diff --git a/server/shell.ts b/server/shell.ts
index 8511adf7..94fa1e09 100644
--- a/server/shell.ts
+++ b/server/shell.ts
@@ -398,14 +398,17 @@ export function renderShell(opts: ShellOptions): string {
     }
 
     // Install banner — browser only (not shown in installed PWA)
-    // Permanently dismissed once closed or installed (localStorage key persists)
+    // Permanently dismissed once closed or installed.
+    // Uses root-domain cookie so dismissal persists across all subdomains.
     if (!isStandalone) {
-      const installDismissed = () => localStorage.getItem('rspace_install_dismissed') === '1';
+      const installDismissed = () => document.cookie.includes('rspace_install_dismissed=1');
       const dismissInstall = () => {
         const b = document.getElementById('pwa-install-banner');
         if (b) b.style.display = 'none';
         document.body.classList.remove('rspace-banner-visible');
-        localStorage.setItem('rspace_install_dismissed', '1');
+        const host = location.hostname;
+        const domainPart = host.includes('.') ? '; domain=.' + host.split('.').slice(-2).join('.') : '';
+        document.cookie = 'rspace_install_dismissed=1; path=/; max-age=315360000; SameSite=Lax' + domainPart;
       };
 
       window.addEventListener("beforeinstallprompt", (e) => {