Detect backend down (#5010)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/core/contexts/AppConfigContext.test.tsx

@@ -51,6 +51,7 @@ describe('AppConfigContext', () => {
 
     expect(apiClient.get).toHaveBeenCalledWith('/api/v1/config/app-config', {
       suppressErrorToast: true,
+      skipAuthRedirect: true,
     });
   });
 
@@ -282,6 +283,7 @@ describe('AppConfigContext', () => {
     await waitFor(() => {
       expect(apiClient.get).toHaveBeenCalledWith('/api/v1/config/app-config', {
         suppressErrorToast: true,
+        skipAuthRedirect: true,
       });
     });
   });

frontend/src/core/contexts/AppConfigContext.tsx

@@ -87,7 +87,8 @@ export const AppConfigProvider: React.FC<AppConfigProviderProps> = ({
   const isBlockingMode = bootstrapMode === 'blocking';
   const [config, setConfig] = useState<AppConfig | null>(initialConfig);
   const [error, setError] = useState<string | null>(null);
-  const [fetchCount, setFetchCount] = useState(0);
+  // Track how many times we've attempted to fetch. useRef avoids re-renders that can trigger loops.
+  const fetchCountRef = React.useRef(0);
   const [hasResolvedConfig, setHasResolvedConfig] = useState(Boolean(initialConfig) && !isBlockingMode);
   const [loading, setLoading] = useState(!hasResolvedConfig);
 
@@ -96,11 +97,14 @@ export const AppConfigProvider: React.FC<AppConfigProviderProps> = ({
 
   const fetchConfig = useCallback(async (force = false) => {
     // Prevent duplicate fetches unless forced
-    if (!force && fetchCount > 0) {
+    if (!force && fetchCountRef.current > 0) {
       console.debug('[AppConfig] Already fetched, skipping');
       return;
     }
 
+    // Mark that we've attempted a fetch to prevent repeated auto-fetch loops
+    fetchCountRef.current += 1;
+
     const shouldBlockUI = !hasResolvedConfig || isBlockingMode;
     if (shouldBlockUI) {
       setLoading(true);
@@ -112,7 +116,6 @@ export const AppConfigProvider: React.FC<AppConfigProviderProps> = ({
         const testConfig = getSimulatedAppConfig();
         if (testConfig) {
           setConfig(testConfig);
-          setFetchCount((prev) => prev + 1);
           setHasResolvedConfig(true);
           setLoading(false);
           return;
@@ -128,12 +131,17 @@ export const AppConfigProvider: React.FC<AppConfigProviderProps> = ({
 
         // apiClient automatically adds JWT header if available via interceptors
         // Always suppress error toast - we handle 401 errors locally
-        const response = await apiClient.get<AppConfig>('/api/v1/config/app-config', { suppressErrorToast: true });
+        const response = await apiClient.get<AppConfig>(
+          '/api/v1/config/app-config',
+          {
+            suppressErrorToast: true,
+            skipAuthRedirect: true,
+          } as any,
+        );
         const data = response.data;
 
         console.debug('[AppConfig] Config fetched successfully:', data);
         setConfig(data);
-        setFetchCount(prev => prev + 1);
         setHasResolvedConfig(true);
         setLoading(false);
         return; // Success - exit function
@@ -170,7 +178,7 @@ export const AppConfigProvider: React.FC<AppConfigProviderProps> = ({
     }
 
     setLoading(false);
-  }, [fetchCount, hasResolvedConfig, isBlockingMode, maxRetries, initialDelay]);
+  }, [hasResolvedConfig, isBlockingMode, maxRetries, initialDelay]);
 
   useEffect(() => {
     // Skip config fetch on auth pages (/login, /signup, /auth/callback, /invite/*)
@@ -209,11 +217,13 @@ export const AppConfigProvider: React.FC<AppConfigProviderProps> = ({
     return () => window.removeEventListener('jwt-available', handleJwtAvailable);
   }, [fetchConfig]);
 
+  const refetch = useCallback(() => fetchConfig(true), [fetchConfig]);
+
   const value: AppConfigContextValue = {
     config,
     loading,
     error,
-    refetch: () => fetchConfig(true),
+    refetch,
   };
 
   return (

frontend/src/core/hooks/useBackendProbe.ts

@@ -0,0 +1,86 @@
+import { useCallback, useEffect, useState } from 'react';
+import { BASE_PATH } from '@app/constants/app';
+
+type BackendStatus = 'up' | 'starting' | 'down';
+
+interface BackendProbeState {
+  status: BackendStatus;
+  loginDisabled: boolean;
+  loading: boolean;
+}
+
+/**
+ * Lightweight backend probe that avoids global axios interceptors.
+ * Used on auth screens to decide whether to show login, anonymous mode, or a backend-starting message.
+ */
+export function useBackendProbe() {
+  const [state, setState] = useState<BackendProbeState>({
+    status: 'starting',
+    loginDisabled: false,
+    loading: true,
+  });
+
+  const probe = useCallback(async () => {
+    const statusUrl = `${BASE_PATH || ''}/api/v1/info/status`;
+    const loginUrl = `${BASE_PATH || ''}/api/v1/proprietary/ui-data/login`;
+
+    const next: BackendProbeState = {
+      status: 'starting',
+      loginDisabled: false,
+      loading: false,
+    };
+
+    try {
+      const res = await fetch(statusUrl, { method: 'GET', cache: 'no-store' });
+      if (res.ok) {
+        const data = await res.json().catch(() => null);
+        if (data && data.status === 'UP') {
+          next.status = 'up';
+          setState(next);
+          return next;
+        }
+        next.status = 'starting';
+      } else if (res.status === 404 || res.status === 503) {
+        next.status = 'starting';
+      } else {
+        next.status = 'down';
+      }
+    } catch {
+      next.status = 'down';
+    }
+
+    // Fallback: proprietary login endpoint to detect disabled login and backend availability
+    try {
+      const res = await fetch(loginUrl, { method: 'GET', cache: 'no-store' });
+      if (res.ok) {
+        next.status = 'up';
+        const data = await res.json().catch(() => null);
+        if (data && data.enableLogin === false) {
+          next.loginDisabled = true;
+        }
+      } else if (res.status === 404) {
+        // Endpoint missing usually means login disabled
+        next.status = 'up';
+        next.loginDisabled = true;
+      } else if (res.status === 503) {
+        next.status = 'starting';
+      } else {
+        next.status = 'down';
+      }
+    } catch {
+      // keep previous inferred state (down/starting)
+    }
+
+    setState(next);
+    return next;
+  }, []);
+
+  useEffect(() => {
+    void probe();
+  }, [probe]);
+
+  return {
+    ...state,
+    probe,
+  };
+}

frontend/src/core/services/httpErrorHandler.ts

@@ -88,6 +88,7 @@ const SPECIAL_SUPPRESS_MS = 1500; // brief window to suppress generic duplicate
  * Returns true if the error should be suppressed (deduplicated), false otherwise
  */
 export async function handleHttpError(error: any): Promise<boolean> {
+  const skipAuthRedirect = error?.config?.skipAuthRedirect === true;
   // Check if this error should skip the global toast (component will handle it)
   if (error?.config?.suppressErrorToast === true) {
     return false; // Don't show global toast, but continue rejection
@@ -105,12 +106,19 @@ export async function handleHttpError(error: any): Promise<boolean> {
                       pathname.includes('/invite/');
 
     // If not on auth page, redirect to login with expired session message
-    if (!isAuthPage) {
+    if (!isAuthPage && !skipAuthRedirect) {
       console.debug('[httpErrorHandler] 401 detected, redirecting to login');
       // Store the current location so we can redirect back after login
       const currentLocation = window.location.pathname + window.location.search;
-      // Redirect to login with state
-      window.location.href = `/login?expired=true&from=${encodeURIComponent(currentLocation)}`;
+      // Redirect to login with state (only show expired when a JWT existed)
+      let hadStoredJwt = false;
+      try {
+        hadStoredJwt = Boolean(localStorage.getItem('stirling_jwt'));
+      } catch {
+        // ignore storage access failures
+      }
+      const expiredPrefix = hadStoredJwt ? 'expired=true&' : '';
+      window.location.href = `/login?${expiredPrefix}from=${encodeURIComponent(currentLocation)}`;
       return true; // Suppress toast since we're redirecting
     }
 
@@ -173,4 +181,4 @@ export async function handleHttpError(error: any): Promise<boolean> {
   }
 
   return false; // Error was handled with toast, continue normal rejection
-}
+}