sandbox-sdk

SKILL.md

Cloudflare Sandbox SDK

Build secure, isolated code execution environments on Cloudflare Workers.

FIRST: Verify Installation

npm install @cloudflare/sandbox
docker info  # Must succeed - Docker required for local dev

Retrieval Sources

Your knowledge of the Sandbox SDK may be outdated. Prefer retrieval over pre-training for any Sandbox SDK task.

Resource
URL

Docs
https://developers.cloudflare.com/sandbox/

API Reference
https://developers.cloudflare.com/sandbox/api/

Examples
https://github.com/cloudflare/sandbox-sdk/tree/main/examples

Get Started
https://developers.cloudflare.com/sandbox/get-started/

When implementing features, fetch the relevant doc page or example first.

Required Configuration

wrangler.jsonc (exact - do not modify structure):

{
  "containers": [{
    "class_name": "Sandbox",
    "image": "./Dockerfile",
    "instance_type": "lite",
    "max_instances": 1
  }],
  "durable_objects": {
    "bindings": [{ "class_name": "Sandbox", "name": "Sandbox" }]
  },
  "migrations": [{ "new_sqlite_classes": ["Sandbox"], "tag": "v1" }]
}

Worker entry - must re-export Sandbox class:

import { getSandbox } from '@cloudflare/sandbox';
export { Sandbox } from '@cloudflare/sandbox';  // Required export

Quick Reference

Task
Method

Get sandbox
getSandbox(env.Sandbox, 'user-123')

Run command
await sandbox.exec('python script.py')

Run code (interpreter)
await sandbox.runCode(code, { language: 'python' })

Write file
await sandbox.writeFile('/workspace/app.py', content)

Read file
await sandbox.readFile('/workspace/app.py')

Create directory
await sandbox.mkdir('/workspace/src', { recursive: true })

List files
await sandbox.listFiles('/workspace')

Expose port
await sandbox.exposePort(8080)

Destroy
await sandbox.destroy()

Core Patterns

Execute Commands

const sandbox = getSandbox(env.Sandbox, 'user-123');
const result = await sandbox.exec('python --version');
// result: { stdout, stderr, exitCode, success }

Code Interpreter (Recommended for AI)

Use runCode() for executing LLM-generated code with rich outputs:

const ctx = await sandbox.createCodeContext({ language: 'python' });

await sandbox.runCode('import pandas as pd; data = [1,2,3]', { context: ctx });
const result = await sandbox.runCode('sum(data)', { context: ctx });
// result.results[0].text = "6"

Languages: python, javascript, typescript

State persists within context. Create explicit contexts for production.

File Operations

await sandbox.mkdir('/workspace/project', { recursive: true });
await sandbox.writeFile('/workspace/project/main.py', code);
const file = await sandbox.readFile('/workspace/project/main.py');
const files = await sandbox.listFiles('/workspace/project');

When to Use What

Need
Use
Why

Shell commands, scripts
exec()
Direct control, streaming

LLM-generated code
runCode()
Rich outputs, state persistence

Build/test pipelines
exec()
Exit codes, stderr capture

Data analysis
runCode()
Charts, tables, pandas

Extending the Dockerfile

Base image (docker.io/cloudflare/sandbox:0.7.0) includes Python 3.11, Node.js 20, and common tools.

Add dependencies by extending the Dockerfile:

FROM docker.io/cloudflare/sandbox:0.7.0

# Python packages
RUN pip install requests beautifulsoup4

# Node packages (global)
RUN npm install -g typescript

# System packages
RUN apt-get update && apt-get install -y ffmpeg && rm -rf /var/lib/apt/lists/*

EXPOSE 8080  # Required for local dev port exposure

Keep images lean - affects cold start time.

Preview URLs (Port Exposure)

Expose HTTP services running in sandboxes:

const { url } = await sandbox.exposePort(8080);
// Returns preview URL for the service

Production requirement: Preview URLs need a custom domain with wildcard DNS (*.yourdomain.com). The .workers.dev domain does not support preview URL subdomains.

See: https://developers.cloudflare.com/sandbox/guides/expose-services/

OpenAI Agents SDK Integration

The SDK provides helpers for OpenAI Agents at @cloudflare/sandbox/openai:

import { Shell, Editor } from '@cloudflare/sandbox/openai';

See examples/openai-agents for complete integration pattern.

Sandbox Lifecycle

getSandbox() returns immediately - container starts lazily on first operation

Containers sleep after 10 minutes of inactivity (configurable via sleepAfter)

Use destroy() to immediately free resources

Same sandboxId always returns same sandbox instance

Anti-Patterns

Don't use internal clients (CommandClient, FileClient) - use sandbox.* methods

Don't skip the Sandbox export - Worker won't deploy without export { Sandbox }

Don't hardcode sandbox IDs for multi-user - use user/session identifiers

Don't forget cleanup - call destroy() for temporary sandboxes

Detailed References

references/api-quick-ref.md - Full API with options and return types

references/examples.md - Example index with use cases