Features
In-Browser Sandbox
The Sandbox lets you run any stdio-based npm MCP server entirely in your browser — no local Node.js, no terminal, no setup.
How it works
The Sandbox is powered by WebContainers — a WASM-based Node.js runtime that runs entirely in your browser tab. When you provide an npm package name:
Boot
WebContainers initializes a WASM-based Node.js environment in your browser. This takes ~5–10 seconds on first load.
Install
npm installs the package you specify (e.g. @modelcontextprotocol/server-filesystem). Dependencies are downloaded from the npm registry.
Spawn
The server process is started with the arguments you provide. A custom MCP transport bridges stdin/stdout to the MCP SDK client in your browser.
Interact
The Playground appears with all tools, resources, and prompts from the running server. Everything stays in your browser tab.
Using the Sandbox
Navigate to /playground/sandbox and enter:
Package name (required)
Any npm package that implements an MCP stdio server.
@modelcontextprotocol/server-filesystemArguments (optional)
Arguments passed to the server process. Separate with spaces.
/tmpExample packages to try
@modelcontextprotocol/server-filesystemargs: /tmpFile system access — read, write, list files in a directory.
@modelcontextprotocol/server-memoryIn-memory key-value store. Useful for testing persistence tools.
@modelcontextprotocol/server-everythingA demo server with every tool type — great for testing form generation.
Security model
The Sandbox is a deliberate security exception to MCP Playground's rule of "never connect to MCP servers from client-side code." It is safe because:
- →WebContainers run in a WASM sandbox with no access to your host filesystem or network
- →Everything executes inside your browser tab — nothing is sent to our servers
- →The sandbox is isolated from the rest of the page via COEP/COOP headers
Limitations
- ×Only npm packages — Python (pypi) servers are not supported
- ×Packages with native C++ addons (node-gyp) will not install
- ×First boot is slower (~5–10 seconds) due to WASM initialization + npm install
- ×The sandbox state is reset when you close or reload the tab
- ×Some packages may not expose a binary that can be executed as a server