diff --git a/frontend/src/api.ts b/frontend/src/api.ts
index 3ae9074..fa93d10 100644
--- a/frontend/src/api.ts
+++ b/frontend/src/api.ts
@@ -4,6 +4,10 @@ const BASE = '/twin-optimiser/api'
 
 async function request<T>(path: string, opts?: RequestInit): Promise<T> {
   const res = await fetch(BASE + path, { credentials: 'include', ...opts })
+  if (res.status === 401) {
+    ;(window.top ?? window).location.href = '/login'
+    throw new Error('Unauthenticated')
+  }
   if (!res.ok) {
     const body = await res.json().catch(() => ({})) as { error?: string }
     throw new Error(body.error || `HTTP ${res.status}`)