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The exposePort() method provides a public HTTPS URL that maps directly to a specific port running inside your sandbox container. Fermion automatically supports ports 3000, 1337, and 1338, allowing you to host web servers, APIs, or other services securely and access them from anywhere. When you start a server inside your sandbox, using commands like node server.js or npm run dev, you can call exposePort(port) to instantly retrieve a URL in the format https://{subdomain}-{port}.run-code.com. This enables you to preview, test, and share live services running within your sandbox without any manual networking setup or tunneling. The returned URLs are ephemeral, meaning they last only for the sandbox’s lifetime. Each new session generates new subdomains for security and isolatio

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exposePort(port)

Description: Returns the public HTTPS URL for a specific port in the sandbox. The sandbox automatically exposes ports 3000, 1337, and 1338 publicly. Parameters:
ParameterTypeRequiredDescription
port3000 | 1337 | 1338YesPort number (must be one of these three)
Returns: Promise<string> - Public HTTPS URL (format: https://{subdomain}-{port}.run-code.com) Example: 
// Start a server
await sandbox.writeFile({
  path: '~/server.js',
  content: `
    const http = require('http');
    http.createServer((req, res) => {
      res.writeHead(200, {'Content-Type': 'text/plain'});
      res.end('Hello from Fermion!');
    }).listen(3000, () => {
      console.log('Server running on port 3000');
    });
  `
})

// Run server in background
sandbox.runStreamingCommand({
  cmd: 'node',
  args: ['server.js'],
  onStdout: (data) => console.log(data.trim())
})

// Wait a moment for server to start
await new Promise(r => setTimeout(r, 2000))

// Get public URL
const url = await sandbox.exposePort(3000)
console.log('Access your server at:', url)
// Output: https://abc123xyz-3000.run-code.com

// Test it
const response = await fetch(url)
const text = await response.text()
console.log(text) // 'Hello from Fermion!'
Multiple Ports: 
// Run services on different ports
// Port 3000: Web app
sandbox.runStreamingCommand({
  cmd: 'npm',
  args: ['run', 'dev'] // Starts on 3000
})

// Port 1337: API server
sandbox.runStreamingCommand({
  cmd: 'node',
  args: ['api-server.js'] // Starts on 1337
})

// Get both URLs
const webUrl = await sandbox.exposePort(3000)
const apiUrl = await sandbox.exposePort(1337)

console.log('Web:', webUrl)
console.log('API:', apiUrl)
Throws:
  • Error: "No container found" - Must call create() or fromSnippet() first
  • TypeScript error if port is not 3000, 1337, or 1338
Important Notes:
  • URLs are available immediately, even before a server starts
  • Accessing URL before server starts will show connection error
  • HTTPS is automatic - no certificate setup needed
  • URLs are stable during sandbox lifetime
  • URLs change on reconnect to same snippet
Best Practices:
  • Wait a moment after starting server before accessing URL
  • Add error handling when fetching URLs
  • Use health check endpoints to verify server is ready