diff --git a/scanslicer/entry.py b/scanslicer/entry.py
index 4f903b9..2e0e858 100644
--- a/scanslicer/entry.py
+++ b/scanslicer/entry.py
@@ -114,3 +114,12 @@ def validate_wayland_environment():
         raise RuntimeError(
             f"Wayland socket not found: {socket_path}"
         )
+
+
+# validate_x11_environment: function definition for ScanSlicer behavior.
+def validate_x11_environment():
+    display_name = os.environ.get("DISPLAY", "").strip()
+    if not display_name:
+        raise RuntimeError(
+            "Missing X11 runtime environment. Set DISPLAY or use --backend wayland."
+        )