diff --git a/lib/rspace-header.ts b/lib/rspace-header.ts
index 9ea66b2..cfe0f8a 100644
--- a/lib/rspace-header.ts
+++ b/lib/rspace-header.ts
@@ -47,7 +47,22 @@ function getSession(): SessionState | null {
   }
 }
 
+
 function clearSession(): void {
+  // Notify server so all other browser sessions get revoked
+  try {
+    const stored = localStorage.getItem(SESSION_KEY);
+    if (stored) {
+      const session = JSON.parse(stored) as SessionState;
+      if (session.accessToken) {
+        fetch(`${ENCRYPTID_URL}/api/session/logout`, {
+          method: 'POST',
+          headers: { 'Authorization': `Bearer ${session.accessToken}` },
+        }).catch(() => { /* best-effort */ });
+      }
+    }
+  } catch { /* ignore */ }
+
   localStorage.removeItem(SESSION_KEY);
   localStorage.removeItem('rspace-username');
 }
@@ -921,6 +936,24 @@ export function mountHeader(options: HeaderOptions): void {
       }
     } catch { /* silently ignore */ }
   })();
+
+  // Validate session with server — detects logout from another browser session
+  (async () => {
+    const session = getSession();
+    if (!session) return;
+    try {
+      const res = await fetch(`${ENCRYPTID_URL}/api/session/verify`, {
+        headers: { 'Authorization': `Bearer ${session.accessToken}` },
+      });
+      if (!res.ok) {
+        // Session revoked or invalid — clear locally and re-render
+        localStorage.removeItem(SESSION_KEY);
+        localStorage.removeItem('rspace-username');
+        renderHeader();
+        onAuthChange?.();
+      }
+    } catch { /* network error — let token expire naturally */ }
+  })();
 }
 
 /**