Why control plane and runtime should split

Notes from building agent platforms where governance and execution cannot share one process.

Most agent demos put the chat UI, auth, billing, and model calls in one service. That works until someone asks for audit logs, Azure AD RBAC, and a Python tool chain that takes thirty seconds.

The pattern that held up: a Node control plane owns tenants, credits, and policy; a FastAPI runtime executes steps. The UI talks to the control plane. The runtime reports progress over Socket.IO. Releases stop blocking each other.

The cost is a versioned step contract. The win is being able to answer “who ran this agent?” without reading model logs.