Add MCP App Support to a Web App

Add MCP App support to an existing web application so it works both as a standalone web app and as an MCP App that renders inline in MCP-enabled hosts like Claude Desktop — from a single codebase.

How It Works

The existing web app stays intact. A thin initialization layer detects whether the app is running inside an MCP host or as a regular web page, and fetches parameters from the appropriate source. A new MCP server wraps the app's bundled HTML as a resource and registers a tool to display it.

Standalone:  Browser loads page → App reads URL params / APIs → renders
MCP App:     Host calls tool → Server returns result → Host renders app in iframe → App reads MCP lifecycle → renders

The app's rendering logic is shared — only the data source changes.

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, 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

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

Reference Examples

Example	Relevant Pattern
examples/map-server/	External API integration + CSP (connectDomains, resourceDomains)
examples/sheet-music-server/	Library that loads external assets (soundfonts)
examples/pdf-server/	Binary content handling + app-only helper tools

Step 1: Analyze the Existing Web App

Before writing any code, examine the existing web app to plan what needs to change.

What to Investigate

1. Data sources — How does the app get its data? (URL params, API calls, props, hardcoded, localStorage)
2. External dependencies — CDN scripts, fonts, API endpoints, iframe embeds, WebSocket connections
3. Build system — Current bundler (Webpack, Vite, Rollup, none), framework (React, Vue, vanilla), entry points
4. User interactions — Does the app have inputs/forms that should map to tool parameters?
5. Runtime detection — How to tell if the app is running inside an MCP host (e.g., check the current origin, a query param, or whether window.parent !== window)

Present findings to the user and confirm the approach.

Data Source Mapping

In hybrid mode, the app keeps its existing data sources for standalone use and adds MCP equivalents:

Standalone data source	MCP App equivalent
URL query parameters	ontoolinput / ontoolresult arguments or structuredContent
REST API calls	app.callServerTool() to server-side tools, or keep direct API calls with CSP connectDomains
Props / component inputs	ontoolinput arguments
localStorage / sessionStorage	Not available in sandboxed iframe — pass via structuredContent or server-side state
WebSocket connections	Keep with CSP connectDomains, or convert to polling via app-only tools
Hardcoded data	Move to tool structuredContent to make it dynamic

Step 2: Investigate CSP Requirements

MCP Apps HTML runs in a sandboxed iframe with no same-origin server. Every external origin must be declared in CSP — missing origins fail silently.

Before writing any code, build the app and investigate all origins it references:

1. Build the app using the existing build command
2. Search the resulting HTML, CSS, and JS for every origin (not just "external" origins — every network request will need CSP approval)
3. For each origin found, trace back to source:
   • If it comes from a constant → universal (same in dev and prod)
   • If it comes from an env var or conditional → note the mechanism and identify both dev and prod values
4. Check for third-party libraries that may make their own requests (analytics, error tracking, etc.)

Document your findings as three lists, and note for each origin whether it's universal, dev-only, or prod-only:

• resourceDomains: origins serving images, fonts, styles, scripts
• connectDomains: origins for API/fetch requests
• frameDomains: origins for nested iframes

If no origins are found, the app may not need custom CSP domains.

Step 3: Set Up the MCP Server

Create a new MCP server with tool and resource registration. This wraps the existing web app for MCP hosts.

Dependencies

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

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.

Server Code

Create server.ts:

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";
import fs from "node:fs/promises";
import path from "node:path";
import { z } from "zod";

const server = new McpServer({ name: "my-app", version: "1.0.0" });

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

// Register the tool — inputSchema maps to the app's data sources
registerAppTool(server, "show-app", {
  description: "Displays the app with the given parameters",
  inputSchema: { query: z.string().describe("The search query") },
  _meta: { ui: { resourceUri } },
}, async (args) => {
  // Process args server-side if needed
  return {
    content: [{ type: "text", text: `Showing app for: ${args.query}` }],
    structuredContent: { query: args.query },
  };
});

// Register the HTML resource
registerAppResource(server, {
  uri: resourceUri,
  name: "My App UI",
  mimeType: RESOURCE_MIME_TYPE,
  // Add CSP domains from Step 2 if needed:
  // _meta: { ui: { connectDomains: ["api.example.com"], resourceDomains: ["cdn.example.com"] } },
}, 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 }] };
});

// Start the server
const transport = new StdioServerTransport();
await server.connect(transport);

Package Scripts

Add to package.json:

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

Step 4: Adapt the Build Pipeline

The MCP App build must produce a single HTML file using vite-plugin-singlefile. The standalone web app build stays unchanged.

Vite Configuration

Create or update vite.config.ts. If the app already uses Vite, add vite-plugin-singlefile and a separate entry point for the MCP App build. If it uses another bundler, add a Vite config alongside for the MCP App build only.

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

export default defineConfig({
  plugins: [viteSingleFile()],
  build: {
    outDir: "dist",
    rollupOptions: {
      input: "mcp-app.html",
    },
  },
});

Add framework-specific Vite plugins as needed (e.g., @vitejs/plugin-react for React, @vitejs/plugin-vue for Vue).

HTML Entry Point

Create mcp-app.html as a separate entry point for the MCP App build. This can point to the same app code — the runtime detection handles the rest:

<!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/main.ts"></script>
  </body>
</html>

Two-Phase Build

1. Vite bundles the UI → dist/mcp-app.html (single file with all assets inlined)
2. Server is compiled separately (TypeScript → JavaScript)

The standalone web app continues to build and deploy as before.

Step 5: Add MCP App Initialization Alongside Existing Logic

This is the core step. Instead of replacing the app's data sources, add an alternative initialization path for MCP mode. The app detects its environment at startup and reads parameters from the right source.

The Hybrid Pattern

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

// Detect whether we're running inside an MCP host.
// Choose a detection method that fits the app:
//   - Origin check: window.location.origin !== 'https://myhost.com'
//   - Null origin (sandboxed iframe): window.location.origin === 'null'
//   - Query param: new URL(location.href).searchParams.has('mcp')
const isMcpApp = window.location.origin === "null";

async function getParameters(): Promise<Record<string, string>> {
  if (isMcpApp) {
    // Running as MCP App — get params from tool lifecycle
    const app = new App({ name: "My App", version: "1.0.0" });

    // Register handlers BEFORE connect()
    const params = await new Promise<Record<string, string>>((resolve) => {
      app.ontoolresult = (result) => resolve(result.structuredContent ?? {});
    });

    await app.connect(new PostMessageTransport());
    return params;
  } else {
    // Running as standalone web app — get params from URL
    return Object.fromEntries(new URL(location.href).searchParams);
  }
}

async function main() {
  const params = await getParameters();
  renderApp(params); // Same rendering logic for both modes
}

main().catch(console.error);

URL Parameters (Hybrid)

// Before (standalone only):
const query = new URL(location.href).searchParams.get("q");
renderApp(query);

// After (hybrid):
async function getQuery(): Promise<string> {
  if (isMcpApp) {
    const app = new App({ name: "My App", version: "1.0.0" });
    return new Promise((resolve) => {
      app.ontoolinput = (params) => resolve(params.arguments?.q ?? "");
      app.connect(new PostMessageTransport());
    });
  }
  return new URL(location.href).searchParams.get("q") ?? "";
}

const query = await getQuery();
renderApp(query); // Unchanged rendering logic

API Calls (Hybrid)

// Before (standalone only):
const data = await fetch("/api/data").then(r => r.json());

// After (hybrid):
async function fetchData(): Promise<any> {
  if (isMcpApp) {
    const result = await app.callServerTool("fetch-data", {});
    return result.structuredContent;
  }
  return fetch("/api/data").then(r => r.json());
}

Or keep direct API calls in both modes with CSP connectDomains:

// API calls can stay unchanged if the API is external and the CSP declares the domain
// Declare connectDomains: ["api.example.com"] in the resource registration

localStorage / sessionStorage (Hybrid)

// Before (standalone only):
const saved = localStorage.getItem("settings");

// After (hybrid) — localStorage isn't available in sandboxed iframes:
function getSettings(): any {
  if (isMcpApp) {
    // Will be provided via tool result
    return null; // or a default
  }
  return JSON.parse(localStorage.getItem("settings") ?? "null");
}

Complete Hybrid Example

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

const isMcpApp = window.location.origin === "null";

async function initMcpApp(): Promise<Record<string, any>> {
  const app = new App({ name: "My App", version: "1.0.0" });

  // Register ALL handlers BEFORE connect()
  const params = await new Promise<Record<string, any>>((resolve) => {
    app.ontoolinput = (input) => resolve(input.arguments ?? {});
  });

  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());
  return params;
}

async function initStandaloneApp(): Promise<Record<string, any>> {
  return Object.fromEntries(new URL(location.href).searchParams);
}

async function main() {
  const params = isMcpApp ? await initMcpApp() : await initStandaloneApp();
  renderApp(params); // Same rendering logic — no fork needed
}

main().catch(console.error);

Step 6: Add Host Styling Integration (MCP Mode Only)

When running as an MCP App, integrate with host styling for theme consistency. Use CSS variable fallbacks so the app looks correct in both modes.

Vanilla JS — use helper functions:

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

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);
};

React — use hooks:

import { useApp, useHostStyles } from "@modelcontextprotocol/ext-apps/react";

const { app } = useApp({ appInfo, capabilities, onAppCreated });
useHostStyles(app);

Using variables in CSS — use var() with fallbacks so standalone mode still looks right:

.container {
  background: var(--color-background-secondary, #f5f5f5);
  color: var(--color-text-primary, #333);
  font-family: var(--font-sans, system-ui);
  border-radius: var(--border-radius-md, 8px);
}

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.

Optional Enhancements

App-Only Helper Tools

For data the UI needs to poll or fetch that the model doesn't need to call directly:

registerAppTool(server, "refresh-data", {
  description: "Fetches 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("refresh-data", {}).

Streaming Partial Input

For large tool inputs, use ontoolinputpartial to show progress during LLM generation:

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

app.ontoolinput = (params) => {
  renderFull(params.arguments);
};

Fullscreen Mode

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;
}

Text Fallback

Always provide a content array for non-UI hosts:

return {
  content: [{ type: "text", text: "Fallback description of the result" }],
  structuredContent: { /* data for the UI */ },
};

Common Mistakes to Avoid

1. Forgetting CSP declarations for external origins — fails silently in the sandboxed iframe
2. Using localStorage / sessionStorage in MCP mode — not available in sandboxed iframe; use fallbacks or pass via structuredContent
3. Missing vite-plugin-singlefile — external assets won't load in the iframe
4. Registering handlers after connect() — register ALL handlers BEFORE calling app.connect()
5. Hardcoding styles without fallbacks — use host CSS variables with var(..., fallback) so both modes look correct
6. Not handling safe area insets — always apply ctx.safeAreaInsets in onhostcontextchanged
7. Forgetting text content fallback — always provide content array for non-UI hosts
8. Forgetting resource registration — the tool references a resourceUri that must have a matching resource
9. Replacing standalone logic instead of branching — keep the original data sources intact; add the MCP path alongside them

Testing

Using basic-host

Test the MCP App mode 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. MCP mode: App loads in basic-host without console errors
2. ontoolinput handler fires with tool arguments
3. ontoolresult handler fires with tool result
4. Host styling (theme, fonts, colors) applies correctly
5. External resources load (if CSP domains are configured)
6. Standalone mode: App still works when opened directly in a browser