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

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

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
    },
  },
});

<!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>

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

server.registerTool(
  "my-tool",
  { inputSchema: z.object({ param: z.string() }) },
  async ({ param }) => {
    const data = await fetchData(param);
    return { content: [{ type: "text", text: JSON.stringify(data) }] };
  },
);

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: z.object({ param: z.string() }),
    _meta: { ui: { resourceUri } },
  },
  async ({ param }) => {
    const data = await fetchData(param);
    return {
      content: [{ type: "text", text: JSON.stringify(data) }], // text fallback for non-UI hosts
      structuredContent: { data }, // structured data for the UI
    };
  },
);

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

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

registerAppResource(
  server,
  "My Tool UI",
  resourceUri,
  {},
  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 }],
    };
  },
);

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());

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

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) }] };
});

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

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
};

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.registerTool(
      "my-tool",
      {
        description: "Shows data",
        inputSchema: z.object({ param: z.string() }),
      },
      plainToolHandler,
    );
  }
};

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;
}

# 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