Boundary
The host owns user interaction and approval. Tools, hooks, and protocols sit behind it.
What this adds
Make the tool stack usable.
A host is where the user experiences the system. It exposes the tool registry, turns approvals into a first-class interaction, and records what the session did.
This is the point where the lower layers start to feel like a product surface instead of a bag of scripts.
Done when
A user can inspect available tools and explicitly approve or deny a tool call before execution.
Real-world analog
Aider is a strong real-world comparison: a CLI host that gathers context, routes capabilities, and keeps the user in the loop.
Why this matters
People do not use protocols directly. They use hosts that make those lower layers feel coherent and inspectable.