+    // Use `clone` to recreate the data with the current realm's classes.
+    // This works because the structured clone algorithm skips prototype chains; classes must be defined in both realms.
+    this.store.set(structuredClone(transaction.payload));
     this.operation = "local";
   }
   revertTransaction(_transaction: RevertedTransaction) {