@grayhaven/sandcastle

@grayhaven/sandcastle

TypeScript SDK for the SandCastle sandbox runtime — lightweight WASM-based sandboxes for AI agent code execution.

Install

bun add @grayhaven/sandcastle
# or
npm install @grayhaven/sandcastle

Quick Start

import { SandCastle } from "@grayhaven/sandcastle";

const sc = new SandCastle();
const result = await sc.run<number>("return 1 + 1;");
// result === 2

With Input

const result = await sc.run<string>(
  'const i = globalThis.__sandcastle_input; return "Hello " + i.name;',
  { name: "World" }
);
// result === "Hello World"

Code Mode (reduce LLM tool calls by 80%)

import { createCodeTool, TwoPassExecutor } from "@grayhaven/sandcastle/codemode";

const executor = new TwoPassExecutor();
const tool = createCodeTool({ tools: [...yourTools], executor });
// Give this single tool to your LLM instead of N separate tools

Guest Type Declarations

Feed these to your LLM so it knows the sandbox API:

import type {} from "@grayhaven/sandcastle/guest";

Requirements

The sandcastle CLI must be installed and in your PATH. See the main repo for installation instructions.

What works out of the box

LLM-generated code just works — no configuration needed:

const _ = require('lodash');          // Built-in shim
const { format } = require('date-fns'); // Built-in shim
const data = await fetch(url);        // Delegates to HTTP capability
setTimeout(() => {}, 100);            // Runs immediately (no event loop)
const clone = structuredClone(obj);   // JSON round-trip
const id = crypto.randomUUID();       // Polyfilled

Features

• Sub-millisecond sandbox creation (~600us, 1,700 ops/sec sustained)
• ES2024+ support (QuickJS-NG) — Object.groupBy, Set.intersection, iterator helpers
• Fuel metering, memory limits, wall-clock timeouts
• Host capability bridge (KV, HTTP, custom) with quota enforcement
• fetch() polyfill — delegates to HTTP capability when registered
• Module shims — lodash, path, uuid, date-fns, qs work via require()
• Streaming output — real-time console.log callback
• Persistent sandboxes — multi-turn state via input passing
• Input/output artifacts (virtual filesystem)
• Promise/async support (return Promise.all([...]) works)
• Polyfills: TextEncoder/TextDecoder, URL, atob/btoa, crypto, setTimeout, structuredClone, performance.now
• Memory protection: MemoryExceeded status instead of opaque WASM traps
• LLM-friendly errors: require('express') explains alternatives instead of crashing
• 140 integration tests, 85 experiments

License

Apache-2.0