diff --git a/proto/message_contents/frames.proto b/proto/message_contents/frames.proto
index 9018f0c..1f4545c 100644
--- a/proto/message_contents/frames.proto
+++ b/proto/message_contents/frames.proto
@@ -48,4 +48,6 @@ message FrameAction {
   bytes installation_signature = 4;
   // The public installation id used to sign.
   bytes installation_id = 5;
+  // The inbox id of the installation used to sign.
+  string inbox_id = 6;
 }