How to reconnect to a running Sandbox

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mkdir sandbox-reconnect-demo && cd sandbox-reconnect-demo
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npm init -y
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npm install @vercel/sandbox ms
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npm install -D @types/node
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vercel link
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vercel env pull

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import ms from 'ms';
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import { Sandbox } from '@vercel/sandbox';
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import { writeFileSync, readFileSync, existsSync, unlinkSync } from 'fs';
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const SANDBOX_FILE = './sandbox-id.txt';
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const TIMING_FILE = './cold-start-time.txt';
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async function main() {
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  if (existsSync(SANDBOX_FILE)) {
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    // Reconnect to existing sandbox
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    const sandboxId = readFileSync(SANDBOX_FILE, 'utf-8').trim();
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    const coldStartTime = existsSync(TIMING_FILE)
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      ? parseInt(readFileSync(TIMING_FILE, 'utf-8').trim())
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      : null;
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    console.log(`Found existing sandbox: ${sandboxId}`);
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    console.log('Reconnecting...\n');
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    try {
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      const reconnectStart = Date.now();
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      const sandbox = await Sandbox.get({ sandboxId });
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      const reconnectTime = Date.now() - reconnectStart;
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      console.log(`Reconnected to: ${sandbox.sandboxId}`);
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      console.log(`Reconnect time: ${(reconnectTime / 1000).toFixed(2)}s`);
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      // Show timing comparison
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      if (coldStartTime) {
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        console.log(`\n--- Timing Comparison ---`);
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        console.log(`Cold start:     ${(coldStartTime / 1000).toFixed(2)}s`);
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        console.log(`Warm reconnect: ${(reconnectTime / 1000).toFixed(2)}s`);
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        console.log(`Speedup:        ${(coldStartTime / reconnectTime).toFixed(1)}x faster`);
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      }
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      // Clean up
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      await sandbox.stop();
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      unlinkSync(SANDBOX_FILE);
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      if (existsSync(TIMING_FILE)) unlinkSync(TIMING_FILE);
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      console.log('\nSandbox stopped and cleaned up');
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    } catch (error) {
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      console.log('Could not reconnect, creating new sandbox...');
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      unlinkSync(SANDBOX_FILE);
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      if (existsSync(TIMING_FILE)) unlinkSync(TIMING_FILE);
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      await createNewSandbox();
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    }
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  } else {
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    await createNewSandbox();
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  }
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}
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async function createNewSandbox() {
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  const startTime = Date.now();
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  console.log('Creating new sandbox...');
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  const sandbox = await Sandbox.create({
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    timeout: ms('10m'),
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    runtime: 'node22',
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  });
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  const totalTime = Date.now() - startTime;
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  // Save the ID and timing for later
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  writeFileSync(SANDBOX_FILE, sandbox.sandboxId);
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  writeFileSync(TIMING_FILE, totalTime.toString());
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  console.log(`Sandbox created: ${sandbox.sandboxId}`);
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  console.log(`Cold start time: ${(totalTime / 1000).toFixed(2)}s`);
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  console.log('\nRun this script again to see reconnect speed!');
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}
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main().catch(console.error);

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npx dotenv-cli -e .env.local -- npx tsx index.ts

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Creating new sandbox...
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Sandbox created: sbx_abc123
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Cold start time: 2.34s
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Run this script again to see reconnect speed!

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Found existing sandbox: sbx_abc123
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Reconnecting...
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Reconnected to: sbx_abc123
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Reconnect time: 0.31s
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--- Timing Comparison ---
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Cold start:     2.34s
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Warm reconnect: 0.31s
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Speedup:        7.5x faster
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Sandbox stopped and cleaned up

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try {
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  const sandbox = await Sandbox.get({ sandboxId });
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  console.log('Reconnected successfully');
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} catch (error) {
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  console.log('Sandbox no longer available, creating a new one...');
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  const sandbox = await Sandbox.create({ runtime: 'node22' });
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}