single-file-bundling

Configure Vite with vite-plugin-singlefile to produce a single self-contained HTML file for MCP Apps running in sandboxed iframes.

Overview

MCP Apps run in sandboxed iframes with no same-origin server. This means:

• No relative asset URLs -- <script src="./main.js"> will not resolve
• No CSS file imports -- <link href="./styles.css"> will not load
• No image/font paths -- relative paths to assets will fail
• The entire UI must be a single HTML file with all JS, CSS, images, and fonts inlined

vite-plugin-singlefile handles this by inlining all assets into one HTML file during the Vite build. This is mandatory for MCP Apps -- without it, the app will show a blank iframe.

Capabilities

Vite Configuration

• Configure Vite with vite-plugin-singlefile
• Set correct base, build.outDir, and entry point
• Handle framework-specific Vite plugins (React, Vue, Svelte, Preact, Solid)

Single HTML File Output

• All JavaScript inlined as <script> tags
• All CSS inlined as <style> tags
• All images converted to data URIs
• All fonts converted to base64

Two-Phase Build Setup

• Phase 1: Vite bundles UI into dist/mcp-app.html
• Phase 2: TypeScript compiles server into dist/server.js
• Combined build script orchestrates both phases

Hybrid Build Pipelines

• Add MCP build alongside existing standalone build
• Separate Vite configs for MCP and standalone if needed
• Separate HTML entry points (mcp-app.html vs index.html)

Usage

Basic Vite Configuration

// vite.config.ts
import { defineConfig } from 'vite';
import { viteSingleFile } from 'vite-plugin-singlefile';

export default defineConfig({
  plugins: [viteSingleFile()],
  build: {
    outDir: 'dist',
    // Entry point for the MCP App UI
    rollupOptions: {
      input: 'mcp-app.html',
    },
  },
});

React Vite Configuration

// vite.config.ts
import { defineConfig } from 'vite';
import react from '@vitejs/plugin-react';
import { viteSingleFile } from 'vite-plugin-singlefile';

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

Vue Vite Configuration

// vite.config.ts
import { defineConfig } from 'vite';
import vue from '@vitejs/plugin-vue';
import { viteSingleFile } from 'vite-plugin-singlefile';

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

Svelte Vite Configuration

// vite.config.ts
import { defineConfig } from 'vite';
import { svelte } from '@sveltejs/vite-plugin-svelte';
import { viteSingleFile } from 'vite-plugin-singlefile';

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

HTML Entry Point

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

For React (.tsx entry):

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

Package.json Build Scripts

{
  "scripts": {
    "build:ui": "vite build",
    "build:server": "tsc --project tsconfig.server.json",
    "build": "npm run build:ui && npm run build:server",
    "dev": "concurrently \"vite\" \"tsx watch src/server.ts\"",
    "serve": "tsx src/server.ts"
  }
}

Server Reading the Bundled HTML

import fs from 'fs';
import path from 'path';
import { registerAppResource, RESOURCE_MIME_TYPE } from '@modelcontextprotocol/ext-apps';

// Read the single-file bundle produced by Vite
const bundledHtml = fs.readFileSync(
  path.join(__dirname, '../dist/mcp-app.html'),
  'utf-8'
);

registerAppResource(server, {
  uri: 'app:///my-app',
  name: 'My App',
  mimeType: RESOURCE_MIME_TYPE,
  async read() {
    return {
      contents: [{
        uri: 'app:///my-app',
        mimeType: RESOURCE_MIME_TYPE,
        text: bundledHtml,
      }],
    };
  },
});

Hybrid Build Pipeline (MCP + Standalone)

When converting a web app that already has its own build:

// vite.config.mcp.ts -- MCP-specific build config
import { defineConfig } from 'vite';
import react from '@vitejs/plugin-react';
import { viteSingleFile } from 'vite-plugin-singlefile';

export default defineConfig({
  plugins: [react(), viteSingleFile()],
  build: {
    outDir: 'dist/mcp',
    rollupOptions: {
      input: 'mcp-app.html',  // Separate entry from index.html
    },
  },
});

{
  "scripts": {
    "build:standalone": "vite build",
    "build:mcp:ui": "vite build --config vite.config.mcp.ts",
    "build:mcp:server": "tsc --project tsconfig.server.json",
    "build:mcp": "npm run build:mcp:ui && npm run build:mcp:server",
    "build:all": "npm run build:standalone && npm run build:mcp"
  }
}

Installing Dependencies

# Required dev dependencies
npm install -D vite vite-plugin-singlefile

# Framework-specific (pick one)
npm install -D @vitejs/plugin-react       # React
npm install -D @vitejs/plugin-vue         # Vue
npm install -D @sveltejs/vite-plugin-svelte  # Svelte

Common Pitfalls

1. Forgetting vite-plugin-singlefile: Without it, Vite produces separate JS/CSS files that won't load in the sandboxed iframe.
2. Wrong entry point: The rollupOptions.input must point to the MCP App HTML file, not the standalone index.html.
3. Large bundle size: Inline images and fonts increase HTML file size. Consider optimizing assets or using CSP resourceDomains for large external resources.
4. TypeScript server in Vite output: The server should be compiled separately (Phase 2), not included in the Vite bundle.
5. Missing type="module": The <script> tag in the HTML entry must have type="module" for Vite to process it.

Verification Checklist

• vite-plugin-singlefile in devDependencies
• vite.config.ts imports and uses viteSingleFile()
• rollupOptions.input points to mcp-app.html (not index.html)
• mcp-app.html entry point exists with <script type="module">
• npm run build:ui produces dist/mcp-app.html
• dist/mcp-app.html is self-contained (no external src= or href= to files)
• Build scripts defined: build:ui, build:server, build
• Server reads from dist/mcp-app.html at runtime
• Framework-specific Vite plugin included (if applicable)

Task Definition

const singleFileBundlingTask = defineTask({
  name: 'single-file-bundling',
  description: 'Configure Vite with vite-plugin-singlefile for MCP App',

  inputs: {
    framework: { type: 'string', required: true },
    entryPoint: { type: 'string', default: 'mcp-app.html' },
    outDir: { type: 'string', default: 'dist' },
    hybrid: { type: 'boolean', default: false }
  },

  outputs: {
    viteConfigCreated: { type: 'boolean' },
    entryPointCreated: { type: 'boolean' },
    buildScriptsAdded: { type: 'boolean' },
    artifacts: { type: 'array' }
  },

  async run(inputs, taskCtx) {
    return {
      kind: 'skill',
      title: `Configure single-file bundling (${inputs.framework})`,
      skill: {
        name: 'single-file-bundling',
        context: {
          framework: inputs.framework,
          entryPoint: inputs.entryPoint,
          outDir: inputs.outDir,
          hybrid: inputs.hybrid,
          instructions: [
            'Install vite and vite-plugin-singlefile',
            'Create vite.config.ts with framework plugin and singlefile',
            'Create mcp-app.html entry point',
            'Add build scripts to package.json',
            inputs.hybrid ? 'Create separate MCP Vite config alongside existing build' : null,
            'Build and verify single-file output'
          ].filter(Boolean)
        }
      },
      io: {
        inputJsonPath: `tasks/${taskCtx.effectId}/input.json`,
        outputJsonPath: `tasks/${taskCtx.effectId}/result.json`
      }
    };
  }
});

Applicable Processes

• create-mcp-app.js
• add-app-to-mcp-server.js
• convert-web-app-to-mcp.js

External Dependencies

• Vite (build tool)
• vite-plugin-singlefile (asset inlining)
• Framework-specific Vite plugins (@vitejs/plugin-react, @vitejs/plugin-vue, etc.)

References

• vite-plugin-singlefile
• Vite Documentation
• MCP Apps SDK Examples

Related Skills

• mcp-app-scaffolding
• mcp-tool-resource-pattern
• mcp-app-verification

Related Agents

• mcp-app-architect
• mcp-ui-developer