beam: the terminal client

beam was a full-screen terminal client for agent sessions: chat, plan, shell and file edits in one scrollback, an approval card that blocked a gated tool call inline, a command palette with argument completion, @ file addressing, and a status bar carrying the live model, session and context pressure.

beam running a real session: the brand header with model and session id, the
/ @ ! ? affordances, an operator prompt asking what is needed to take Stripe
checkout live, and the agent's local_fs.read_file calls resolving inline as
it reads the setup doc and the deployment manifest before answering

It was the instrument that exercised every seam of the system from a client’s side.

What it settled

The seam is the protocol, not the surface. Almost none of beam knew which agent it was talking to. What it had been all along, without being asked to be one, was a client for the Agent Client Protocol.
A surface must not own policy. Where beam did reach past the protocol, that was a governance boundary having quietly come to live inside a client. A boundary that only holds while one particular surface is running is not a boundary.
Argument completion belongs to the protocol. Beam’s command palette filled its value domains from what the agent already advertised — the same thing a browser client receives. Nothing had to be added to the protocol for it, and the web client was built from beam’s model rather than designed fresh.