--- a/sys/bus/u4b/serial/umoscom.c (production, verbatim)
+++ b/umoscom_prod_guarded.c (harness copy)
@@ -44,6 +44,23 @@
 #include <bus/u4b/usb_process.h>
 
 #include <bus/u4b/serial/usb_serial.h>
+
+#ifdef UMOSCOM_HARNESS
+/*
+ * DF-5455 HARNESS GUARD, the ONLY change to production (see
+ * prod_guard.diff): reroute
+ * the EP0 control-request macro to a shim that reproduces the traced
+ * usbd_do_request_flags() failure semantics (usb_request.c:642-650 timeout
+ * breaks out BEFORE the usbd_copy_out at :680-681; a zero-length data
+ * stage copies nothing at :666-683) so the caller's buffer stays
+ * untouched, exactly like a silent/stalling/short-replying evil MCS7703.
+ * All production call sites stay byte-identical.
+ */
+void		mo_shim_request(struct usb_device_request *, void *, int);
+#undef	ucom_cfg_do_request
+#define	ucom_cfg_do_request(udev, com, req, ptr, flags, timo)	\
+	mo_shim_request((req), (ptr), (timo))
+#endif
 
 #ifdef USB_DEBUG
 static int umoscom_debug = 0;