diff --git a/backend/workflows/static/workflows/js/session_warning.js b/backend/workflows/static/workflows/js/session_warning.js
index 6ea9ca5..9b460d5 100644
--- a/backend/workflows/static/workflows/js/session_warning.js
+++ b/backend/workflows/static/workflows/js/session_warning.js
@@ -18,6 +18,8 @@
   let lastConfirmedAt = Date.now();
   let warningVisible = false;
   let keepaliveInFlight = false;
+  let timeoutCheckInFlight = false;
+  let redirectInFlight = false;
 
   function getCsrfToken() {
     const cookie = document.cookie
@@ -44,11 +46,22 @@
     status.textContent = "";
   }
 
+  function redirectToLogin() {
+    if (redirectInFlight) return;
+    redirectInFlight = true;
+    window.location.href = config.loginUrl;
+  }
+
   function readStoredConfirmedAt() {
     try {
       const raw = window.localStorage.getItem(storageKey);
       const parsed = raw ? Number.parseInt(raw, 10) : NaN;
-      return Number.isFinite(parsed) ? parsed : null;
+      if (!Number.isFinite(parsed)) return null;
+      const maxAgeMs = config.idleTimeoutSeconds * 1000;
+      if (Date.now() - parsed >= maxAgeMs) {
+        return null;
+      }
+      return parsed;
     } catch (_error) {
       return null;
     }
@@ -95,8 +108,9 @@
         credentials: "same-origin",
         body: JSON.stringify({ keepalive: true }),
       });
-      if (!response.ok) {
-        window.location.href = config.loginUrl;
+      const contentType = (response.headers.get("content-type") || "").toLowerCase();
+      if (!response.ok || response.redirected || !contentType.includes("application/json")) {
+        redirectToLogin();
         return;
       }
       syncConfirmedAt(Date.now(), "self");
@@ -108,12 +122,46 @@
         hideStatus();
       }, 1200);
     } catch (_error) {
-      window.location.href = config.loginUrl;
+      redirectToLogin();
     } finally {
       keepaliveInFlight = false;
     }
   }
 
+  async function confirmSessionOrRedirect() {
+    if (timeoutCheckInFlight || keepaliveInFlight || redirectInFlight) return;
+    timeoutCheckInFlight = true;
+    try {
+      const response = await fetch(config.keepaliveUrl, {
+        method: "POST",
+        headers: {
+          "Content-Type": "application/json",
+          "X-CSRFToken": getCsrfToken(),
+          "X-Requested-With": "XMLHttpRequest",
+        },
+        credentials: "same-origin",
+        body: JSON.stringify({ keepalive: true, timeout_check: true }),
+      });
+      const contentType = (response.headers.get("content-type") || "").toLowerCase();
+      if (!response.ok || response.redirected || !contentType.includes("application/json")) {
+        try {
+          window.localStorage.removeItem(storageKey);
+        } catch (_error) {
+          // Ignore storage cleanup failures.
+        }
+        redirectToLogin();
+        return;
+      }
+      syncConfirmedAt(Date.now(), "self");
+      hideWarning();
+      hideStatus();
+    } catch (_error) {
+      redirectToLogin();
+    } finally {
+      timeoutCheckInFlight = false;
+    }
+  }
+
   const storedConfirmedAt = readStoredConfirmedAt();
   if (storedConfirmedAt) {
     lastConfirmedAt = storedConfirmedAt;
@@ -149,7 +197,7 @@
     const elapsedSeconds = Math.floor((Date.now() - lastConfirmedAt) / 1000);
     const secondsLeft = config.idleTimeoutSeconds - elapsedSeconds;
     if (secondsLeft <= 0) {
-      window.location.href = config.loginUrl;
+      confirmSessionOrRedirect();
       return;
     }
     if (secondsLeft <= warningLeadSeconds) {