Use a Horizon tunnel with Expo

Call your local API from an Expo app running on a real phone, with an HTTPS URL that reaches your machine.

Before you beginCopy Anchor Link

• Node.js 18 or later
• A Horizon account and the CLI (see Getting started)
• A reserved subdomain for -s. Reserve one on the Subdomains page.
• An Expo app, and a local API that listens on port 3000

How this differs from expo start --tunnelCopy Anchor Link

The two tools tunnel different servers.

• npx expo start --tunnel is for Expo's development server (Metro, port 8081). Expo says it uses ngrok and needs @expo/ngrok. It lets a device load your app bundle from anywhere.
• A Horizon tunnel points at any local HTTP server. This page points it at your API, so the app can call that API from a phone.

You can use both: --tunnel to load the app, Horizon for the API. This page doesn't cover tunneling Metro with Horizon.

hrzn tunnel http://localhost:3000 -s my-api

HORIZON: Tunnel connected
  URL          https://my-api.hrzn.run (reserved)
  Forwarding   http://localhost:3000
  Request log  https://hrzn.run/dashboard/tunnels/my-api

EXPO_PUBLIC_API_URL=https://my-api.hrzn.run

const apiUrl = process.env.EXPO_PUBLIC_API_URL;

export async function getHealth() {
  const response = await fetch(`${apiUrl}/health`);
  return response.json();
}

Check it worksCopy Anchor Link

Trigger the call in the app. The Horizon terminal prints a line for it:

  GET     200  /health

TroubleshootingCopy Anchor Link

Network request failedCopy Anchor Link

React Native's fetch prints Network request failed when it can't reach the URL. Open https://my-api.hrzn.run/health in the phone's browser. If that fails, check that the Horizon terminal is running and that EXPO_PUBLIC_API_URL has the right subdomain.

The app still calls the old URLCopy Anchor Link

Expo inlines EXPO_PUBLIC_ values into your code. Do a full reload in Expo Go or your development build after you change .env.

The URL changed after a restartCopy Anchor Link

You started the tunnel without -s, so Horizon gave you a new random subdomain. Restart with -s my-api and the .env value stays valid.

Next stepsCopy Anchor Link

• Read Expo's guide to environment variables and the Expo CLI reference.