diff --git a/apps/server/src/services/threads/thread-parent.ts b/apps/server/src/services/threads/thread-parent.ts
index 2fc71c394..d061dc9f1 100644
--- a/apps/server/src/services/threads/thread-parent.ts
+++ b/apps/server/src/services/threads/thread-parent.ts
@@ -158,9 +158,7 @@ export function assertValidParentThread(
   }
   const liveParentThread: Thread = parentThread;
 
-  if (liveParentThread.projectId !== args.projectId) {
-    throwParentThreadInvalid("wrong_project");
-  }
+  // EXPERIMENT (#1711): cross-project parents allowed.
   if (liveParentThread.archivedAt !== null) {
     throwParentThreadInvalid("archived");
   }