diff --git a/specification/grpc/a2a.proto b/specification/grpc/a2a.proto
index 0c32eb358..41eaa0341 100644
--- a/specification/grpc/a2a.proto
+++ b/specification/grpc/a2a.proto
@@ -198,6 +198,7 @@ message Part {
     FilePart file = 2;
     DataPart data = 3;
   }
+  // Optional metadata associated with this part.
   google.protobuf.Struct metadata = 4;
 }