The Apps SDK is OpenAI's framework for building apps that work inside ChatGPT. It is based on the Model Context Protocol (MCP), an open standard that lets an AI model find and call tools on an external server. OpenAI now documents it together with plugins for ChatGPT and Codex, under the heading "ChatGPT plugins and Apps SDK".
A developer builds an MCP server that defines the tools, their input and output schemas, and the authentication they need; ChatGPT calls these tools during a conversation. A tool can also return a UI resource: a web component that runs in an iframe in the chat and talks to ChatGPT through the MCP Apps bridge (JSON-RPC messages sent with postMessage). Extra ChatGPT features are reached through `window.openai`. The server, with optional skills, is packaged as a plugin, uploaded as a ZIP and submitted for review; after approval the developer publishes it to the directory.
OpenAI publishes the documentation and an examples repository on GitHub. The pages read state no fee for building or publishing.