mcp-js (V8 sandbox) — Install & Live Demo

Why use it

Key features

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

Live Demo

What it looks like in practice

mcp-js-v8.replay ▶ ready

0/0

Install

Pick your client

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

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

Open Claude Desktop → Settings → Developer → Edit Config. Restart after saving.

~/.cursor/mcp.json · .cursor/mcp.json

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

Cursor uses the same mcpServers schema as Claude Desktop. Project config wins over global.

VS Code → Cline → MCP Servers → Edit

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

Click the MCP Servers icon in the Cline sidebar, then "Edit Configuration".

~/.codeium/windsurf/mcp_config.json

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

Same shape as Claude Desktop. Restart Windsurf to pick up changes.

~/.continue/config.json

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

Continue uses an array of server objects rather than a map.

~/.config/zed/settings.json

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

Add to context_servers. Zed hot-reloads on save.

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

One-liner. Verify with claude mcp list. Remove with claude mcp remove.

Use Cases

Real-world ways to use mcp-js (V8 sandbox)

Iteratively shape a JSON dataset across multiple tool calls

👤 Data wranglers ⏱ ~15 min intermediate

When to use: Multi-step transform where you want to keep intermediate variables.

Prerequisites

Server/skill installed and authenticated — See repo README

Flow

Load data

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

→ Length printed
Iterate

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

→ Filtered count — data still in heap
Snapshot

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

→ Saved

Outcome: Notebook-style JS sessions without a notebook.

Pitfalls

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.

Combine with: filesystem

Combinations

Pair with other MCPs for X10 leverage

mcp-js-v8 + filesystem

Pair with filesystem for complementary capabilities

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

Tools

What this MCP exposes

Tool	Inputs	When to call	Cost
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

Cost & Limits

What this costs to run

API quota: See provider docs for rate limits
Tokens per call: Varies by tool
Monetary: See repo README for pricing details
Tip: Cache tool results and avoid repeated identical calls.

Security

Permissions, secrets, blast radius

Credential storage: Use environment variables; never commit secrets

Data egress: Tool calls go to the provider's API as documented

Troubleshooting

Common errors and fixes

context expired

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

Verify: Reload from snapshot_load

memory limit exceeded

Raise MCP_JS_MAX_HEAP_MB or chunk the input.

Verify: Reduce dataset size and retry

Alternatives

mcp-js (V8 sandbox) vs others

Alternative	When to use it instead	Tradeoff
yepcode/mcp-server-js	You want hosted execution with secrets	Hosted/paid vs local/free

More

Resources

📖 Read the official README on GitHub

🐙 Browse open issues

🔍 Browse all 400+ MCP servers and Skills