diff --git a/src/frontend/src/App.tsx b/src/frontend/src/App.tsx
index ee62e12..23b6530 100644
--- a/src/frontend/src/App.tsx
+++ b/src/frontend/src/App.tsx
@@ -25,6 +25,7 @@ import {
   clearUserId,
   ensureUserId,
   hasStoredUserId,
+  USER_SESSION_INVALIDATED_EVENT,
   updateUserId,
 } from "./lib/userIdentity";
 import { THEME_MODE_KEY } from "./lib/constants";
@@ -363,6 +364,28 @@ export default function App() {
     };
   }, []);
 
+  useEffect(() => {
+    const handleSessionInvalidated = () => {
+      setUserId("");
+      setUserEmail("");
+      setIsOnboardingOpen(true);
+      navigateTo("/");
+      message.warning("Your session is no longer valid. Please sign in again.");
+    };
+
+    window.addEventListener(
+      USER_SESSION_INVALIDATED_EVENT,
+      handleSessionInvalidated,
+    );
+
+    return () => {
+      window.removeEventListener(
+        USER_SESSION_INVALIDATED_EVENT,
+        handleSessionInvalidated,
+      );
+    };
+  }, []);
+
   const themeConfig = useMemo(
     () => ({
       algorithm: themeMode === "dark" ? darkAlgorithm : defaultAlgorithm,
diff --git a/src/frontend/src/lib/api.ts b/src/frontend/src/lib/api.ts
index 0b9a0d1..5ec9433 100644
--- a/src/frontend/src/lib/api.ts
+++ b/src/frontend/src/lib/api.ts
@@ -1,4 +1,4 @@
-import { ensureUserId } from "./userIdentity";
+import { ensureUserId, invalidateUserSession } from "./userIdentity";
 import type { ApiError } from "./types";
 
 const API_BASE = "/api";
@@ -17,6 +17,15 @@ export function isNotFoundError(error: unknown) {
   return !!(maybeError && (maybeError.isNotFound || maybeError.status === 404));
 }
 
+function shouldClearStoredUserId(status: number, detail: unknown) {
+  if (status !== 401) {
+    return false;
+  }
+
+  const normalizedDetail = String(detail || "").trim().toLowerCase();
+  return normalizedDetail === "invalid user id" || normalizedDetail === "missing user id";
+}
+
 export async function apiFetch<T = any>(url: string, options: RequestInit = {}): Promise<T> {
   const fullUrl = url.startsWith("/api") ? API_BASE + url.slice(4) : url;
   const userId = ensureUserId();
@@ -45,6 +54,10 @@ export async function apiFetch<T = any>(url: string, options: RequestInit = {}):
 
   if (!response.ok) {
     const payload = await response.json().catch(() => ({} as { detail?: string }));
+    if (shouldClearStoredUserId(response.status, payload?.detail)) {
+      invalidateUserSession();
+    }
+
     throw createApiError(payload.detail || `Request failed (${response.status})`, {
       status: response.status,
     });
diff --git a/src/frontend/src/lib/userIdentity.ts b/src/frontend/src/lib/userIdentity.ts
index fbd52f2..d9c57e8 100644
--- a/src/frontend/src/lib/userIdentity.ts
+++ b/src/frontend/src/lib/userIdentity.ts
@@ -2,6 +2,7 @@ import { USER_TOKEN_KEY } from "./constants";
 import { getStoredValue, removeStoredValue, setStoredValue } from "./storage";
 
 const emailByUserId = new Map<string, string>();
+export const USER_SESSION_INVALIDATED_EVENT = "lunchtime:user-session-invalidated";
 
 function generateGuid() {
   if (typeof crypto !== "undefined" && typeof crypto.randomUUID === "function") {
@@ -76,3 +77,11 @@ export function regenerateUserId() {
 export function clearUserId() {
   removeStoredValue(USER_TOKEN_KEY);
 }
+
+export function invalidateUserSession() {
+  clearUserId();
+
+  if (typeof window !== "undefined") {
+    window.dispatchEvent(new Event(USER_SESSION_INVALIDATED_EVENT));
+  }
+}