diff --git a/src/components/Navbar.jsx b/src/components/Navbar.jsx
index b30360d..09f4db8 100644
--- a/src/components/Navbar.jsx
+++ b/src/components/Navbar.jsx
@@ -19,12 +19,13 @@ import {
 } from '@tabler/icons-react'
 import { openProjectAdmin } from '../lib/employeeAdminApi'
 import { serverpulsApi } from '../lib/serverpulsApi'
+import { startsCollapsed } from '../lib/sidebar'
 
 export default function Sidebar() {
   const { user, logout, isAdmin } = useAuth()
   const navigate = useNavigate()
   const location = useLocation()
-  const [collapsed, setCollapsed] = useState(false)
+  const [collapsed, setCollapsed] = useState(() => startsCollapsed())
   const [switching, setSwitching] = useState(false)
   const [pulseLevel, setPulseLevel] = useState('ok')
 
diff --git a/src/lib/sidebar.js b/src/lib/sidebar.js
new file mode 100644
index 0000000..5ab119b
--- /dev/null
+++ b/src/lib/sidebar.js
@@ -0,0 +1,12 @@
+// Seitenmenue: auf schmalen Bildschirmen (Handy) startet es eingeklappt, sonst bleiben vom Inhalt bei 390 px nur
+// etwa 140 px und jede Seite scrollt waagerecht. Ausklappen geht weiterhin per Knopf.
+export const AUTO_COLLAPSE_MAX_WIDTH = 720
+
+/** true, wenn das Menue eingeklappt starten soll (Fensterbreite bis 720 px); ohne window (Test, Server) false. */
+export function startsCollapsed(win = typeof window !== 'undefined' ? window : null) {
+  try {
+    return Boolean(win?.matchMedia?.(`(max-width: ${AUTO_COLLAPSE_MAX_WIDTH}px)`)?.matches)
+  } catch {
+    return false
+  }
+}
diff --git a/tests/unit/sidebar.test.mjs b/tests/unit/sidebar.test.mjs
new file mode 100644
index 0000000..7f832ce
--- /dev/null
+++ b/tests/unit/sidebar.test.mjs
@@ -0,0 +1,27 @@
+import test from 'node:test'
+import assert from 'node:assert/strict'
+import { loadModule } from './load.mjs'
+
+const { startsCollapsed, AUTO_COLLAPSE_MAX_WIDTH } = await loadModule('src/lib/sidebar.js')
+
+// Fenster mit matchMedia, das "(max-width: Npx)" gegen eine feste Breite prueft
+const windowOf = (width) => ({
+  matchMedia: (query) => ({ matches: width <= Number(/max-width:\s*(\d+)px/.exec(query)?.[1]) }),
+})
+
+test('startsCollapsed: Handy (390 px) und Grenze (720 px) starten eingeklappt', () => {
+  assert.equal(AUTO_COLLAPSE_MAX_WIDTH, 720)
+  assert.equal(startsCollapsed(windowOf(390)), true)
+  assert.equal(startsCollapsed(windowOf(720)), true)
+})
+
+test('startsCollapsed: breitere Fenster starten ausgeklappt', () => {
+  assert.equal(startsCollapsed(windowOf(721)), false)
+  assert.equal(startsCollapsed(windowOf(1280)), false)
+})
+
+test('startsCollapsed: ohne window oder matchMedia (oder bei Fehler) ausgeklappt', () => {
+  assert.equal(startsCollapsed(null), false)
+  assert.equal(startsCollapsed({}), false)
+  assert.equal(startsCollapsed({ matchMedia: () => { throw new Error('kaputt') } }), false)
+})