A2UI is an open-source declarative UI protocol, created by Google with contributions from CopilotKit and the open-source community, that lets AI agents generate rich, interactive user interfaces rendered natively across web, mobile, and desktop without executing arbitrary code. It targets host app developers, agent developers, and renderer developers, who send declarative component descriptions that clients render using their own pre-approved catalogs and native widgets. Key capabilities include streaming progressive rendering, framework-agnostic renderers (Angular, Flutter, Lit, React, Markdown), transports such as A2A, and integration with MCP and MCP Apps.