diff --git a/packages/ui/src/theme/layers.ts b/packages/ui/src/theme/layers.ts
index 94bf0398a..6135bd546 100644
--- a/packages/ui/src/theme/layers.ts
+++ b/packages/ui/src/theme/layers.ts
@@ -1,5 +1,6 @@
-import { getDefaultZIndex } from "@mantine/core";
+// Keep this module server-safe: Mantine's getDefaultZIndex helper is client-only.
+const mantineModalZIndex = 200;
 
-export const appShellHeaderZIndex = getDefaultZIndex("modal") + 1;
+export const appShellHeaderZIndex = mantineModalZIndex + 1;
 export const modalZIndex = appShellHeaderZIndex + 1;
 export const managedModalZIndex = modalZIndex + 1;