diff --git a/src/components/layout/auth-shell.tsx b/src/components/layout/auth-shell.tsx
index 829a742ce..23fb7b3a7 100644
--- a/src/components/layout/auth-shell.tsx
+++ b/src/components/layout/auth-shell.tsx
@@ -48,7 +48,7 @@ export function AuthShell({
    */
   demoMode?: boolean;
 }) {
-  const { user, isAuthenticated, isLoading } = useAuth();
+  const { user, isAuthenticated, isAuthUnknown, isLoading } = useAuth();
   const { t } = useTranslations();
   const pathname = usePathname();
   const router = useRouter();
@@ -129,17 +129,35 @@ export function AuthShell({
 
   useEffect(() => {
     if (!isLoading && !isAuthenticated && !isPublicPage) {
-      // Session END (expired / invalidated cookie → `/api/auth/me` 401). Wipe
-      // the in-memory query cache (the cross-user guard — the QueryClient
-      // outlives the navigation on this SPA and the health-data families are
-      // not user-scoped) plus the persisted IndexedDB + SW caches, so nothing
-      // leaks to the next account on a shared browser. Logout does the same
-      // from `useLogout`; this covers the expiry path logout never runs
-      // through. Best-effort; the redirect proceeds regardless.
-      clearCachesForSessionEnd(queryClient);
+      // Session END (expired / invalidated cookie → `/api/auth/me` 401/403).
+      // Wipe the in-memory query cache (the cross-user guard — the
+      // QueryClient outlives the navigation on this SPA and the health-data
+      // families are not user-scoped) plus the persisted IndexedDB + SW
+      // caches, so nothing leaks to the next account on a shared browser.
+      // Logout does the same from `useLogout`; this covers the expiry path
+      // logout never runs through. Best-effort; the redirect proceeds
+      // regardless.
+      //
+      // `isAuthUnknown` = the auth probe failed WITHOUT a session verdict
+      // (offline relaunch, network blip, 5xx) and no prior-session marker
+      // exists. Nothing to protect and nothing to render — send the visitor
+      // to the login gate, but never run the wipe off a non-verdict: the
+      // wipe on a network failure is exactly what used to destroy the
+      // offline caches. (With the marker present, `isAuthenticated` holds
+      // the last-known state and this branch never runs.)
+      if (!isAuthUnknown) {
+        clearCachesForSessionEnd(queryClient);
+      }
       router.replace("/auth/login");
     }
-  }, [isLoading, isAuthenticated, isPublicPage, router, queryClient]);
+  }, [
+    isLoading,
+    isAuthenticated,
+    isAuthUnknown,
+    isPublicPage,
+    router,
+    queryClient,
+  ]);
 
   // Redirect non-admins away from /admin
   useEffect(() => {
diff --git a/src/hooks/__tests__/use-auth-offline-gate.test.ts b/src/hooks/__tests__/use-auth-offline-gate.test.ts
new file mode 100644
index 000000000..cb5b54533
--- /dev/null
+++ b/src/hooks/__tests__/use-auth-offline-gate.test.ts
@@ -0,0 +1,144 @@
+/**
+ * Offline auth gate — a failed `/api/auth/me` is only a session END when the
+ * server said so (401/403).
+ *
+ * Pre-fix, ANY auth-probe error read as "logged out": an airplane-mode
+ * relaunch redirected to /auth/login and `clearCachesForSessionEnd()` wiped
+ * the SW data/page caches and the IndexedDB query snapshot — destroying
+ * exactly the installed-PWA-opened-offline scenario the offline plumbing
+ * exists for. These tests pin the classification (`isAuthVerdictUnknown`),
+ * the last-known-state fallback (`readWasAuthenticated` marker), and the
+ * marker's session-end lifecycle.
+ */
+
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { QueryClient } from "@tanstack/react-query";
+
+const clearOfflineCachesForSessionEnd = vi.fn().mockResolvedValue(undefined);
+vi.mock("@/lib/pwa/query-persister", () => ({
+  clearOfflineCachesForSessionEnd: () => clearOfflineCachesForSessionEnd(),
+}));
+
+vi.mock("next/navigation", () => ({
+  useRouter: () => ({ push: vi.fn(), replace: vi.fn() }),
+}));
+vi.mock("@/lib/i18n/context", () => ({
+  useTranslations: () => ({ t: (key: string) => key }),
+}));
+
+// Drivable query state so the hook's classification can be exercised for
+// success / network-error / 401 without a fetch layer.
+interface MockQueryState {
+  data: unknown;
+  isError: boolean;
+  error: unknown;
+  isLoading: boolean;
+}
+let queryState: MockQueryState;
+vi.mock("@tanstack/react-query", async () => {
+  const actual = await vi.importActual<
+    typeof import("@tanstack/react-query")
+  >("@tanstack/react-query");
+  return {
+    ...actual,
+    useQuery: () => ({ ...queryState, refetch: vi.fn() }),
+    useQueryClient: () => ({}),
+    useMutation: () => ({ mutate: () => undefined, isPending: false }),
+  };
+});
+
+import { ApiError } from "@/lib/api/api-fetch";
+import {
+  clearCachesForSessionEnd,
+  isAuthVerdictUnknown,
+  readWasAuthenticated,
+  useAuth,
+} from "../use-auth";
+
+// Node test env has no localStorage — back it with a Map.
+const store = new Map<string, string>();
+beforeEach(() => {
+  store.clear();
+  queryState = { data: undefined, isError: false, error: null, isLoading: false };
+  vi.stubGlobal("localStorage", {
+    getItem: (k: string) => store.get(k) ?? null,
+    setItem: (k: string, v: string) => void store.set(k, v),
+    removeItem: (k: string) => void store.delete(k),
+  });
+});
+afterEach(() => {
+  vi.unstubAllGlobals();
+  clearOfflineCachesForSessionEnd.mockClear();
+});
+
+describe("isAuthVerdictUnknown — failure classification", () => {
+  it("treats a fetch TypeError (offline / DNS) as no verdict", () => {
+    expect(isAuthVerdictUnknown(new TypeError("Failed to fetch"))).toBe(true);
+  });
+
+  it("treats the SW's synthetic 503 offline envelope as no verdict", () => {
+    expect(
+      isAuthVerdictUnknown(new ApiError("offline", 503, { offline: true })),
+    ).toBe(true);
+  });
+
+  it("treats an abort/timeout as no verdict", () => {
+    expect(
+      isAuthVerdictUnknown(new DOMException("timeout", "TimeoutError")),
+    ).toBe(true);
+  });
+
+  it("treats a 5xx as no verdict — a server hiccup is not a logout", () => {
+    expect(isAuthVerdictUnknown(new ApiError("boom", 500))).toBe(true);
+  });
+
+  it("treats 401 and 403 as the real session-end verdict", () => {
+    expect(isAuthVerdictUnknown(new ApiError("unauthorized", 401))).toBe(false);
+    expect(isAuthVerdictUnknown(new ApiError("forbidden", 403))).toBe(false);
+  });
+});
+
+describe("useAuth — last-known state on a non-verdict failure", () => {
+  it("stays authenticated on a network failure when a prior session marker exists", () => {
+    store.set("healthlog-was-authenticated", "1");
+    queryState.isError = true;
+    queryState.error = new TypeError("Failed to fetch");
+
+    const auth = useAuth();
+    expect(auth.isAuthenticated).toBe(true);
+    expect(auth.isAuthUnknown).toBe(true);
+    expect(auth.user).toBeNull();
+  });
+
+  it("reads unauthenticated-but-unknown on a network failure without a marker", () => {
+    queryState.isError = true;
+    queryState.error = new TypeError("Failed to fetch");
+
+    const auth = useAuth();
+    expect(auth.isAuthenticated).toBe(false);
+    // The shell must NOT run the session-end wipe off this state.
+    expect(auth.isAuthUnknown).toBe(true);
+  });
+
+  it("reads a true 401 as session end — wipe and redirect may proceed", () => {
+    store.set("healthlog-was-authenticated", "1");
+    queryState.isError = true;
+    queryState.error = new ApiError("unauthorized", 401);
+
+    const auth = useAuth();
+    expect(auth.isAuthenticated).toBe(false);
+    expect(auth.isAuthUnknown).toBe(false);
+  });
+});
+
+describe("session-end marker lifecycle", () => {
+  it("clearCachesForSessionEnd drops the marker so the next cold offline launch lands on the login gate", () => {
+    store.set("healthlog-was-authenticated", "1");
+    expect(readWasAuthenticated()).toBe(true);
+
+    clearCachesForSessionEnd(new QueryClient());
+
+    expect(readWasAuthenticated()).toBe(false);
+    expect(clearOfflineCachesForSessionEnd).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/src/hooks/use-auth.ts b/src/hooks/use-auth.ts
index 7330c70c1..c4bb806f7 100644
--- a/src/hooks/use-auth.ts
+++ b/src/hooks/use-auth.ts
@@ -9,7 +9,7 @@ import {
 import { useRouter } from "next/navigation";
 import { toast } from "sonner";
 import { queryKeys } from "@/lib/query-keys";
-import { apiGet, apiFetchRaw } from "@/lib/api/api-fetch";
+import { apiGet, apiFetchRaw, ApiError } from "@/lib/api/api-fetch";
 import { retryOnceOnTransientError } from "@/lib/queries/retry-transient";
 import { useTranslations } from "@/lib/i18n/context";
 import type {
@@ -149,6 +149,64 @@ export interface AuthUser {
   moduleAvailability?: Partial<Record<ModuleKey, boolean>>;
 }
 
+/**
+ * Non-sensitive "a session existed here" marker (localStorage boolean, no
+ * identity data). A cold offline launch cannot reach `/api/auth/me`, so the
+ * auth query errors without ever answering the session question — the marker
+ * is the last-known state that lets the shell render the cached app chrome
+ * instead of bouncing to the login gate and wiping the offline caches. Set on
+ * every successful `/api/auth/me`, cleared on every session END (logout and
+ * true 401/403).
+ */
+const WAS_AUTHENTICATED_KEY = "healthlog-was-authenticated";
+
+export function readWasAuthenticated(): boolean {
+  try {
+    return localStorage.getItem(WAS_AUTHENTICATED_KEY) === "1";
+  } catch {
+    return false;
+  }
+}
+
+function storeWasAuthenticated(): void {
+  try {
+    localStorage.setItem(WAS_AUTHENTICATED_KEY, "1");
+  } catch {
+    /* storage unavailable — the marker is best-effort */
+  }
+}
+
+function clearWasAuthenticated(): void {
+  try {
+    localStorage.removeItem(WAS_AUTHENTICATED_KEY);
+  } catch {
+    /* best effort */
+  }
+}
+
+/**
+ * True when an `/api/auth/me` failure carries NO verdict on the session —
+ * a network-level failure (fetch `TypeError` offline, abort/timeout), the
+ * service worker's synthetic 503 offline envelope (`meta.offline`), the
+ * browser reporting itself offline, or a server-side 5xx. None of these mean
+ * "logged out"; only an explicit 401/403 does. Treating them as session end
+ * used to redirect an offline relaunch to /auth/login and wipe the offline
+ * caches — destroying exactly the installed-PWA-opened-offline scenario the
+ * SW + query persister exist for.
+ */
+export function isAuthVerdictUnknown(error: unknown): boolean {
+  if (error instanceof ApiError) {
+    // 5xx (incl. the SW's synthetic 503 with `meta.offline: true`) and any
+    // other non-auth status: the server never ruled on the session. Only an
+    // explicit 401/403 is a verdict.
+    return error.status !== 401 && error.status !== 403;
+  }
+  // Everything below the HTTP layer is transport-level by construction:
+  // fetch `TypeError` (offline / DNS / connection refused), `AbortError` /
+  // `TimeoutError` DOMException from the 15 s default signal.
+  return true;
+}
+
 async function fetchMe(): Promise<AuthUser> {
   // v1.16.4 — routed through the typed wrapper; a non-OK /me (401)
   // throws `ApiError`, which `useAuth` treats as "not authenticated"
@@ -183,6 +241,8 @@ async function fetchMe(): Promise<AuthUser> {
     ? data.dateFormat
     : "AUTO";
   storeDateFormat(dateFormat);
+  // A confirmed session: refresh the non-sensitive offline-relaunch marker.
+  storeWasAuthenticated();
   return {
     ...(data as AuthUser),
     disableCoach: data.disableCoach ?? false,
@@ -227,10 +287,30 @@ export function useAuth() {
     staleTime: 5 * 60 * 1000,
   });
 
+  // v1.27.x offline fix — classify a failed `/api/auth/me` before acting on
+  // it. Only an explicit 401/403 means "session ended"; a network-level
+  // failure (offline relaunch, SW 503 offline envelope, timeout) or a 5xx
+  // carries no verdict, so the hook holds the LAST-KNOWN auth state (the
+  // localStorage marker) instead of flipping to unauthenticated. Pre-fix, an
+  // airplane-mode relaunch read as a logout: the shell redirected to
+  // /auth/login and wiped every offline cache (data 7→0 entries, pages 1→0
+  // in the audit measurement), making the whole offline story unreachable.
+  const verdictUnknown = query.isError && isAuthVerdictUnknown(query.error);
+  const isAuthenticated =
+    !!query.data || (verdictUnknown && readWasAuthenticated());
+
   return {
     user: query.data ?? null,
     isLoading: query.isLoading,
-    isAuthenticated: !!query.data,
+    isAuthenticated,
+    /**
+     * True while the session question is genuinely unanswered — the auth
+     * probe failed at the transport level (or 5xx) and no cached `/me`
+     * payload exists. The shell uses it to suppress the session-end cache
+     * wipe: a wipe without a 401/403 verdict would destroy the offline
+     * caches on every network blip.
+     */
+    isAuthUnknown: !query.data && verdictUnknown,
     error: query.error,
     refetch: query.refetch,
   };
@@ -257,6 +337,10 @@ export function useAuth() {
  */
 export function clearCachesForSessionEnd(queryClient: QueryClient): void {
   queryClient.clear();
+  // The offline-relaunch marker is session state too: after a real session
+  // end the next cold offline launch must land on the login gate, not the
+  // cached shell.
+  clearWasAuthenticated();
   void clearOfflineCachesForSessionEnd();
 }