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: "text", text: JSON.stringify(data) }] };
});

After (App tool with UI):

import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";

const resourceUri = "ui://my-tool/mcp-app.html";

registerAppTool(server, "my-tool", {
  description: "Shows data with an interactive UI",
  inputSchema: { param: z.string() },
  _meta: { ui: { resourceUri } },
}, async (args) => {
  const data = await fetchData(args.param);
  return {
    content: [{ type: "text", text: JSON.stringify(data) }],   // text fallback for non-UI hosts
    structuredContent: { data },                                 // structured data for the UI
  };
});

Key guidance:

• Always keep the content array with a text fallback for text-only clients
• Add structuredContent for data the UI needs to render
• Link the tool to its resource via _meta.ui.resourceUri
• Leave tools that don't benefit from UI unchanged — they stay as plain tools

Step 5: Register Resources

Register the HTML resource so the host can fetch it:

import fs from "node:fs/promises";
import path from "node:path";

const resourceUri = "ui://my-tool/mcp-app.html";

registerAppResource(server, {
  uri: resourceUri,
  name: "My Tool UI",
  mimeType: RESOURCE_MIME_TYPE,
}, async () => {
  const html = await fs.readFile(
    path.resolve(import.meta.dirname, "dist", "mcp-app.html"),
    "utf-8",
  );
  return { contents: [{ uri: resourceUri, mimeType: RESOURCE_MIME_TYPE, text: html }] };
});

If multiple tools share the same UI, they can reference the same resourceUri and the same resource registration.

Step 6: Build the UI

Handler Registration

Register ALL handlers BEFORE calling app.connect():

import { App, PostMessageTransport, applyDocumentTheme, applyHostStyleVariables, applyHostFonts } from "@modelcontextprotocol/ext-apps";

const app = new App({ name: "My App", version: "1.0.0" });

app.ontoolinput = (params) => {
  // Render the UI using params.arguments and/or params.structuredContent
};

app.ontoolresult = (result) => {
  // Update UI with final tool result
};

app.onhostcontextchanged = (ctx) => {
  if (ctx.theme) applyDocumentTheme(ctx.theme);
  if (ctx.styles?.variables) applyHostStyleVariables(ctx.styles.variables);
  if (ctx.styles?.css?.fonts) applyHostFonts(ctx.styles.css.fonts);
  if (ctx.safeAreaInsets) {
    const { top, right, bottom, left } = ctx.safeAreaInsets;
    document.body.style.padding = `${top}px ${right}px ${bottom}px ${left}px`;
  }
};

app.onteardown = async () => {
  return {};
};

await app.connect(new PostMessageTransport());

Host Styling

Use host CSS variables for theme integration:

.container {
  background: var(--color-background-secondary);
  color: var(--color-text-primary);
  font-family: var(--font-sans);
  border-radius: var(--border-radius-md);
}

Key variable groups: --color-background-*, --color-text-*, --color-border-*, --font-sans, --font-mono, --font-text-*-size, --font-heading-*-size, --border-radius-*. See src/spec.types.ts for the full list.

For React apps, use the useApp and useHostStyles hooks instead — see basic-server-react/ for the pattern.

Optional Enhancements

App-Only Helper Tools

Tools the UI calls but the model doesn't need to invoke directly (polling, pagination, chunk loading):

registerAppTool(server, "poll-data", {
  description: "Polls latest data for the UI",
  _meta: { ui: { resourceUri, visibility: ["app"] } },
}, async () => {
  const data = await getLatestData();
  return { content: [{ type: "text", text: JSON.stringify(data) }] };
});

The UI calls these via app.callServerTool("poll-data", {}).

CSP Configuration

If the UI needs to load external resources (fonts, APIs, CDNs), declare the domains:

registerAppResource(server, {
  uri: resourceUri,
  name: "My Tool UI",
  mimeType: RESOURCE_MIME_TYPE,
  _meta: {
    ui: {
      connectDomains: ["api.example.com"],      // fetch/XHR targets
      resourceDomains: ["cdn.example.com"],      // scripts, styles, images
      frameDomains: ["embed.example.com"],        // nested iframes
    },
  },
}, async () => { /* ... */ });

Streaming Partial Input

For large tool inputs, show progress during LLM generation:

app.ontoolinputpartial = (params) => {
  const args = params.arguments; // Healed partial JSON - always valid
  // Render preview with partial data
};

app.ontoolinput = (params) => {
  // Final complete input - switch to full render
};

Graceful Degradation with getUiCapability()

Conditionally register App tools only when the client supports UI, falling back to text-only tools:

import { getUiCapability, registerAppTool, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";

server.server.oninitialized = () => {
  const clientCapabilities = server.server.getClientCapabilities();
  const uiCap = getUiCapability(clientCapabilities);

  if (uiCap?.mimeTypes?.includes(RESOURCE_MIME_TYPE)) {
    // Client supports UI — register App tool
    registerAppTool(server, "my-tool", {
      description: "Shows data with interactive UI",
      _meta: { ui: { resourceUri } },
    }, appToolHandler);
  } else {
    // Text-only client — register plain tool
    server.tool("my-tool", "Shows data", { param: z.string() }, plainToolHandler);
  }
};

Fullscreen Mode

Allow the UI to expand to fullscreen:

app.onhostcontextchanged = (ctx) => {
  if (ctx.availableDisplayModes?.includes("fullscreen")) {
    fullscreenBtn.style.display = "block";
  }
  if (ctx.displayMode) {
    container.classList.toggle("fullscreen", ctx.displayMode === "fullscreen");
  }
};

async function toggleFullscreen() {
  const newMode = currentMode === "fullscreen" ? "inline" : "fullscreen";
  const result = await app.requestDisplayMode({ mode: newMode });
  currentMode = result.mode;
}

Common Mistakes to Avoid

1. Forgetting text content fallback — Always include content array with text for non-UI hosts
2. Registering handlers after connect() — Register ALL handlers BEFORE calling app.connect()
3. Missing vite-plugin-singlefile — Without it, assets won't load in the sandboxed iframe
4. Forgetting resource registration — The tool references a resourceUri that must have a matching resource
5. Hardcoding styles — Use host CSS variables (var(--color-*)) for theme integration
6. Not handling safe area insets — Always apply ctx.safeAreaInsets in onhostcontextchanged

Testing

Using basic-host

Test the enhanced server with the basic-host example:

# Terminal 1: Build and run your server
npm run build && npm run serve

# Terminal 2: Run basic-host (from cloned repo)
cd /tmp/mcp-ext-apps/examples/basic-host
npm install
SERVERS='["http://localhost:3001/mcp"]' npm run start
# Open http://localhost:8080

Configure SERVERS with a JSON array of your server URLs (default: http://localhost:3001/mcp).

Verify

1. Plain tools still work and return text output
2. App tools render their UI in the iframe
3. ontoolinput handler fires with tool arguments
4. ontoolresult handler fires with tool result
5. Host styling (theme, fonts, colors) applies correctly