mcpserver-migrate-mcpapps

name: mcpserver-migrate-mcpapps description: "Migrates an MCP server with interactive widgets from the OpenAI Apps SDK (window.openai, text/html+skybridge) to the MCP Apps standard (@modelcontextprotocol/ext-apps), covering server-side and client-side changes."

Skill: Migrate OpenAI Apps SDK → MCP Apps

Migrate an MCP server with interactive widgets from the OpenAI Apps SDK (window.openai, text/html+skybridge, flat _meta["openai/..."] keys) to the MCP Apps standard (@modelcontextprotocol/ext-apps).

When to Use

Use this skill when:

• An MCP server uses text/html+skybridge MIME type for widget resources
• Widget code references window.openai globals (e.g. window.openai.callTool, window.openai.toolOutput, window.openai.theme)
• Server code uses flat _meta["openai/outputTemplate"] or _meta["openai/widgetAccessible"] keys
• The goal is to make the server compatible with MCP Apps hosts (Claude, ChatGPT, Microsoft 365 Copilot, etc.)

References

• MCP Apps repo: https://github.com/modelcontextprotocol/ext-apps
• API docs: https://modelcontextprotocol.github.io/ext-apps/api/
• Migration guide: https://modelcontextprotocol.github.io/ext-apps/api/documents/Migrate_OpenAI_App.html
• Patterns: https://modelcontextprotocol.github.io/ext-apps/api/documents/Patterns.html
• Quickstart: https://modelcontextprotocol.github.io/ext-apps/api/documents/Quickstart.html
• React module: https://modelcontextprotocol.github.io/ext-apps/api/modules/_modelcontextprotocol_ext-apps_react.html
• Examples: https://github.com/modelcontextprotocol/ext-apps/tree/main/examples

Packages

Migration Mapping

MIME Type

Server: _meta Keys

Server: Class & Helpers

Server: Tool Registration

Widget tools (tools that render UI) use registerAppTool:

import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";
import { z } from "zod";

const WIDGET_URI = "ui://myapp/widget.html";

registerAppResource(server, "Widget Name", WIDGET_URI, {
  mimeType: RESOURCE_MIME_TYPE,
  description: "Description of the widget",
}, async (): Promise<ReadResourceResult> => {
  const html = await fs.readFile(widgetPath, "utf-8");
  return { contents: [{ uri: WIDGET_URI, mimeType: RESOURCE_MIME_TYPE, text: html }] };
});

registerAppTool(server, "show-widget", {
  title: "Show Widget",
  description: "Displays the widget",
  inputSchema: {
    filter: z.string().optional().describe("Optional filter"),
  },
  annotations: { readOnlyHint: true },
  _meta: { ui: { resourceUri: WIDGET_URI } },
}, async ({ filter }): Promise<CallToolResult> => {
  const data = await fetchData(filter);
  return {
    content: [{ type: "text", text: `Loaded ${data.length} items.` }],
    structuredContent: { items: data },
  };
});

Data-only tools (no UI) use server.tool() directly:

server.tool("update-item", "Updates an item.", {
  id: z.string().describe("Item ID"),
  status: z.string().describe("New status"),
}, async ({ id, status }) => {
  await db.update(id, { status });
  return { content: [{ type: "text" as const, text: `Updated ${id}.` }] };
});

Client (Widget): Global API

Client (Widget): React Hook

Client (Widget): useApp Hook API

The useApp hook from @modelcontextprotocol/ext-apps/react has the following signature:

interface UseAppOptions {
  appInfo: { name: string; version: string };
  capabilities: McpUiAppCapabilities; // usually {}
  onAppCreated?: (app: App) => void;  // register handlers BEFORE connect
}

interface AppState {
  app: App | null;       // null while connecting
  isConnected: boolean;
  error: Error | null;
}

function useApp(options: UseAppOptions): AppState;

Important: Event handlers (ontoolresult, onhostcontextchanged, ontoolinput, etc.) must be set in the onAppCreated callback, which fires before the connection handshake. The app in the returned AppState is App | null, so always use optional chaining (app?.callServerTool(...)).

Client (Widget): Built-in React Hooks

The SDK also provides these hooks (no custom wrapper needed):

Step-by-Step Migration Process

1. Update Dependencies

Server package.json — add:

"@modelcontextprotocol/ext-apps": "^1.0.0",
"zod": "^3.25.0"

Widgets package.json — add:

"@modelcontextprotocol/ext-apps": "^1.0.0"

2. Create MCP Apps React Context (Widgets)

Create a shared hook file (e.g. hooks/useMcpApp.tsx) that wraps the useApp hook.

Key API rules:

• useApp takes { appInfo: { name, version }, capabilities: {}, onAppCreated? }
• Event handlers (ontoolresult, onhostcontextchanged) must be registered in onAppCreated
• onAppCreated fires before connection, so use refs to bridge into React state
• useApp returns { app: App | null, isConnected: boolean, error: Error | null }

import React, { createContext, useContext, useEffect, useRef, useState } from "react";
import { useApp, type McpUiHostContext } from "@modelcontextprotocol/ext-apps/react";
import type { App } from "@modelcontextprotocol/ext-apps";

interface McpAppContextValue {
  app: App | null;
  isConnected: boolean;
  toolData: unknown;
  theme: "light" | "dark";
  hostContext: McpUiHostContext | undefined;
}

const McpAppContext = createContext<McpAppContextValue | null>(null);

export function McpAppProvider({ name, children }: { name: string; children: React.ReactNode }) {
  const [toolData, setToolData] = useState<unknown>(null);
  const [theme, setTheme] = useState<"light" | "dark">("light");
  const [hostContext, setHostContext] = useState<McpUiHostContext | undefined>(undefined);

  // Refs so the onAppCreated callback can update React state
  const setToolDataRef = useRef(setToolData);
  const setThemeRef = useRef(setTheme);
  const setHostContextRef = useRef(setHostContext);
  setToolDataRef.current = setToolData;
  setThemeRef.current = setTheme;
  setHostContextRef.current = setHostContext;

  const { app, isConnected, error } = useApp({
    appInfo: { name, version: "1.0.0" },
    capabilities: {},
    onAppCreated: (app) => {
      app.ontoolresult = (result) => {
        if (result?.structuredContent) {
          setToolDataRef.current(result.structuredContent);
        }
      };
      app.onhostcontextchanged = (ctx) => {
        setHostContextRef.current((prev) => ({ ...prev, ...ctx }));
        if (ctx?.theme === "dark" || ctx?.theme === "light") {
          setThemeRef.current(ctx.theme);
        }
      };
    },
  });

  // Set initial host context after connection
  useEffect(() => {
    if (app) {
      const initial = app.getHostContext();
      if (initial) {
        setHostContext(initial);
        if (initial.theme === "dark" || initial.theme === "light") {
          setTheme(initial.theme);
        }
      }
    }
  }, [app]);

  return (
    <McpAppContext.Provider value={{ app, isConnected, toolData, theme, hostContext }}>
      {children}
    </McpAppContext.Provider>
  );
}

export function useMcpApp() {
  const ctx = useContext(McpAppContext);
  if (!ctx) throw new Error("useMcpApp must be used within McpAppProvider");
  return ctx;
}

export function useMcpToolData<T = unknown>(): T | null {
  const { toolData } = useMcpApp();
  return toolData as T | null;
}

export function useMcpTheme(): "light" | "dark" {
  const { theme } = useMcpApp();
  return theme;
}

3. Update Widget Entry Points (main.tsx)

Wrap the app in <McpAppProvider> instead of reading window.openai:

import { McpAppProvider, useMcpTheme } from "../hooks/useMcpApp";

function ThemedApp() {
  const theme = useMcpTheme();
  return (
    <FluentProvider theme={theme === "dark" ? webDarkTheme : webLightTheme}>
      <MyWidget />
    </FluentProvider>
  );
}

createRoot(document.getElementById("root")!).render(
  <McpAppProvider name="My Widget">
    <ThemedApp />
  </McpAppProvider>
);

4. Update Widget Components

Replace all window.openai references:

// BEFORE
const toolOutput = useOpenAiGlobal("toolOutput");
window.openai.callTool("update-item", { id: "1", status: "done" });
window.openai.requestDisplayMode(
  window.openai.displayMode === "expanded" ? "default" : "expanded"
);

// AFTER
const toolData = useMcpToolData<MyDataType>();
const { app, hostContext } = useMcpApp();
// app is App | null — use optional chaining
await app?.callServerTool({ name: "update-item", arguments: { id: "1", status: "done" } });
await app?.requestDisplayMode({
  mode: hostContext?.displayMode === "fullscreen" ? "inline" : "fullscreen"
});

Note: requestDisplayMode takes an object { mode: string }, not a raw string. Display modes are "inline" | "fullscreen" | "pip". Check hostContext?.availableDisplayModes before requesting.

Fullscreen toggle checklist:

• useCallback deps must include app and hostContext?.displayMode — an empty [] captures the initial null/undefined values and the MCP SDK path silently fails every time
• Guard with if (app) (not optional chaining app?.requestDisplayMode(...)) so a missing app falls through to the browser fallback instead of returning undefined and exiting
• Sync isFullscreen state from hostContext.displayMode via a useEffect — otherwise the button icon won't update when the host confirms the mode change

// Sync fullscreen state from MCP host context changes
useEffect(() => {
  if (hostContext?.displayMode !== undefined) {
    setIsFullscreen(hostContext.displayMode === "fullscreen");
  }
}, [hostContext?.displayMode]);

const toggleFullscreen = useCallback(async () => {
  // 1. MCP Apps SDK
  try {
    if (app) {
      const current = hostContext?.displayMode;
      await app.requestDisplayMode({ mode: current === "fullscreen" ? "inline" : "fullscreen" });
      return;
    }
  } catch { /* not available */ }
  // 2. Browser Fullscreen API
  try {
    if (!document.fullscreenElement) {
      await document.documentElement.requestFullscreen();
    } else {
      await document.exitFullscreen();
    }
    return;
  } catch { /* sandboxed */ }
  // 3. CSS fallback
  setIsFullscreen((prev) => !prev);
}, [app, hostContext?.displayMode]);

5. Rewrite Server (mcp-server.ts)

1. Replace Server with McpServer
2. Replace manual setRequestHandler with registerAppTool / registerAppResource / server.tool()
3. Use RESOURCE_MIME_TYPE instead of "text/html+skybridge"
4. Use zod schemas for tool input definitions
5. Return structuredContent (object) alongside content (text array) from widget tools

6. Update Server Entry Point (index.ts)

Switch from server.connect(transport) with a low-level Server to McpServer:

import { createMcpServer } from "./mcp-server.js";

app.all("/mcp", async (req, res) => {
  const server = createMcpServer(); // returns McpServer
  const transport = new StreamableHTTPServerTransport({
    sessionIdGenerator: undefined,
    enableJsonResponse: true,
  });
  await server.connect(transport);
  await transport.handleRequest(req, res, req.body);
});

7. Build & Test

npm run install:all
npm run build:widgets
npm run dev:server

Verify with the MCP Inspector (npx @modelcontextprotocol/inspector) or connect from a host like Claude.

Common Pitfalls

Files Typically Changed