AG-UI (Agent-User Interaction protocol)

An open protocol for streaming an agent’s messages, state, tool calls and approval requests to a frontend as one ordered stream of typed events.

MCP connects an agent to tools and A2A connects agents to each other; AG-UI covers the last hop, from the agent to the person using the app. Instead of a custom format per project, the backend emits standard events (text chunks, tool calls, state updates) over a stream such as SSE, and the UI renders them, including steps where the user must approve an action. It started at CopilotKit and has SDKs for TypeScript and Python.

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Related terms

Server-Sent Events (SSE): A standard way for a server to push a stream of updates to the browser over one long HTTP response.
MCP (Model Context Protocol): An open protocol for plugging tools and data into AI apps: write a server once, use it from any client.
A2A (Agent2Agent protocol): An open protocol for one AI agent to hand a task to another agent, possibly from another vendor, and follow it until it is done.
Human in the loop: A person approves, edits or rejects the agent’s action before it takes effect.

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