/add-app-to-server

name: add-app-to-server
description: This skill should be used when the user asks to "add an app to my MCP server", "add UI to my MCP server", "add a view to my MCP tool", "enrich MCP tools with UI", "add interactive UI to existing server", "add MCP Apps to my server", or needs to add interactive UI capabilities to an existing MCP server that already has tools. Provides guidance for analyzing existing tools and adding MCP Apps UI resources.

Add UI to MCP Server

Enrich an existing MCP server's tools with interactive UIs using the MCP Apps SDK (`@modelcontextprotocol/ext-apps`).

How It Works

Existing tools get paired with HTML resources that render inline in the host's conversation. The tool continues to work for text-only clients — UI is an enhancement, not a replacement. Each tool that benefits from UI gets linked to a resource via `_meta.ui.resourceUri`, and the host renders that resource in a sandboxed iframe when the tool is called.

Getting Reference Code

Clone the SDK repository for working examples and API documentation:

git clone --branch "v$(npm view @modelcontextprotocol/ext-apps version)" --depth 1 https://github.com/modelcontextprotocol/ext-apps.git /tmp/mcp-ext-apps

API Reference (Source Files)

Read JSDoc documentation directly from `/tmp/mcp-ext-apps/src/`:

| File | Contents | |------|----------| | `src/app.ts` | `App` class, handlers (`ontoolinput`, `ontoolresult`, `onhostcontextchanged`, `onteardown`), lifecycle | | `src/server/index.ts` | `registerAppTool`, `registerAppResource`, `getUiCapability`, tool visibility options | | `src/spec.types.ts` | All type definitions: `McpUiHostContext`, CSS variable keys, display modes | | `src/styles.ts` | `applyDocumentTheme`, `applyHostStyleVariables`, `applyHostFonts` | | `src/react/useApp.tsx` | `useApp` hook for React apps | | `src/react/useHostStyles.ts` | `useHostStyles`, `useHostStyleVariables`, `useHostFonts` hooks |

Key Examples (Mixed Tool Patterns)

These examples demonstrate servers with both App-enhanced and plain tools — the exact pattern you're adding:

| Example | Pattern | |---------|---------| | `examples/map-server/` | `show-map` (App tool) + `geocode` (plain tool) | | `examples/pdf-server/` | `display_pdf` (App tool) + `list_pdfs` (plain tool) + `read_pdf_bytes` (app-only tool) | | `examples/system-monitor-server/` | `get-system-info` (App tool) + `poll-system-stats` (app-only polling tool) |

Framework Templates

Learn and adapt from `/tmp/mcp-ext-apps/examples/basic-server-{framework}/`:

| Template | Key Files | |----------|-----------| | `basic-server-vanillajs/` | `server.ts`, `src/mcp-app.ts`, `mcp-app.html` | | `basic-server-react/` | `server.ts`, `src/mcp-app.tsx` (uses `useApp` hook) | | `basic-server-vue/` | `server.ts`, `src/App.vue` | | `basic-server-svelte/` | `server.ts`, `src/App.svelte` | | `basic-server-preact/` | `server.ts`, `src/mcp-app.tsx` | | `basic-server-solid/` | `server.ts`, `src/mcp-app.tsx` |

Step 1: Analyze Existing Tools

Before writing any code, analyze the server's existing tools and determine which ones benefit from UI.

1. Read the server source and list all registered tools 2. For each tool, assess whether it would benefit from UI (returns data that could be visualized, involves user interaction, etc.) vs. is fine as text-only (simple lookups, utility functions) 3. Identify tools that could become **app-only helpers** (data the UI needs to poll/fetch but the model doesn't need to call directly) 4. Present the analysis to the user and confirm which tools to enhance

Decision Framework

| Tool output type | UI benefit | Example | |---|---|---| | Structured data / lists / tables | High — interactive table, search, filtering | List of items, search results | | Metrics / numbers over time | High — charts, gauges, dashboards | System stats, analytics | | Media / rich content | High — viewer, player, renderer | Maps, PDFs, images, video | | Simple text / confirmations | Low — text is fine | "File created", "Setting updated" | | Data for other tools | Consider app-only | Polling endpoints, chunk loaders |

Step 2: Add Dependencies

npm install @modelcontextprotocol/ext-apps
npm install -D vite vite-plugin-singlefile

Plus framework-specific dependencies if needed (e.g., `react`, `react-dom`, `@vitejs/plugin-react` for React).

Use `npm install` to add dependencies rather than manually writing version numbers. This lets npm resolve the latest compatible versions. Never specify version numbers from memory.

Step 3: Set Up the Build Pipeline

Vite Configuration

Create `vite.config.ts` with `vite-plugin-singlefile` to bundle the UI into a single HTML file:

import { defineConfig } from "vite";
import { viteSingleFile } from "vite-plugin-singlefile";

export default defineConfig({
  plugins: [viteSingleFile()],
  build: {
    outDir: "dist",
    rollupOptions: {
      input: "mcp-app.html", // one per UI, or one shared entry
    },
  },
});

HTML Entry Point

Create `mcp-app.html` (or one per distinct UI if tools need different views):

<!doctype html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>MCP App</title>
  </head>
  <body>
    <div id="root"></div>
    <script type="module" src="./src/mcp-app.ts"></script>
  </body>
</html>

Build Scripts

Add build scripts to `package.json`. The UI must be built before the server code bundles it:

{
  "scripts": {
    "build:ui": "vite build",
    "build:server": "tsc",
    "build": "npm run build:ui && npm run build:server",
    "serve": "tsx server.ts"
  }
}

Step 4: Convert Tools to App Tools

Transform plain MCP tools into App tools with UI.

**Before** (plain MCP tool):

server.tool("my-tool", { param: z.string() }, async (args) => {
  const data = await fetchData(args.param);
  return { content: [{ type: