claude-plugins-official/plugins/mcp-server-dev/skills/build-mcp-app/SKILL.md

name, description, version

name	description	version
build-mcp-app	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.	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">
  import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";

  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:
  function onPick(contact) {
    app.sendMessage({
      role: "user",
      content: [{ type: "text", text: `Selected contact: ${contact.id}` }],
    });
  }
</script>

Method	Direction	Use for
app.ontoolresult = fn	Host → widget	Receive the tool's return value
app.ontoolinput = fn	Host → widget	Receive the tool's input args (what Claude passed)
app.sendMessage({...})	Widget → host	Inject a message into the conversation
app.updateModelContext({...})	Widget → host	Update context silently (no visible message)
app.callServerTool({name, arguments})	Widget → server	Call another tool on your server

sendMessage is the typical "user picked something, tell Claude" path. updateModelContext is for state that Claude should know about but shouldn't clutter the chat.

What widgets cannot do:

• Access the host page's DOM, cookies, or storage
• Make network calls to arbitrary origins (CSP-restricted — route through callServerTool)

Keep widgets small and single-purpose. A picker picks. A chart displays. Don't build a whole sub-app inside the iframe — split it into multiple tools with focused widgets.

---

Scaffold: minimal picker widget

Install:

npm install @modelcontextprotocol/sdk @modelcontextprotocol/ext-apps zod

Server (src/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 { readFileSync } from "node:fs";
import { z } from "zod";

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

const pickerHtml = readFileSync("./widgets/picker.html", "utf8");

registerAppTool(server, "pick_contact", {
  description: "Open an interactive contact picker. User selects one contact.",
  inputSchema: { filter: z.string().optional().describe("Name/email prefix filter") },
  _meta: { ui: { resourceUri: "ui://widgets/picker.html" } },
}, async ({ filter }) => {
  const contacts = await db.contacts.search(filter ?? "");
  return { content: [{ type: "text", text: JSON.stringify(contacts) }] };
});

registerAppResource(server, "Contact Picker", "ui://widgets/picker.html", {},
  async () => ({
    contents: [{ uri: "ui://widgets/picker.html", mimeType: RESOURCE_MIME_TYPE, text: pickerHtml }],
  }),
);

await server.connect(new StdioServerTransport());

Widget (widgets/picker.html):

<!doctype html>
<meta charset="utf-8" />
<style>
  body { font: 14px system-ui; margin: 0; }
  ul { list-style: none; padding: 0; margin: 0; max-height: 300px; overflow-y: auto; }
  li { padding: 10px 14px; cursor: pointer; border-bottom: 1px solid #eee; }
  li:hover { background: #f5f5f5; }
  .sub { color: #666; font-size: 12px; }
</style>
<ul id="list"></ul>
<script type="module">
  import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";

  const app = new App({ name: "ContactPicker", version: "1.0.0" }, {});
  const ul = document.getElementById("list");

  app.ontoolresult = ({ content }) => {
    const contacts = JSON.parse(content[0].text);
    ul.innerHTML = "";
    for (const c of contacts) {
      const li = document.createElement("li");
      li.innerHTML = `<div>${c.name}</div><div class="sub">${c.email}</div>`;
      li.addEventListener("click", () => {
        app.sendMessage({
          role: "user",
          content: [{ type: "text", text: `Selected contact: ${c.id} (${c.name})` }],
        });
      });
      ul.append(li);
    }
  };

  await app.connect();
</script>

See references/widget-templates.md for more widget shapes.

---

Design notes that save you a rewrite

One widget per tool. Resist the urge to build one mega-widget that does everything. One tool → one focused widget → one clear result shape. Claude reasons about these far better.

Tool description must mention the widget. Claude only sees the tool description when deciding what to call. "Opens an interactive picker" in the description is what makes Claude reach for it instead of guessing an ID.

Widgets are optional at runtime. Hosts that don't support the apps surface simply ignore _meta.ui and render the tool's text content normally. Since your tool handler already returns meaningful text/JSON (the widget's data), degradation is automatic — Claude sees the data directly instead of via the widget.

Don't block on widget results for read-only tools. A widget that just displays data (chart, preview) shouldn't require a user action to complete. Return the display widget and a text summary in the same result so Claude can continue reasoning without waiting.

---

Testing

• Local: point Claude desktop's MCP config at your server, trigger the tool, check the widget renders and sendMessage flows back into the chat.
• Host fallback: disable the apps surface (or use a host without it) and confirm the tool degrades gracefully.
• CSP: open browser devtools on the iframe — CSP violations are the #1 reason widgets silently fail.

---

Reference files

• references/widget-templates.md — reusable HTML scaffolds for picker / confirm / progress / display
• references/apps-sdk-messages.md — the App class API: widget ↔ host ↔ server messaging