Choosing a protocol

Five options, three genuinely different problems. Most bad choices come from answering the wrong question.

The three problems

1. "A tool result should render as a widget inside someone else's chat host." → MCP Apps family (MCP Apps, mcp-ui, OpenAI Apps SDK).
2. "An agent should describe UI that my client renders with its own native components." → A2UI.
3. "My frontend needs to show what a long-running agent is doing, share state with it, and let the user interrupt." → AG-UI.

If the answer is more than one, you need more than one. They compose cleanly.

Decision tree