diff --git a/pxtsim/embed.ts b/pxtsim/embed.ts
index 0f3ecf99a416..59ed1873a763 100644
--- a/pxtsim/embed.ts
+++ b/pxtsim/embed.ts
@@ -83,6 +83,7 @@ namespace pxsim {
     export interface SimulatorBroadcastMessage extends SimulatorMessage {
         broadcast: boolean;
         toParentIFrameOnly?: boolean;
+        srcFrameIndex?: number;
     }
 
     export interface SimulatorControlMessage extends SimulatorBroadcastMessage {
diff --git a/pxtsim/simdriver.ts b/pxtsim/simdriver.ts
index 525189be2a48..66bd18a54aee 100644
--- a/pxtsim/simdriver.ts
+++ b/pxtsim/simdriver.ts
@@ -319,6 +319,8 @@ namespace pxsim {
 
             const broadcastmsg = msg as pxsim.SimulatorBroadcastMessage;
             if (source && broadcastmsg?.broadcast) {
+                // include index of the source iframe
+                broadcastmsg.srcFrameIndex = frames.findIndex((item) => item.contentWindow === source)
                 // if the editor is hosted in a multi-editor setting
                 // don't start extra frames
                 const single = !!this._currentRuntime?.single;