Migrating from OpenAI Apps SDK to MCP Apps SDK

This reference maps OpenAI Apps SDK concepts to their MCP Apps SDK (@modelcontextprotocol/ext-apps) equivalents. Use the tables below for quick lookup during migration, and refer to the code examples for complete before/after comparisons.

This guide covers server-side changes first (metadata, tools, resources), then client-side changes (setup, context, events).

Server-Side

The server-side changes involve updating metadata structure and using helper functions.

Quick Start Comparison

OpenAI Apps SDK	MCP Apps SDK
Flat metadata keys (_meta["openai/..."])	Nested metadata structure (_meta.ui.*)
Direct server.registerTool()/server.registerResource()	Helper functions: registerAppTool(), registerAppResource()
UI Resource MIME type: text/html+skybridge	UI Resource MIME type: text/html;profile=mcp-app

Tool Metadata

OpenAI	MCP Apps	Notes
_meta["openai/outputTemplate"]	_meta.ui.resourceUri	URI of UI resource
_meta["openai/toolInvocation/invoking"]	—	Not yet implemented
_meta["openai/toolInvocation/invoked"]	—	Not yet implemented
_meta["openai/widgetAccessible"] (boolean)	_meta.ui.visibility (string[])	true/false → include/exclude "app" in array
_meta["openai/visibility"] (string)	_meta.ui.visibility (string[])	"public"/"private" → include/exclude "model" in array

Resource Metadata

OpenAI	MCP Apps	Notes
_meta["openai/widgetCSP"]	_meta.ui.csp	See CSP field mapping below
—	_meta.ui.permissions	MCP adds: permissions for camera, microphone, geolocation, clipboard
_meta["openai/widgetDomain"]	_meta.ui.domain	Dedicated sandbox origin
_meta["openai/widgetPrefersBorder"]	_meta.ui.prefersBorder	Visual boundary preference
_meta["openai/widgetDescription"]	—	Not yet implemented; use app.updateModelContext() for dynamic context

Resource MIME Type

OpenAI	MCP Apps	Notes
text/html+skybridge	text/html;profile=mcp-app	Auto-set by registerAppResource(); use RESOURCE_MIME_TYPE constant if manual

CSP Field Mapping

OpenAI	MCP Apps	Notes
resource_domains	resourceDomains	Origins for static assets (images, fonts, styles, scripts)
connect_domains	connectDomains	Origins for fetch/XHR/WebSocket requests
frame_domains	frameDomains	Origins for nested iframes
redirect_domains	—	OpenAI-only: origins for openExternal redirects
—	baseUriDomains	MCP-only: base-uri CSP directive

Server-Side Migration Example

Before (OpenAI)

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";

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

  // Register tool with OpenAI metadata
  server.registerTool(
    "shopping-cart",
    {
      title: "Shopping Cart",
      description: "Display the user's shopping cart",
      inputSchema: { userId: z.string() },
      annotations: { readOnlyHint: true },
      _meta: {
        "openai/outputTemplate": "ui://view/cart.html",
        "openai/toolInvocation/invoking": "Loading cart...",
        "openai/toolInvocation/invoked": "Cart ready",
        "openai/widgetAccessible": true,
      },
    },
    async (args) => {
      const cart = await getCart(args.userId);
      return {
        content: [{ type: "text", text: JSON.stringify(cart) }],
        structuredContent: { cart },
      };
    },
  );

  // Register UI resource
  server.registerResource(
    "Cart View",
    "ui://view/cart.html",
    { mimeType: "text/html+skybridge" },
    async () => ({
      contents: [
        {
          uri: "ui://view/cart.html",
          mimeType: "text/html+skybridge",
          text: getCartHtml(),
          _meta: {
            "openai/widgetCSP": {
              resource_domains: ["https://cdn.example.com"],
              connect_domains: ["https://api.example.com"],
              frame_domains: ["https://embed.example.com"],
            },
          },
        },
      ],
    }),
  );

  return server;
}
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After (MCP Apps)

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

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

  // Register tool with MCP Apps metadata
  registerAppTool(
    server,
    "shopping-cart",
    {
      title: "Shopping Cart",
      description: "Display the user's shopping cart",
      inputSchema: { userId: z.string() },
      annotations: { readOnlyHint: true },
      _meta: { ui: { resourceUri: "ui://view/cart.html" } },
    },
    async (args) => {
      const cart = await getCart(args.userId);
      return {
        content: [{ type: "text", text: JSON.stringify(cart) }],
        structuredContent: { cart },
      };
    },
  );

  // Register UI resource
  registerAppResource(
    server,
    "Cart View",
    "ui://view/cart.html",
    { description: "Shopping cart UI" },
    async () => ({
      contents: [
        {
          uri: "ui://view/cart.html",
          mimeType: RESOURCE_MIME_TYPE,
          text: getCartHtml(),
          _meta: {
            ui: {
              csp: {
                resourceDomains: ["https://cdn.example.com"],
                connectDomains: ["https://api.example.com"],
                frameDomains: ["https://embed.example.com"],
              },
            },
          },
        },
      ],
    }),
  );

  return server;
}
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Key Differences Summary

1. Metadata Structure: OpenAI uses flat _meta["openai/..."] properties; MCP uses nested _meta.ui.* structure
2. Tool Visibility: OpenAI uses boolean/string (true/"public"); MCP uses string arrays (["app", "model"])
3. CSP Field Names: snake_case → camelCase (e.g., connect_domains → connectDomains)
4. App Permissions: MCP adds _meta.ui.permissions for camera, microphone, geolocation, clipboard (not in OpenAI)
5. Resource MIME Type: text/html+skybridge → text/html;profile=mcp-app (use RESOURCE_MIME_TYPE constant)
6. Helper Functions: MCP provides registerAppTool() and registerAppResource() helpers
7. Not Yet Implemented: _meta["openai/toolInvocation/invoking"], _meta["openai/toolInvocation/invoked"], and _meta["openai/widgetDescription"] don't have MCP equivalents yet

Client-Side

Client-side migration involves replacing the implicit window.openai global with an explicit App instance.

Quick Start Comparison

OpenAI Apps SDK	MCP Apps SDK
Implicit global (window.openai)	Explicit instance (new App(...))
Properties pre-populated on load	Async connection + notifications
Sync property access	Getters + event handlers

Setup & Connection

OpenAI	MCP Apps	Notes
window.openai (auto-available)	const app = new App({name, version})	MCP requires explicit instantiation
(implicit)	Vanilla: await app.connect() / React: useApp()	MCP requires async connection; auto-detects OpenAI env
—	await app.connect(new OpenAITransport())	Force OpenAI mode (not yet available, see PR #172)
—	await app.connect(new PostMessageTransport(...))	Force MCP mode explicitly

Host Context Properties

OpenAI	MCP Apps	Notes
window.openai.theme	app.getHostContext()?.theme	"light" | "dark"
window.openai.locale	app.getHostContext()?.locale	BCP 47 language tag (e.g., "en-US")
window.openai.displayMode	app.getHostContext()?.displayMode	"inline" | "pip" | "fullscreen"
window.openai.maxHeight	app.getHostContext()?.viewport?.maxHeight	Max container height in px
window.openai.safeArea	app.getHostContext()?.safeAreaInsets	{ top, right, bottom, left }
window.openai.userAgent	app.getHostContext()?.userAgent	Host user agent string
—	app.getHostContext()?.availableDisplayModes	MCP adds: which modes host supports
—	app.getHostContext()?.toolInfo	MCP adds: tool metadata during call

Tool Data (Input/Output)

OpenAI	MCP Apps	Notes
window.openai.toolInput	app.ontoolinput = (params) => { params.arguments }	Tool arguments; MCP uses callback
window.openai.toolOutput	app.ontoolresult = (params) => { params.structuredContent }	Tool result; MCP uses callback
window.openai.toolResponseMetadata	app.ontoolresult → params._meta	Widget-only metadata from server
—	app.ontoolinputpartial = (params) => {...}	MCP adds: streaming partial args
—	app.ontoolcancelled = (params) => {...}	MCP adds: cancellation notification

Calling Tools

OpenAI	MCP Apps	Notes
await window.openai.callTool(name, args)	await app.callServerTool({ name, arguments: args })	Call another MCP server tool

Sending Messages

OpenAI	MCP Apps	Notes
await window.openai.sendFollowUpMessage({ prompt })	await app.sendMessage({ role: "user", content: [{ type: "text", text: prompt }] })	MCP uses structured content array

External Links

OpenAI	MCP Apps	Notes
await window.openai.openExternal({ href })	await app.openLink({ url: href })	Different param name: href → url

Display Mode

OpenAI	MCP Apps	Notes
await window.openai.requestDisplayMode({ mode })	await app.requestDisplayMode({ mode })	Same API
—	Check app.getHostContext()?.availableDisplayModes first	MCP lets you check what's available

Size Reporting

OpenAI	MCP Apps	Notes
window.openai.notifyIntrinsicHeight(height)	app.sendSizeChanged({ width, height })	MCP includes width
Manual only	new App(appInfo, capabilities, { autoResize: true /* default */ })	MCP auto-reports via ResizeObserver

State Persistence

OpenAI	MCP Apps	Notes
window.openai.widgetState	—	Not directly available in MCP
window.openai.setWidgetState(state)	—	Use alternative mechanisms (localStorage, server-side state, etc.)

File Operations (Not Yet in MCP Apps)

OpenAI	MCP Apps	Notes
await window.openai.uploadFile(file)	—	Not yet implemented
await window.openai.getFileDownloadUrl({ fileId })	—	Not yet implemented

Other (Not Yet in MCP Apps)

OpenAI	MCP Apps	Notes
await window.openai.requestModal(options)	—	Not yet implemented
window.openai.requestClose()	—	Not yet implemented
window.openai.setOpenInAppUrl({ href })	—	Not yet implemented
window.openai.view	—	Not yet mapped

Event Handling

OpenAI	MCP Apps	Notes
window.openai.toolInput (read on load)	app.ontoolinput = (params) => {...}	OpenAI: sync property; MCP: callback (register before connect())
window.openai.toolOutput (read on load)	app.ontoolresult = (params) => {...}	OpenAI: sync property; MCP: callback (register before connect())
addEventListener("openai:set_globals", ...)	app.onhostcontextchanged = (ctx) => {...}	Both push updates; MCP uses callback, OpenAI uses DOM event
—	app.onteardown = async () => {...}	MCP adds: cleanup before unmount

Logging

OpenAI	MCP Apps	Notes
console.log(...)	app.sendLog({ level: "info", data: "..." })	MCP provides structured logging

Host Info

OpenAI	MCP Apps	Notes
—	app.getHostVersion()	Returns { name, version } of host
—	app.getHostCapabilities()	Check serverTools, openLinks, logging, etc.

Client-Side Migration Example

Before (OpenAI)

// OpenAI Apps SDK
applyTheme(window.openai.theme);
console.log("Tool args:", window.openai.toolInput);
console.log("Tool result:", window.openai.toolOutput);

// Call a tool
const result = await window.openai.callTool("get_weather", { city: "Tokyo" });

// Send a message
await window.openai.sendFollowUpMessage({ prompt: "Weather updated!" });

// Report height
window.openai.notifyIntrinsicHeight(400);

// Open link
await window.openai.openExternal({ href: "https://example.com" });
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After (MCP Apps)

import { App } from "@modelcontextprotocol/ext-apps";

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

// Register handlers BEFORE connect (events may occur immediately after connect)
app.ontoolinput = (params) => {
  console.log("Tool args:", params.arguments);
};

app.ontoolresult = (params) => {
  console.log("Tool result:", params.structuredContent);
};

app.onhostcontextchanged = (ctx) => {
  if (ctx.theme) applyTheme(ctx.theme);
};

// Connect (auto-detects OpenAI vs MCP)
await app.connect();

// Access context
applyTheme(app.getHostContext()?.theme);

// Call a tool
const result = await app.callServerTool({
  name: "get_weather",
  arguments: { city: "Tokyo" },
});

// Send a message
await app.sendMessage({
  role: "user",
  content: [{ type: "text", text: "Weather updated!" }],
});

// Open link (note: url not href)
await app.openLink({ url: "https://example.com" });
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Key Differences Summary

1. Initialization: OpenAI is implicit; MCP requires new App() + await app.connect()
2. Data Flow: OpenAI pre-populates; MCP uses async notifications (register handlers before connect())
3. Auto-resize: MCP has built-in ResizeObserver support via autoResize option
4. Structured Content: MCP uses { type: "text", text: "..." } arrays for messages
5. Context Changes: MCP pushes updates via onhostcontextchanged; no polling needed
6. Capabilities: MCP lets you check what the host supports before calling methods