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mcp-js (V8 sandbox)

Name: mcp-js (V8 sandbox) MCP Server
Author: r33drichards

Run untrusted JS in a V8 isolate — persistent heap, S3-backed snapshots.

mcp-js exposes a V8 isolate as an MCP code-execution tool. It can persist the heap across calls (so variables survive between tool calls) and snapshot to S3 — useful for long-running data-prep sessions where you want JavaScript state without standing up a notebook.

▶ 데모 보기 📦 설치 💡 사용 사례

왜 쓰나요

핵심 기능

Isolated V8 runtime, no Node syscalls by default
Persistent heap snapshots (local FS or S3)
Multiple named contexts per session
Built in Rust — fast startup, low memory overhead
Predictable timeouts and memory caps

라이브 데모

실제 사용 모습

mcp-js-v8.replay ▶ 준비됨

0/0

설치

클라이언트 선택

~/Library/Application Support/Claude/claude_desktop_config.json · Windows: %APPDATA%\Claude\claude_desktop_config.json

{
  "mcpServers": {
    "mcp-js-v8": {
      "command": "mcp-js",
      "args": []
    }
  }
}

Claude Desktop → Settings → Developer → Edit Config 열기. 저장 후 앱 재시작.

~/.cursor/mcp.json · .cursor/mcp.json

{
  "mcpServers": {
    "mcp-js-v8": {
      "command": "mcp-js",
      "args": []
    }
  }
}

Cursor는 Claude Desktop과 동일한 mcpServers 스키마 사용. 프로젝트 설정이 전역보다 우선.

VS Code → Cline → MCP Servers → Edit

{
  "mcpServers": {
    "mcp-js-v8": {
      "command": "mcp-js",
      "args": []
    }
  }
}

Cline 사이드바의 MCP Servers 아이콘 클릭 후 "Edit Configuration" 선택.

~/.codeium/windsurf/mcp_config.json

{
  "mcpServers": {
    "mcp-js-v8": {
      "command": "mcp-js",
      "args": []
    }
  }
}

Claude Desktop과 같은 형식. Windsurf 재시작 후 적용.

~/.continue/config.json

{
  "mcpServers": [
    {
      "name": "mcp-js-v8",
      "command": "mcp-js",
      "args": []
    }
  ]
}

Continue는 맵이 아닌 서버 오브젝트 배열 사용.

~/.config/zed/settings.json

{
  "context_servers": {
    "mcp-js-v8": {
      "command": {
        "path": "mcp-js",
        "args": []
      }
    }
  }
}

context_servers에 추가. 저장 시 Zed가 핫 리로드.

claude mcp add mcp-js-v8 -- mcp-js

한 줄 명령. claude mcp list로 확인, claude mcp remove로 제거.

사용 사례

실전 활용법: mcp-js (V8 sandbox)

Iteratively shape a JSON dataset across multiple tool calls

👤 Data wranglers ⏱ ~15 min intermediate

언제 쓸까: Multi-step transform where you want to keep intermediate variables.

사전 조건

Server/skill installed and authenticated — See repo README

흐름

Load data

eval_js: const data = await fetch('https://example.com/data.json').then(r => r.json()); data.length✓ 복사됨

→ Length printed
Iterate

eval_js: const filtered = data.filter(x => x.status === 'active'); filtered.length✓ 복사됨

→ Filtered count — data still in heap
Snapshot

snapshot_save name=cleanup-2026-05-21✓ 복사됨

→ Saved

결과: Notebook-style JS sessions without a notebook.

함정

V8 isolate has no Node APIs by default. If you need fs/net, opt in explicitly — V8 isolate has no Node APIs by default. If you need fs/net, opt in explicitly — and remember Claude can then write to disk.

함께 쓰기: filesystem

조합

다른 MCP와 조합해 10배 효율

mcp-js-v8 + filesystem

Pair with filesystem for complementary capabilities

Use this server together with filesystem to complete a multi-step task.✓ 복사됨

도구

이 MCP가 노출하는 것

도구	입력	언제 호출	비용
eval_js	context_id, code	Run a JS snippet, optionally with state from prior calls	free (local)
snapshot_save	context_id, name	Persist current heap	free / S3 PUT
snapshot_load	name	Resume an earlier session	free / S3 GET

비용 및 제한

운영 비용

API 쿼터: See provider docs for rate limits
호출당 토큰: Varies by tool
금액: See repo README for pricing details
팁: Cache tool results and avoid repeated identical calls.

보안

권한, 시크릿, 파급범위

자격 증명 저장: Use environment variables; never commit secrets

데이터 외부 송신: Tool calls go to the provider's API as documented

문제 해결

자주 발생하는 오류와 해결

context expired

Contexts expire after the configured idle window. Use snapshot_save before long pauses.

확인: Reload from snapshot_load

memory limit exceeded

Raise MCP_JS_MAX_HEAP_MB or chunk the input.

확인: Reduce dataset size and retry

대안

mcp-js (V8 sandbox) 다른 것과 비교

대안	언제 쓰나	단점/장점
yepcode/mcp-server-js	You want hosted execution with secrets	Hosted/paid vs local/free