Example: Basic Server (Vanilla JS)

An MCP App example with a vanilla JavaScript UI (no framework).

Looking for a React-based example? See basic-server-react!

Overview

Tool registration with a linked UI resource
Vanilla JS UI using the App class directly
App communication APIs: callServerTool, sendMessage, sendLog, openLink
Theme integration via applyDocumentTheme(), applyHostStyleVariables(), and applyHostFonts()

Key Files

server.ts - MCP server with tool and resource registration
mcp-app.html / src/mcp-app.ts - Vanilla JS UI using App class

Getting Started

npm install
npm run dev

How It Works

The server registers a get-time tool with metadata linking it to a UI HTML resource (ui://get-time/mcp-app.html).
When the tool is invoked, the Host renders the UI from the resource.
The UI uses the MCP App SDK API to communicate with the host and call server tools.

Build System

This example bundles into a single HTML file using Vite with vite-plugin-singlefile — see vite.config.ts. This allows all UI content to be served as a single MCP resource. Alternatively, MCP apps can load external resources by defining _meta.ui.csp.resourceDomains in the UI resource metadata.