diff --git a/chrome-extension/popup.js b/chrome-extension/popup.js
index 1ee352e..eff88cc 100644
--- a/chrome-extension/popup.js
+++ b/chrome-extension/popup.js
@@ -14,6 +14,8 @@ let connected = false;
   if (stored.config) config = { ...config, ...stored.config };
   if (stored.token && stored.token !== null) authToken = stored.token;
   if (stored.mode) uploadMode = stored.mode;
+  // Default to HTTP if no mode saved
+  if (!stored.mode) uploadMode = 'http';
 
   document.getElementById('cfg-server').value = config.serverUrl;
   document.getElementById('cfg-user').value = config.username;
@@ -232,6 +234,17 @@ function updateBtn() {
 // ==================== UPLOAD ====================
 async function startUpload() {
   if (!connected) { showStatus('Not connected to server', 'error'); return; }
+  // If UDP mode selected but relay not configured, fall back to HTTP silently
+  if (uploadMode === 'udp') {
+    try {
+      const health = await apiFetch('GET', '/api/health');
+      if (!health.relayConfigured) {
+        showStatus('⚠️ UDP relay not configured — switching to HTTP', 'loading');
+        setMode('http');
+        await new Promise(r => setTimeout(r, 1000));
+      }
+    } catch (_) {}
+  }
   const pending = selectedFiles.filter(f => f.status === 'pending');
   document.getElementById('upload-btn').disabled = true;
   const prefix = document.getElementById('folder-select').value;