Remix 3 MCP Demo

This project demonstrates how to build interactive MCP (Model Context Protocol) widgets that run on Cloudflare Workers and can be embedded in AI chat interfaces like ChatGPT. It showcases the power of combining MCP with modern web technologies to create rich, stateful experiences within AI conversations.

Demo Video

See the calculator widget in action with ChatGPT, including the hidden TRON easter egg:

https://github.com/user-attachments/assets/5df110d8-f40b-4c6a-8820-c2dbf3ff79c8

Watch the demo on X/Twitter

How the Demo Works

Architecture Overview

This demo implements a calculator widget as an MCP tool that can be invoked by AI assistants. The architecture consists of several key components:

1. MCP Server - A Cloudflare Durable Object that implements the Model Context Protocol
2. Widget System - Interactive UI components built with Remix 3 that can be embedded in AI chats
3. Two-way Communication - Widgets can both receive initial state from the AI and send messages back
4. Static Assets - Widget bundles served from Cloudflare's CDN

The Calculator Widget

The calculator is a fully functional, beautifully styled calculator with a retro-futuristic aesthetic inspired by Tron. Here's what makes it special:

Initial State Configuration

When an AI assistant invokes the calculator tool, it can pass initial state parameters:

• display - The initial display value
• previousValue - A value already entered (e.g., "I want to add 5 to a number")
• operation - The pending operation (+, -, *, /)
• waitingForNewValue - Whether the calculator is ready for new input
• errorState - Whether to start in an error state

This means the AI can pre-configure the calculator based on the user's request. For example, if a user says "I want to add 5 to something," the AI can invoke the calculator with previousValue: 5, operation: '+', and waitingForNewValue: true.

Interactive UI

The calculator widget is a fully interactive Remix application that:

• Renders using JSX/TSX with CSS-in-JS styling
• Supports keyboard shortcuts (Enter, Escape, number keys, operators, etc.)
• Features a Tron-style initialization sequence with animated loading messages
• Updates in real-time as users interact with it
• Uses Remix 3's experimental DOM renderer for efficient updates

The Easter Egg: The Master Control Program

There's a hidden feature in the calculator: when the result equals 1982 (the year the original Tron film was released), the calculator sends an MCP prompt message to the AI assistant, instructing it to adopt the persona of the Master Control Program (MCP) from Tron.

This demonstrates the widget's ability to dynamically influence the conversation by sending messages back to the AI.

Technical Implementation

MCP Server with Durable Objects

The MathMCP class extends McpAgent and uses Cloudflare's Durable Objects to maintain state:

export class MathMCP extends McpAgent<Env, State, Props> {
	server = new McpServer(
		{
			name: 'MathMCP',
			version: '1.0.0',
		},
		{
			instructions: `Use this server to solve math problems reliably and accurately.`,
		},
	)
	async init() {
		await registerTools(this)
		await registerWidgets(this)
	}
}

The server registers two types of capabilities:

1. Tools - A do_math tool that performs arithmetic operations server-side
2. Widgets - Interactive UI resources that can be embedded in the chat

Widget Registration

Widgets are registered as MCP Apps resources (for the HTML/JS bundle) and MCP tools (for invocation). The registration includes:

• Input Schema - Zod schemas defining what parameters the widget accepts
• Output Schema - Zod schemas defining what the widget can return
• HTML Bundle - The rendered HTML with script references
• MCP Apps Metadata - Standard _meta.ui.resourceUri and resource CSP settings for any MCP Apps-compatible host

registerAppResource(agent.server, name, uri, {
	description: widget.description,
	_meta: {
		ui: {
			csp: {
				connectDomains: [],
				resourceDomains: [baseUrl],
			},
		},
	},
}, async () => ({
	contents: [
		createUIResource({
			uri,
			encoding: 'text',
			content: {
				type: 'rawHtml',
				htmlString: await widget.getHtml(),
			},
		}).resource,
	],
}))

registerAppTool(agent.server, name, {
	title: widget.title,
	description: widget.description,
	inputSchema: widget.inputSchema,
	outputSchema: widget.outputSchema,
	_meta: {
		ui: { resourceUri: uri },
	},
}, handler)

Separate Build Process

The project uses two separate build processes:

1. Widget Build (Vite) - Builds the calculator UI into standalone JavaScript bundles

   • Input: worker/widgets/calculator/index.tsx
   • Output: dist/public/widgets/calculator.js
   • Format: ES modules with all dependencies bundled

2. Worker Build (Wrangler) - Builds the Cloudflare Worker with MCP server

   • Input: worker/index.tsx
   • Output: Worker bundle deployed to Cloudflare
   • Includes: MCP protocol handlers, tool registration, widget serving

Communication Protocol

Widgets communicate with their parent frame (the AI chat interface) using the MCP Apps JSON-RPC bridge (ui/* methods) via the App class:

• Initialization - Widget calls ui/initialize with App.connect()
• Tool Input - Host sends ui/notifications/tool-input
• Tool Calls - Widget calls tools/call via app.callServerTool()
• Messages - Widget sends ui/message via app.sendMessage()
• Links - Widget requests ui/open-link via app.openLink()

const app = new App({ name: 'calculator-widget', version: '1.0.0' })
app.ontoolinput = ({ arguments: toolInput }) => {
	console.log('toolInput', toolInput)
}
await app.connect()

// Widget sends a prompt to the AI
await app.sendMessage({
	role: 'user',
	content: [{ type: 'text', text: MCP_PROMPT }],
})

The User Experience

Here's what happens when a user interacts with this MCP server in ChatGPT:

1. User asks: "Can I get a calculator?"
2. ChatGPT invokes the calculator tool via MCP
3. The tool metadata links to the UI resource via _meta.ui.resourceUri
4. ChatGPT fetches the ui:// resource, renders the widget in an iframe, and sends tool input via ui/notifications/tool-input
5. The widget loads, shows a Tron-style initialization sequence, then displays the calculator
6. User interacts with the calculator (clicking buttons or using keyboard)
7. If the result is 1982, the widget sends a prompt back to ChatGPT
8. ChatGPT adopts the MCP persona and responds accordingly

Running on Your Own

Prerequisites

• Node.js (v18 or later)
• npm or yarn
• A Cloudflare account (for deployment)

Local Development

1. Clone and Install

   npm install
   

2. Start Development Server

   npm run dev
   

   This runs two processes concurrently:

   • Widget build in watch mode (Vite)
   • Worker with local Durable Objects (Wrangler)

3. Test the Calculator Widget

   Visit http://localhost:8787/__dev/widgets to see the calculator widget in isolation.

4. Connect to MCP Inspector

   Use the MCP Inspector to test the MCP server:

   npm run inspect
   

   Then connect to http://localhost:8787/mcp in the inspector.

Deployment

1. Build for Production

   npm run build
   

2. Deploy to Cloudflare

   npm run deploy
   

3. Use with ChatGPT

   Once deployed, you can add this MCP server to ChatGPT by providing the deployment URL + /mcp endpoint.

Project Structure

├── worker/
│   ├── index.tsx              # Main worker entry point
│   ├── tools.ts               # MCP tool definitions (do_math)
│   ├── widgets.tsx            # Widget registration system
│   ├── utils.ts               # CORS and utility functions
│   └── widgets/
│       ├── utils.ts           # Widget communication utilities
│       └── calculator/
│           ├── index.tsx      # Calculator UI component
│           ├── calculator.ts  # Calculator business logic
│           └── mcp-prompt.ts  # The MCP easter egg prompt
├── dist/
│   └── public/
│       └── widgets/
│           └── calculator.js  # Built calculator bundle
├── vite.config.widgets.ts     # Vite config for widget builds
└── wrangler.jsonc             # Cloudflare Workers config

Key Technologies

• Cloudflare Workers - Edge computing platform
• Durable Objects - Stateful coordination primitives
• Model Context Protocol - Protocol for AI-to-service communication
• Remix 3 - React framework (experimental DOM renderer)
• Vite - Fast build tool for widget bundles
• Zod - TypeScript-first schema validation

Environment & Configuration

The wrangler.jsonc configures:

• Durable Object binding (MATH_MCP_OBJECT)
• Assets binding for serving widget bundles
• Node.js compatibility for MCP SDK
• Observability for production monitoring

Development Tips

• Widget Development: Changes to widget code will hot-reload automatically
• Worker Changes: Wrangler will restart the worker on file changes
• Type Safety: Run npm run typecheck to validate TypeScript
• Linting: Run npm run lint to check code style

Credits

This demo showcases cutting-edge web technologies including experimental Remix 3 features, MCP widgets, and Cloudflare's edge computing platform. The calculator design pays homage to the aesthetic of Tron, with its distinctive orange glow and retro-futuristic style.