add-app-to-server

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• SKILL.md

SKILL.md (first lines)

# 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:

```bash
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

```bash
npm install @modelcontextprotocol/ext-apps @modelcontextprotocol/client@^2.0.0 @modelcontextprotocol/server@^2.0.0 zod@^4.2.0
npm install -D vite vite-plugin-singlefile
```

Plus framework-specific dependencies if needed (e.g., `react`, `react-dom`, `@vitejs/plugin-react` for React), and `@modelcontextprotocol/node` / `@modelcontextprotocol/express` if the server uses HTTP transports.

ext-apps 2.x requires the split base MCP SDK packages at `^2.0.0`
(`@modelcontextprotocol/core` comes in transitively). Existing servers that
still import `@modelcontextprotocol/sdk` v1 must migrate those imports and
handler schemas before adding the App integration:

| SDK v1 (`@modelcontextprotocol/sdk`) | SDK v2 |
|---|---|
| `sdk/server/mcp.js` (`McpServer`) | `@modelcontextprotocol/server` |
| `sdk/server/streamableHttp.js` (`StreamableHTTPServerTransport`) | `NodeStreamableHTTPServerTransport` from `@modelcontextprotocol/node` |
| Express wiring by hand | `createMcpExpressApp` from `@modelcontextprotocol/express` |
| `sdk/server/stdio.js` | `@modelcontextprotocol/server/stdio` |
| `sdk/types.js` types (`CallToolResult`, …) | `@modelcontextprotocol/client` or `@modelcontextprotocol/server` |
| `sdk/types.js` zod schemas (`CallToolResultSchema`, …) | `@modelcontextprotocol/core` |
| Raw zod shapes: `inputSchema: { q: z.string() }` | `inputSchema: z.object({ q: z.string() })` |
| `extra.signal` in tool callbacks | `extra.mcpReq.signal` |
| `setRequestHandler(SomeRequestSchema, handler)` | `setRequestHandler("some/method", { params: ParamsSchema }, handler)` (the 2-arg form is only for spec methods such as `"tools/call"`) |

## 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:

```typescript
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

From Velclaw/OCodejs-VelClaw, MIT.