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An MCP App is a server tool that returns UI the host renders in the chat. How that differs from a text tool, and where those servers sit in this directory.
A normal MCP tool returns text or JSON. An MCP App also returns a UI resource. Claude, ChatGPT, or VS Code renders that UI in a sandbox. Clicks and filters become the next message. The spec lives on modelcontextprotocol.io. Host catalogs are Claude Connectors, ChatGPT Apps, and the VS Code MCP gallery.
PlugMCP does not invent a second catalog. An App is still a server. It gets a listing at /mcp/{slug}. The Apps page groups the ones whose README talks about mcp-ui or a ui:// resource. Host catalogs stay at Claude, ChatGPT, and VS Code.