If you have started building tools for AI chat hosts, you have probably hit the same fork in the road. There are two ways to add a user interface to a tool right now. The first is MCP Apps, the open standard from the Model Context Protocol. The second is the OpenAI Apps SDK, the framework OpenAI shipped for the ChatGPT app store.

They both let your tool render a UI inside the chat. They both run inside a sandboxed iframe. They both talk to your server over JSON-RPC. So the question almost every developer is asking is the obvious one: are these competing standards?

The short answer

No. The OpenAI Apps SDK is built on top of MCP. It uses MCP for the wire format, the tool definitions, the UI rendering, and the security model. It then adds a thin layer with ChatGPT-specific extensions: a payment hook (window.openai.requestCheckout), a store directory, and a few helpers that only run inside ChatGPT.

So the choice is not "pick one or the other." The honest framing is:

• Build to MCP Apps first. Your tool then works in Claude, Claude Desktop, VS Code, Goose, Postman, MCPJam, and any host that supports the open standard.
• Light up OpenAI Apps SDK extensions only when you need ChatGPT-specific features. Mostly that means payments and the ChatGPT app store.

The four real differences

Strip away the marketing and there are only four things the OpenAI Apps SDK gives you that MCP Apps do not:

1. A store and a directory — the ChatGPT app store will list your app and surface it to ChatGPT users. The open MCP spec has no centralised discovery.
2. In-chat payments — window.openai.requestCheckout() runs a payment flow without leaving the chat. MCP Apps has no checkout primitive today.
3. Conversation-aware helpers — tighter access to surrounding conversation state.
4. Distribution to ChatGPT users specifically — ChatGPT has hundreds of millions of users; the Apps SDK is the only way to reach them through OpenAI's surface.

Everything else — the sandboxing, the iframe rendering, the JSON-RPC audit trail, the tool declarations — is the same on both sides because the Apps SDK uses MCP for them.

Which one to pick

If your audience is ChatGPT users specifically and you need in-chat payments, build with the OpenAI Apps SDK. For everyone else, build to MCP Apps and your tool runs across every host that supports the open standard.

I wrote the full comparison — code examples for both sides, the smart progressive-enhancement pattern, a decision tree, and the two pitfalls most teams hit — on my blog.

Read the full guide on moeed.app →