WebMCP

A proposal and code for websites to support client side LLMs

WebMCP allows websites to share tools, resources, prompts, etc. to LLMs. In other words, WebMCP allows a website to be an MCP server. No sharing API Keys. Use any model you want.

Here's a simple website I built that is WebMCP-enabled

It comes in the form of a widget that a website owner can put on their site and expose tools to give client-side LLMs what they need to provide a great UX for the user or agent.

The look, feel, how it's used, and security are all absolutely open for contribution / constructive criticism. MCP Clients directly building WebMCP functionality seems like an ideal outcome.

An end-user can connect to any number of websites at a time - and tools are "scoped" (by name) based on the domain to simplify organization.

Super Quick Demo (20 seconds, Sound on 🔊)

https://github.com/user-attachments/assets/61229470-1242-401e-a7d9-c0d762d7b519

Getting started (using your LLM with websites using WebMCP)

Installation

Just specify your MCP client (claude, cursor, cline, windsurf, or a path to json)

npx -y @jason.today/webmcp@latest --config claude

If you're interested in setting it up manually, use the command npx -y @jason.today/webmcp@latest --mcp.

Auto-install was inspired by Smithery, but their code is AGPL so I wrote this myself. If it doesn't work for you or you don't see your mcp client, please file an issue.

Using WebMCP

When you're ready to connect to a website, you can ask your model to generate you an mcp token.

Copy the token and paste it to the website's input. As soon as the website registers with it, it's thrown away and cannot be used for subsequent registrations or anything else. The website will receive its own session token for making requests.

If you'd rather your model / service never see the token, you can manually execute npx @jason.today/webmcp --new instead.

Some MCP clients, including Claude Desktop, need to be restarted to get access to new tools. (at least at time of writing)

To disconnect, you can close the browser tab, click "disconnect", or shut down the server with npx @jason.today/webmcp -q.

All configuration files are stored in ~/.webmcp directory.

Getting started (adding WebMCP to your website)

To use WebMCP, simply include webmcp.js on your page (via src or directly):

<script src="webmcp.js"></script>

The WebMCP widget will automatically initialize and appear in the bottom right corner of your page. Clicking on it will ask for a webmcp token which the end-user will generate.

Full Demo (3 minutes)

https://github.com/user-attachments/assets/43ad160a-846d-48ad-9af9-f6d537e78473

More Info About How It Works

The bridge between the MCP client and the website is a localhost-only (not accessible to requests outside your computer) websocket server. Because it is configured to allow requests from your local web browser, authentication / token exchange is required, in case you visit a website attempting to abuse this.

Ideally the web browser itself would have an explicit permission for this, like webcam or microphone use.

1. The MCP client connects to the /mcp path using the server token from .env (auto-generated)
2. The server generates a registration token (instigated via the built-in mcp tool by a model or the --new command)
3. Web clients connect to the /register endpoint with this token and its domain.
4. Web pages connect to their assigned channel based on their domain.
5. When an LLM wants to use a tool / resource / prompt, the request flows from:
   • MCP Client → MCP Server → WebSocket Server → Web Page with the tool / resource / prompt
   • (similar for requesting a list of tools / resources / prompts)
6. The web page performs the request (e.g. call tool) and sends the result back through the same path
7. Multiple web pages can be connected simultaneously, each with their own set of tools and tokens
8. The MCP client sees all tools as a unified list, with channel prefixes to avoid name collisions

sequenceDiagram
    participant User
    participant MCP as MCP Client
    participant Server as MCP Server
    participant WS as WebSocket Server
    participant Web as Website
    
    %% Initial connection
    MCP->>Server: Connect to /mcp with internal server token
    
    %% Website registration token
    User->>MCP: Request registration token
    MCP->>Server: Request registration token
    Server-->>MCP: Return registration token
    MCP-->>User: Display registration token
    
    %% Website registration
    User->>Web: Paste registration token
    Web->>WS: Connect to /register with token & domain (registration token deleted)
    WS-->>Web: Assign channel & session token
    Web->>WS: Connect to assigned channel
    
    %% Tool interaction
    MCP->>Server: Request tools list
    Server->>WS: Forward request
    WS->>Web: Request tools
    Web-->>WS: Return tools list
    WS-->>Server: Forward tools list
    Server-->>MCP: Return tools list
    
    %% Tool execution
    MCP->>Server: Tool request
    Server->>WS: Forward request
    WS->>Web: Execute tool
    Web-->>WS: Return result
    WS-->>Server: Forward result
    Server-->>MCP: Return result
    
    %% Disconnection
    User->>Web: Disconnect
    Web->>WS: Close connection

Security

This is a super early project. I'm very interested in hardening security to prevent malicious extensions etc. from being able to perform prompt injection attacks and similar. If you have constructive ideas, please reach out or open an issue.

Built in tools

• Token generator (for connecting to WebMCP websites)
• MCP Tool Definer (to simplify building the schema of a tool for use with MCP)
  • You can ask for the javascript (if relevant) in a follow-up message for use with WebMCP

Docker

There is a Dockerfile specifically for Smithery deployment.

If you'd like to use docker to run the websocket server, I've added a docker-compose.yml for demonstration purposes.

If --docker is provided to the mcp client config alongside --mcp, it will assume the server is running. This will allow you to dockerize the main process (websocket server), and your mcp client will connect to your docker container via websocket. Similarly, websites will communicate with your docker container.