What it solves
AG-UI standardizes the live interaction boundary between an agent and a user-facing application. Its event stream covers run lifecycle, messages, tool calls, shared state, activities and custom extensions. That makes it an interaction protocol, not a visual component library.
Where it sits
Place AG-UI between the agent runtime and the frontend. A declarative UI payload such as A2UI can travel over AG-UI, while a renderer such as json-render or OpenUI can turn structured output into components. MCP and A2A sit on adjacent boundaries for tools/data and agent-to-agent communication.
Security boundary
Treat every event and tool argument as untrusted input. Validate event shapes, authorize tool effects on the server, avoid exposing raw chain-of-thought, and reconnect from known state snapshots rather than replaying arbitrary client state.
Minimal example
user action → RunAgentInput → agent runtime
agent runtime → ordered AG-UI events → frontend state and UI
Use the event lab to step through a deterministic lifecycle before connecting a real model.