Learning from Mature Coding Agents (16): Runtime Abstraction and Capability Handshake
Five external CLIs expose five different machine interfaces: JSONL event streams, JSON-RPC handshake, HTTP API, ACP, stream-json. The right way to support them is not one interface that pretends they're identical — it's a narrow runtime boundary plus an honest capability matrix. Availability means installed, not authenticated; protocol drift fails closed.