diff --git a/test/gtest/ucp/test_ucp_proto_mock.cc b/test/gtest/ucp/test_ucp_proto_mock.cc
index 512d26a9b3a..91025c2feea 100644
--- a/test/gtest/ucp/test_ucp_proto_mock.cc
+++ b/test/gtest/ucp/test_ucp_proto_mock.cc
@@ -292,6 +292,7 @@ class test_ucp_proto_mock_rcx : public test_ucp_proto_mock {
     {
         set_mock_iface_attr("rc_mlx5",
             [](uct_iface_attr_t &iface_attr) {
+                iface_attr.cap.flags       |= UCT_IFACE_FLAG_AM_SHORT;
                 iface_attr.cap.am.max_short = 208;
                 iface_attr.bandwidth.shared = 10000000000;
                 iface_attr.latency.c        = 0.000006;