Cloudflare Sandbox SDK Usage

This skill covers using the Cloudflare Sandbox SDK in cloudx.sh for secure container execution.

Core Concepts

Sandbox Lifecycle

1. Get sandbox instance via Durable Object binding
2. Execute operations (git clone, file writes, commands)
3. Expose ports for user access
4. Proxy requests to the running container

Session Management

Each GitHub repository gets a unique session ID (UUID). Sessions are tracked in KV:

// Session cache keys
`session:${owner}/${repo}` → sessionId      // 2hr TTL
`info:${sessionId}` → { status, repo, ... } // 2hr TTL
`preview:${sessionId}` → previewUrl         // 2hr TTL
`lock:${owner}/${repo}` → lockValue         // 30s TTL (race prevention)

SDK Patterns

Getting a Sandbox Instance

import { Sandbox, getSandbox, proxyToSandbox } from '@cloudflare/sandbox';

// Re-export for Durable Object registration
export { Sandbox };

// Get sandbox for a specific session
const sandbox = getSandbox(env.SANDBOX, sessionId);

Safe Git Operations

Always use gitCheckout() instead of shell exec for cloning:

// GOOD - Uses SDK method, prevents command injection
await sandbox.gitCheckout(repoUrl, {
  targetDir: '/home/user/repo',
  depth: 1,  // Shallow clone for speed
});

// BAD - Vulnerable to command injection
await sandbox.exec(`git clone ${repoUrl}`);  // NEVER DO THIS

File Operations

// Write configuration files
await sandbox.writeFile('/home/user/repo/.opencode.json', JSON.stringify({
  provider: { anthropic: { apiKey: env.ANTHROPIC_API_KEY } },
  model: { provider: 'anthropic', model: 'claude-opus-4-5-20250514' },
}, null, 2));

// Read files
const content = await sandbox.readFile('/path/to/file');

Command Execution

// Run commands with timeout
await sandbox.exec('npm install', { timeout: 60000 });

// Background processes (for servers)
await sandbox.exec(
  'cd /home/user/repo && nohup opencode serve --port 4096 > /tmp/opencode.log 2>&1 &',
  { timeout: 30000 }
);

Port Exposure

// Expose a port for external access
const portInfo = await sandbox.exposePort(4096);
console.log(portInfo.url);  // Preview URL for the service

Proxying Requests

// In fetch handler, proxy matching requests to sandbox
const proxyResponse = await proxyToSandbox(request, env);
if (proxyResponse) {
  return proxyResponse;
}

Security Considerations

Input Validation

Always validate user inputs before using them:

// GitHub owner validation
const GITHUB_OWNER_REGEX = /^[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,37}[a-zA-Z0-9])?$/;
function isValidGitHubOwner(owner: string): boolean {
  return GITHUB_OWNER_REGEX.test(owner) && !owner.includes('--');
}

// GitHub repo validation
const GITHUB_REPO_REGEX = /^[a-zA-Z0-9._-]{1,100}$/;
function isValidGitHubRepo(repo: string): boolean {
  return GITHUB_REPO_REGEX.test(repo) && repo !== '.' && repo !== '..';
}

// Session ID validation (UUID format)
const UUID_REGEX = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
function isValidSessionId(sessionId: string): boolean {
  return UUID_REGEX.test(sessionId);
}

Sanitization for Shell

When shell execution is unavoidable:

function sanitizeForShell(input: string): string {
  return input.replace(/[^a-zA-Z0-9._-]/g, '');
}

Race Condition Prevention

Use KV-based locking to prevent duplicate sessions:

// Try to acquire lock
const lockValue = crypto.randomUUID();
const existingLock = await env.CACHE.get(lockKey);

if (existingLock) {
  // Another request is creating session, wait and retry
  await new Promise(resolve => setTimeout(resolve, 1000));
  // ... check for existing session
}

// Set lock with short TTL
await env.CACHE.put(lockKey, lockValue, { expirationTtl: 30 });

try {
  // Create session...
} finally {
  await env.CACHE.delete(lockKey);
}

Container Configuration

Wrangler Config

{
  "containers": [{
    "class_name": "Sandbox",
    "image": "./Dockerfile",
    "max_instances": 10
  }],
  "durable_objects": {
    "bindings": [{
      "name": "SANDBOX",
      "class_name": "Sandbox"
    }]
  },
  "migrations": [{
    "tag": "v1",
    "new_sqlite_classes": ["Sandbox"]
  }]
}

Env Interface

interface Env {
  SANDBOX: DurableObjectNamespace<Sandbox>;
  CACHE: KVNamespace;
  ANTHROPIC_API_KEY: string;
  ENVIRONMENT: string;
}

Status Tracking

Track session status through initialization:

type SessionStatus = 'initializing' | 'cloning' | 'starting' | 'running' | 'error';

async function updateSessionStatus(
  env: Env,
  sessionId: string,
  status: SessionStatus,
  error?: string
): Promise<void> {
  const infoKey = `info:${sessionId}`;
  const existing = await env.CACHE.get(infoKey, 'json');

  if (existing) {
    await env.CACHE.put(infoKey, JSON.stringify({
      ...existing,
      status,
      error,
      updatedAt: Date.now(),
    }), { expirationTtl: 7200 });
  }
}

Troubleshooting

Container Not Enabled

Check that containers[].class_name matches durable_objects.bindings[].class_name.

Image Registry Error

Use ./Dockerfile for image path. Cloudflare builds and pushes to their registry.

Port Exposure Fails

Ensure the container exposes the port in Dockerfile (EXPOSE 4096).

Session Not Found

Sessions expire after 2 hours. Check KV for info:{sessionId} to see status.