diff --git a/src/App.tsx b/src/App.tsx
index 564ee1a3d..09545b5db 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -8,7 +8,76 @@ import { WebSocketProvider } from './contexts/WebSocketContext';
 import AppContent from './components/app/AppContent';
 import i18n from './i18n/config.js';
 
+/**
+ * Detect the router basename from explicit runtime config or deployment hints.
+ */
+function detectRouterBasename() {
+  const explicitBasename = typeof window !== 'undefined' ? window.__ROUTER_BASENAME__ || '' : '';
+  if (explicitBasename) {
+    return explicitBasename.replace(/\/+$/, '');
+  }
+
+  if (typeof window === 'undefined' || typeof document === 'undefined') {
+    return '';
+  }
+
+  const candidatePaths = [
+    { kind: 'manifest' as const, value: document.querySelector('link[rel="manifest"]')?.getAttribute('href') },
+    { kind: 'script' as const, value: document.querySelector('script[type="module"][src]')?.getAttribute('src') },
+    ...Array.from(
+      document.querySelectorAll(
+        'link[rel~="icon"][href], link[rel="apple-touch-icon"][href], link[rel="apple-touch-icon-precomposed"][href], link[rel="mask-icon"][href]'
+      )
+    ).map((node) => ({
+      kind: 'icon' as const,
+      value: node.getAttribute('href'),
+    })),
+  ].filter((candidate): candidate is { kind: 'manifest' | 'script' | 'icon'; value: string } => Boolean(candidate.value));
+
+  let detectedBasename = '';
+  for (const candidate of candidatePaths) {
+    try {
+      const candidateUrl = new URL(candidate.value, document.baseURI || window.location.href);
+      if (candidateUrl.origin !== window.location.origin) {
+        continue;
+      }
+
+      const pathname = candidateUrl.pathname;
+      const normalizedPathname = pathname.replace(/\/+$/, '');
+
+      let normalized = '';
+      if (candidate.kind === 'script') {
+        const match = normalizedPathname.match(/^(.*)\/assets\//);
+        normalized = match?.[1] ? match[1].replace(/\/+$/, '') : '';
+      } else {
+        const manifestMatch = normalizedPathname.match(/^(.*)\/(?:manifest\.json|site\.webmanifest)$/);
+        const iconMatch = normalizedPathname.match(
+          /^(.*)\/(?:favicon(?:\.[^/]+)?|apple-touch-icon(?:-[^/]+)?(?:\.[^/]+)?|mask-icon(?:\.[^/]+)?|[^/]*icon[^/]*)$/
+        );
+        const match = candidate.kind === 'manifest' ? manifestMatch : iconMatch;
+        if (match?.[1]) {
+          const segments = match[1].split('/').filter(Boolean);
+          while (segments.length > 1 && ['assets', 'static', 'icons', 'images'].includes(segments[segments.length - 1])) {
+            segments.pop();
+          }
+          normalized = segments.length > 0 ? `/${segments.join('/')}` : '';
+        }
+      }
+
+      if (normalized.length > detectedBasename.length) {
+        detectedBasename = normalized;
+      }
+    } catch {
+      // Ignore invalid candidate URLs and continue checking other hints.
+    }
+  }
+
+  return detectedBasename;
+}
+
 export default function App() {
+  const routerBasename = detectRouterBasename();
+
   return (
     <I18nextProvider i18n={i18n}>
       <ThemeProvider>
@@ -17,7 +86,7 @@ export default function App() {
             <TasksSettingsProvider>
               <TaskMasterProvider>
                 <ProtectedRoute>
-                  <Router basename={window.__ROUTER_BASENAME__ || ''}>
+                  <Router basename={routerBasename}>
                     <Routes>
                       <Route path="/" element={<AppContent />} />
                       <Route path="/session/:sessionId" element={<AppContent />} />