mcp-js (V8 sandbox) — Instalar & Demo en vivo

Por qué usarlo

Características clave

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

Demo en vivo

Cómo se ve en la práctica

mcp-js-v8.replay ▶ listo

0/0

Instalar

Elige tu cliente

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

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

Abre Claude Desktop → Settings → Developer → Edit Config. Reinicia después de guardar.

~/.cursor/mcp.json · .cursor/mcp.json

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

Cursor usa el mismo esquema mcpServers que Claude Desktop. La configuración del proyecto prevalece sobre la global.

VS Code → Cline → MCP Servers → Edit

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

Haz clic en el icono MCP Servers de la barra lateral de Cline y luego en "Edit Configuration".

~/.codeium/windsurf/mcp_config.json

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

Mismo formato que Claude Desktop. Reinicia Windsurf para aplicar.

~/.continue/config.json

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

Continue usa un array de objetos de servidor en lugar de un mapa.

~/.config/zed/settings.json

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

Añádelo a context_servers. Zed recarga en caliente al guardar.

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

Un solo comando. Verifica con claude mcp list. Quita con claude mcp remove.

Casos de uso

Usos del mundo real: mcp-js (V8 sandbox)

Iteratively shape a JSON dataset across multiple tool calls

👤 Data wranglers ⏱ ~15 min intermediate

Cuándo usarlo: Multi-step transform where you want to keep intermediate variables.

Requisitos previos

Server/skill installed and authenticated — See repo README

Flujo

Load data

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

→ Length printed
Iterate

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

→ Filtered count — data still in heap
Snapshot

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

→ Saved

Resultado: Notebook-style JS sessions without a notebook.

Errores comunes

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.

Combinar con: filesystem

Combinaciones

Combínalo con otros MCPs para multiplicar por 10

mcp-js-v8 + filesystem

Pair with filesystem for complementary capabilities

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

Herramientas

Lo que expone este MCP

Herramienta	Entradas	Cuándo llamar	Coste
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

Coste y límites

Lo que cuesta ejecutarlo

Cuota de API: See provider docs for rate limits
Tokens por llamada: Varies by tool
Monetario: See repo README for pricing details
Consejo: Cache tool results and avoid repeated identical calls.

Seguridad

Permisos, secretos, alcance

Almacenamiento de credenciales: Use environment variables; never commit secrets

Salida de datos: Tool calls go to the provider's API as documented

Resolución de problemas

Errores comunes y soluciones

context expired

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

Verificar: Reload from snapshot_load

memory limit exceeded

Raise MCP_JS_MAX_HEAP_MB or chunk the input.

Verificar: Reduce dataset size and retry

Alternativas

mcp-js (V8 sandbox) vs otros

Alternativa	Cuándo usarla	Contrapartida
yepcode/mcp-server-js	You want hosted execution with secrets	Hosted/paid vs local/free

Más

Recursos

📖 Lee el README oficial en GitHub

🐙 Ver issues abiertas

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