CopilotKit created AG-UI as the agent-to-user counterpart to MCP (agent-to-tool) and A2A (agent-to-agent). Any endpoint that speaks AG-UI can be wired into a chat surface, a Slack channel, or a product like OpenBot without rewriting the agent. LangChain, CrewAI, Mastra, Pydantic AI, Google ADK, AWS Strands, and Microsoft Agent Framework are among the runtimes that interoperate through it. Governance and UI features ride the protocol rather than a single vendor's harness.