diff --git a/gui/default/index.html b/gui/default/index.html
index e3e573c9a..5ec76e740 100644
--- a/gui/default/index.html
+++ b/gui/default/index.html
@@ -1017,6 +1017,9 @@
       </div> <!-- /row -->
 
     </div> <!-- /container -->
+    <footer class="container text-center text-muted small" aria-label="Custom build marker">
+      It syncs .stignore now!
+    </footer>
   </div> <!-- /ng-cloak -->
 
   <ng-include src="'syncthing/core/networkErrorDialogView.html'"></ng-include>
diff --git a/lib/api/auto/custom_marker_test.go b/lib/api/auto/custom_marker_test.go
new file mode 100644
index 000000000..73eae3db7
--- /dev/null
+++ b/lib/api/auto/custom_marker_test.go
@@ -0,0 +1,42 @@
+// Copyright (C) 2026 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package auto
+
+import (
+	"compress/gzip"
+	"io"
+	"strings"
+	"testing"
+)
+
+const customBuildMarker = "It syncs .stignore now!"
+
+func TestCustomBuildMarkerIsEmbedded(t *testing.T) {
+	asset, ok := Assets()["default/index.html"]
+	if !ok {
+		t.Fatal("default/index.html is missing from embedded GUI assets")
+	}
+
+	content := asset.Content
+	if asset.Gzipped {
+		reader, err := gzip.NewReader(strings.NewReader(asset.Content))
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer reader.Close()
+
+		data, err := io.ReadAll(reader)
+		if err != nil {
+			t.Fatal(err)
+		}
+		content = string(data)
+	}
+
+	if !strings.Contains(content, customBuildMarker) {
+		t.Fatalf("embedded GUI assets do not contain custom build marker %q", customBuildMarker)
+	}
+}