diff --git a/src/server.ts b/src/server.ts
index 3b0d948..eb7c9e0 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -78,17 +78,34 @@ async function startServer() {
   const app = express()
 
   // Apply security headers with helmet
+  // Get Helmet's default CSP directives and remove upgrade-insecure-requests
+  // This ensures the directive is never added, even in edge cases
+  // Create a copy to avoid mutating Helmet's defaults
+  const defaultCspDirectives = { ...helmet.contentSecurityPolicy.getDefaultDirectives() }
+  delete defaultCspDirectives['upgrade-insecure-requests']
+  
+  // Build custom CSP directives, overriding defaults as needed
+  const cspDirectives = {
+    ...defaultCspDirectives,
+    // Override default-src from defaults
+    'default-src': ["'self'"],
+    // Override script-src for webpack
+    'script-src': ["'self'", "'unsafe-inline'", "'unsafe-eval'"], // unsafe-eval required for webpack runtime
+    // Override style-src for Material-UI
+    'style-src': ["'self'", "'unsafe-inline'"], // Required for Material-UI
+    // Add WebSocket support
+    'connect-src': ["'self'", 'ws:', 'wss:'], // Allow WebSocket connections
+    // Allow data URIs for images
+    'img-src': ["'self'", 'data:', 'blob:'],
+    // Only add upgrade-insecure-requests if explicitly enabled via env var
+    ...(enableUpgradeInsecure && { 'upgrade-insecure-requests': [] }),
+  }
+
   app.use(
     helmet({
       contentSecurityPolicy: {
-        directives: {
-          defaultSrc: ["'self'"],
-          scriptSrc: ["'self'", "'unsafe-inline'", "'unsafe-eval'"], // unsafe-eval required for webpack runtime
-          styleSrc: ["'self'", "'unsafe-inline'"], // Required for Material-UI
-          connectSrc: ["'self'", 'ws:', 'wss:'], // Allow WebSocket connections
-          imgSrc: ["'self'", 'data:', 'blob:'],
-          upgradeInsecureRequests: enableUpgradeInsecure ? [] : null, // Only enable when behind HTTPS reverse proxy
-        },
+        useDefaults: false, // Don't merge with Helmet's defaults to ensure full control
+        directives: cspDirectives,
       },
       hsts: isProduction
         ? {