Why can't my Flutter app on the Android emulator reach my local API at localhost:8000?

Cause

Inside the Android emulator, localhost (127.0.0.1) refers to the emulator itself, not your computer.

Base URLs by target

• Android emulator: http://10.0.2.2:8000 (special alias for the host machine).
• iOS simulator: http://localhost:8000 (it shares the host network).
• Physical device: http://<your-computer-LAN-ip>:8000, both on the same Wi-Fi.
• Or with a USB device on Android: adb reverse tcp:8000 tcp:8000, then use http://localhost:8000 on the device.

String get baseUrl {
  const fromEnv = String.fromEnvironment('API_URL');
  if (fromEnv.isNotEmpty) return fromEnv;
  if (Platform.isAndroid) return 'http://10.0.2.2:8000';
  return 'http://localhost:8000';
}

Pass it at run time: flutter run --dart-define=API_URL=http://192.168.1.20:8000.

Make the server listen on the right interface

Bind to all interfaces, otherwise other devices cannot connect: php artisan serve --host=0.0.0.0, or node / uvicorn --host 0.0.0.0.

Cleartext HTTP is blocked

Android 9+ and iOS (App Transport Security) block plain http:// by default for native networking, and Flutter applies the same policy on mobile: some requests can fail with "Insecure HTTP is not allowed by platform" or "Cleartext HTTP traffic not permitted". For development only, allow it in a debug-only manifest at android/app/src/debug/AndroidManifest.xml:

<application android:usesCleartextTraffic="true" />

(never ship this in release). On iOS use an NSExceptionDomains entry for your dev host in a debug-only plist. Use HTTPS in production.

Pitfalls

• Firewalls on the host machine can block port 8000 for LAN devices.
• Add the INTERNET permission in the main manifest (the Flutter template only adds it to debug and profile).
• dart:io Platform is unavailable on Flutter web; use kIsWeb first.