sunpeak is MCP-App-First

MCP Apps now run in ChatGPT, Claude, Goose, and VS Code. That happened fast. Claude announced MCP App support on January 26, and ChatGPT followed on February 4. Two weeks, two major hosts, one standard.

TL;DR: sunpeak’s APIs are built around the MCP App standard. ChatGPT and Claude-specific features are layered on top as optional imports. Write your app once, run it everywhere—even localhost.

Why MCP-App-First

When ChatGPT Apps launched in October 2025, they had their own proprietary SDK. Building for ChatGPT meant building only for ChatGPT.

That changed when OpenAI contributed to and aligned on MCP Apps as the open standard. The rendering model, the iframe sandbox, the UI functionality — all of it became portable. And as of February 2026, the major hosts actually implemented it.

sunpeak followed the same trajectory. We started as a ChatGPT App framework because ChatGPT was the only major host supporting embedded UIs. Now we’re an MCP App framework, because the standard is real and the host list is growing.

What that means in practice: sunpeak’s core APIs target the MCP App interface, not any single host. At the same time, sunpeak layers in the major host-specific functionality developers need to seamlessly support differentiated platforms (think React Native).

How It Works

sunpeak separates standard MCP App APIs from host-specific ones at the import level.

Core APIs come from the top-level sunpeak import. These work everywhere:

import { useToolData, useHostContext, useDisplayMode, AppProvider } from 'sunpeak';
import type { ResourceConfig } from 'sunpeak';

export const resource: ResourceConfig = {
  name: 'dashboard',
  description: 'Show analytics dashboard',
};

export function DashboardResource() {
  const { output } = useToolData();
  const context = useHostContext();
  const displayMode = useDisplayMode();

  return <div>{/* Your UI — runs in ChatGPT, Claude, Goose, VS Code */}</div>;
}

Host-specific APIs come from subpath imports. Right now that’s sunpeak/chatgpt for ChatGPT-specific tooling:

import { ChatGPTSimulator, buildDevSimulations } from 'sunpeak/chatgpt';

The ChatGPT simulator, dev simulation builder, and any ChatGPT-only runtime features live here. They’re first-class — not afterthoughts or community plugins — but they don’t pollute your app code. Your resource components stay portable.

What Changed, What Didn’t

If you’re already using sunpeak, you’ll notice v0.13 changes many APIs to be based on MCP App abstractions and nomenclature. Fortunately, a lot of Apps SDK knowledge is easily portable to the MCP App interface. Refer to the release notes for more specific migration instructions, and refer to the sunpeak docs for a complete overview of sunpeak and MCP Apps.

With these changes, your app renders in Claude (and others) today. The sunpeak dev simulator at localhost:6767 replicates the MCP App runtime that all hosts implement, so what works locally works in production across hosts.

The Host Landscape

Here’s where MCP Apps run today:

• ChatGPT — OpenAI contributed elements of the original ChatGPT Apps protocol to MCP and now supports the open standard alongside their existing SDK.
• Claude — Anthropic’s web and desktop clients render MCP Apps natively.
• Goose — Block’s open-source AI agent supports MCP Apps.
• VS Code Insiders — Microsoft’s editor renders MCP Apps in the chat sidebar.

More hosts will follow. The MCP App standard is under the Linux Foundation now, and the spec is actively developed at modelcontextprotocol/ext-apps.

Platform-Specific Features Are First-Class

MCP-App-first doesn’t mean lowest-common-denominator. ChatGPT has features that Claude doesn’t, and vice versa. sunpeak treats these as first-class extensions, not hacks.

For ChatGPT, that means full access to OpenAI’s apps-sdk-ui component library, the ChatGPT simulator for local development, and any ChatGPT-specific runtime APIs. These are maintained alongside the core framework, tested in CI, and documented.

As Claude and other hosts ship their own platform-specific features, sunpeak will add corresponding subpath imports. The pattern scales: core stays portable, extensions stay organized.

Get Started

sunpeak is open source and free.

pnpm add -g sunpeak && sunpeak new

Your app works across ChatGPT, Claude, and every other MCP App host from the first line of code.

• Documentation: guides, API reference, and tutorials
• GitHub: source code and issue tracker
• MCP App Framework: overview of sunpeak’s capabilities