diff --git a/web/eslint.config.mjs b/web/eslint.config.mjs
index ff4c08c8..c0c7930e 100644
--- a/web/eslint.config.mjs
+++ b/web/eslint.config.mjs
@@ -103,6 +103,20 @@ export default tseslint.config(
       'simple-import-sort/exports': 'error',
     },
   },
+  {
+    // Service workers run in a ServiceWorkerGlobalScope, not a window. Matched by a
+    // cwd-independent glob so this applies whether eslint runs from web/ or the repo root.
+    files: ['**/public/service-worker.js'],
+    languageOptions: {
+      globals: {
+        self: 'readonly',
+        clients: 'readonly',
+        caches: 'readonly',
+        skipWaiting: 'readonly',
+        registration: 'readonly',
+      },
+    },
+  },
   {
     ignores: [
       'public/mtg/*',
@@ -111,7 +125,6 @@ export default tseslint.config(
       'eslint.config.mjs',
       'out/*',
       'coverage/*',
-      'public/service-worker.js',
       'pages_unused/*',
       'tailwind.config.js',
     ],
diff --git a/web/public/service-worker.js b/web/public/service-worker.js
index 473259e4..dedc7de0 100644
--- a/web/public/service-worker.js
+++ b/web/public/service-worker.js
@@ -46,6 +46,9 @@ self.addEventListener('push', (event) => {
     icon: '/icons/icon-192x192.png',
     badge: '/icons/badge-72x72.png',
     data: data.url || '/',
+    // Notifications sharing a tag replace each other, so a conversation only ever
+    // occupies one slot instead of stacking one entry per message.
+    tag: data.collapseKey || undefined,
   }
   event.waitUntil(self.registration.showNotification(title, options))
 })