/build-mcp-app

name: build-mcp-app
description: This skill should be used when the user wants to build an "MCP app", add "interactive UI" or "widgets" to an MCP server, "render components in chat", build "MCP UI resources", make a tool that shows a "form", "picker", "dashboard" or "confirmation dialog" inline in the conversation, or mentions "apps SDK" in the context of MCP. Use AFTER the build-mcp-server skill has settled the deployment model, or when the user already knows they want UI widgets.
version: 0.1.0

Build an MCP App (Interactive UI Widgets)

An MCP app is a standard MCP server that **also serves UI resources** — interactive components rendered inline in the chat surface. Build once, runs in Claude *and* ChatGPT and any other host that implements the apps surface.

The UI layer is **additive**. Under the hood it's still tools, resources, and the same wire protocol. If you haven't built a plain MCP server before, the `build-mcp-server` skill covers the base layer. This skill adds widgets on top.

---

When a widget beats plain text

Don't add UI for its own sake — most tools are fine returning text or JSON. Add a widget when one of these is true:

| Signal | Widget type | |---|---| | Tool needs structured input Claude can't reliably infer | Form | | User must pick from a list Claude can't rank (files, contacts, records) | Picker / table | | Destructive or billable action needs explicit confirmation | Confirm dialog | | Output is spatial or visual (charts, maps, diffs, previews) | Display widget | | Long-running job the user wants to watch | Progress / live status |

If none apply, skip the widget. Text is faster to build and faster for the user.

---

Widgets vs Elicitation — route correctly

Before building a widget, check if **elicitation** covers it. Elicitation is spec-native, zero UI code, works in any compliant host.

| Need | Elicitation | Widget | |---|---|---| | Confirm yes/no | ✅ | overkill | | Pick from short enum | ✅ | overkill | | Fill a flat form (name, email, date) | ✅ | overkill | | Pick from a large/searchable list | ❌ (no scroll/search) | ✅ | | Visual preview before choosing | ❌ | ✅ | | Chart / map / diff view | ❌ | ✅ | | Live-updating progress | ❌ | ✅ |

If elicitation covers it, use it. See `../build-mcp-server/references/elicitation.md`.

---

Architecture: two deployment shapes

Remote MCP app (most common)

Hosted streamable-HTTP server. Widget templates are served as **resources**; tool results reference them. The host fetches the resource, renders it in an iframe sandbox, and brokers messages between the widget and Claude.

┌──────────┐  tools/call   ┌────────────┐
│  Claude  │─────────────> │ MCP server │
│   host   │<── result ────│  (remote)  │
│          │  + widget ref │            │
│          │               │            │
│          │ resources/read│            │
│          │─────────────> │  widget    │
│ ┌──────┐ │<── template ──│  HTML/JS   │
│ │iframe│ │               └────────────┘
│ │widget│ │
│ └──────┘ │
└──────────┘

MCPB-packaged MCP app (local + UI)

Same widget mechanism, but the server runs locally inside an MCPB bundle. Use this when the widget needs to drive a **local** application — e.g., a file picker that browses the actual local disk, a dialog that controls a desktop app.

For MCPB packaging mechanics, defer to the **`build-mcpb`** skill. Everything below applies to both shapes.

---

How widgets attach to tools

A widget-enabled tool has **two separate registrations**:

1. **The tool** declares a UI resource via `_meta.ui.resourceUri`. Its handler returns plain text/JSON — NOT the HTML. 2. **The resource** is registered separately and serves the HTML.

When Claude calls the tool, the host sees `_meta.ui.resourceUri`, fetches that resource, renders it in an iframe, and pipes the tool's return value into the iframe via the `ontoolresult` event.

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE }
  from "@modelcontextprotocol/ext-apps/server";
import { z } from "zod";

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

// 1. The tool — returns DATA, declares which UI to show
registerAppTool(server, "pick_contact", {
  description: "Open an interactive contact picker",
  inputSchema: { filter: z.string().optional() },
  _meta: { ui: { resourceUri: "ui://widgets/contact-picker.html" } },
}, async ({ filter }) => {
  const contacts = await db.contacts.search(filter);
  // Plain JSON — the widget receives this via ontoolresult
  return { content: [{ type: "text", text: JSON.stringify(contacts) }] };
});

// 2. The resource — serves the HTML
registerAppResource(
  server,
  "Contact Picker",
  "ui://widgets/contact-picker.html",
  {},
  async () => ({
    contents: [{
      uri: "ui://widgets/contact-picker.html",
      mimeType: RESOURCE_MIME_TYPE,
      text: pickerHtml,  // your HTML string
    }],
  }),
);

The URI scheme `ui://` is convention. The mime type MUST be `RESOURCE_MIME_TYPE` (`"text/html;profile=mcp-app"`) — this is how the host knows to render it as an interactive iframe, not just display the source.

---

Widget runtime — the `App` class

Inside the iframe, your script talks to the host via the `App` class from `@modelcontextprotocol/ext-apps`. This is a **persistent bidirectional connection** — the widget stays alive as long as the conversation is active, receiving new tool results and sending user actions.

<script type="module">
  /* ext-apps bundle inlined at build time → globalThis.ExtApps */
  /*__EXT_APPS_BUNDLE__*/
  const { App } = globalThis.ExtApps;

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

  // Set handlers BEFORE connecting
  app.ontoolresult = ({ content }) => {
    const contacts = JSON.parse(content[0].text);
    render(contacts);
  };

  await app.connect();

  // Later, when the user clicks something:
  functio