A proof-of-concept demonstrating Cloudflare's Dynamic Worker Loader API in local development. This allows you to spin up Worker isolates programmatically without deploying them.

• Runs a main Hono worker that serves a web interface
• Accepts custom JavaScript code from users
• Creates dynamic Worker isolates on-demand using the Worker Loader API
• Executes user code in sandboxed environments with no network access
• Persists prompt history using Durable Objects

Dynamic Worker Loaders enable several interesting patterns:

• Runtime code execution: Run user-provided scripts without deployment
• Per-tenant isolation: Spin up separate compute environments for different customers
• A/B testing: Load different code versions dynamically
• Sandbox environments: Execute untrusted code safely with globalOutbound: null
• Ephemeral compute: Create workers, use them, and let them be garbage collected

Main Worker (Hono app)
├── Serves web interface
├── Accepts user code + prompt
├── Creates dynamic isolate keyed by code hash
└── Executes user code with prompt as ?q= parameter

Dynamic Isolate
├── Runs user's ESM code
├── No network access (globalOutbound: null)
├── Receives prompt via URL search params
└── Returns response to main worker

Durable Object
└── Stores recent prompts for easy reuse

# Install dependencies
bun install

# Start development server
bun dev

# Visit http://localhost:8787

Worker Loader Configuration (wrangler.jsonc):

{
  "worker_loaders": [
    { "binding": "LOADER" }
  ]
}

Dynamic Isolate Creation:

const worker = c.env.LOADER.get(isolateId, async () => ({
  compatibilityDate: "2025-06-01",
  mainModule: "main.js",
  modules: { "main.js": userModule },
  env: { WHO: "dynamic-sandbox" },
  globalOutbound: null  // No network access
}));

Calling the Dynamic Worker:

const endpoint = worker.getEntrypoint();
const response = await endpoint.fetch(`http://sandbox/?q=${prompt}`);

Important: Dynamic Worker Loaders are currently in closed beta for production deployment.

The project deploys as-is with wrangler deploy.

You'll get this error: binding LOADER of type worker_loader is invalid [code: 10021]

Option 1: Apply for beta access at Cloudflare's beta program

Option 2: Deploy without Worker Loaders for demo purposes:

# Use the production config without worker_loaders
wrangler deploy --config wrangler.prod.jsonc

The app will work except for the code execution feature - users can still see the interface and examples.

Works immediately with Wrangler 4.x - no beta access required.

• Runtime: Cloudflare Workers
• Framework: Hono with JSX
• Bundler: Wrangler 4.x
• Styling: Tailwind CSS (CDN)
• Validation: Effect Schema
• Storage: Durable Objects
• Package Manager: Bun

1. Code playground: Let users test Worker scripts without deployment
2. User-defined transforms: Allow customers to write data processing logic
3. Multi-tenant applications: Isolate per-customer business logic
4. Dynamic routing: Create request handlers on the fly
5. Compute-as-a-Service: Serverless functions with user-provided code

src/
├── worker.tsx          # Main Hono application
├── do.tsx              # Durable Object for prompt storage
└── env.d.ts            # TypeScript environment definitions
wrangler.jsonc          # Wrangler configuration

MIT