Protocol Detection

Overview

Studio checks widget compatibility with ChatGPT and Claude by detecting which communication protocol the widget uses. Results appear as compatibility badges after tool execution.

Badge	Meaning
ChatGPT ✓	Widget uses window.openai (legacy) or ext-apps protocol
Claude ✓	Widget uses the ext-apps protocol (MCP Apps)

Two widget protocols

Legacy OpenAI (window.openai)

The original ChatGPT widget protocol. The host injects a window.openai global with toolInput, toolOutput, and action methods.

const data = window.openai.toolOutput;

Supported by: ChatGPT only.

ext-apps (MCP Apps)

The JSON-RPC protocol used by Claude and ChatGPT. The widget communicates with the host via window.parent.postMessage, starting with a ui/initialize handshake.

window.parent.postMessage({
  jsonrpc: "2.0",
  id: 1,
  method: "ui/initialize",
  params: {
    protocolVersion: "2026-01-26",
    appInfo: { name: "my-widget", version: "1.0.0" },
    appCapabilities: {},
  },
}, "*");

Supported by: ChatGPT and Claude.

How Studio detects each protocol

Static detection (before render)

Runs when widget HTML is loaded, before the iframe executes JavaScript.

Signal	Detects	Source
window.openai in HTML	legacy OpenAI	Regex scan
ui/initialize in HTML	ext-apps	Regex scan
@modelcontextprotocol/ext-apps in HTML	ext-apps	SDK import
Resource MIME text/html;profile=mcp-app	ext-apps	MCP resource metadata
_meta.ui in tool/resource response	ext-apps	MCP response metadata

Runtime detection (during render)

Fires when the widget actually accesses an API inside the iframe.

Signal	Detects	Mechanism
Widget reads window.openai.toolInput or toolOutput	legacy OpenAI	Getter trap on injected mock
Widget sends ui/initialize via postMessage	ext-apps	Message listener on iframe

Runtime detection is a fallback for cases where static analysis misses (e.g., minified code or dynamic protocol selection).