diff --git a/gnu/tests.scm b/gnu/tests.scm
index 1e3dbf0944..84298de448 100644
--- a/gnu/tests.scm
+++ b/gnu/tests.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2016-2020, 2022-2024 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016-2020, 2022-2025 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2017 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
@@ -48,6 +48,7 @@
 
             marionette-service-type
             marionette-operating-system
+            marionette-program
             define-os-with-source
 
             %simple-os
@@ -72,11 +73,15 @@
 ;;;
 ;;; Code:
 
+(define %default-marionette-device
+  ;; Default marionette device in the guest.
+  "/dev/virtio-ports/org.gnu.guix.port.0")
+
 (define-record-type* <marionette-configuration>
   marionette-configuration make-marionette-configuration
   marionette-configuration?
   (device           marionette-configuration-device ;string
-                    (default "/dev/virtio-ports/org.gnu.guix.port.0"))
+                    (default %default-marionette-device))
   (imported-modules marionette-configuration-imported-modules
                     (default '()))
   (extensions       marionette-configuration-extensions
@@ -92,7 +97,10 @@
     (with-extensions extensions
       gexp)))
 
-(define (marionette-program device imported-modules extensions)
+(define* (marionette-program #:optional
+                             (device %default-marionette-device)
+                             (imported-modules '())
+                             (extensions '()))
   "Return the program that runs the marionette REPL on DEVICE.  Ensure
 IMPORTED-MODULES and EXTENSIONS are accessible from the REPL."
   (define code