The Xema Shell is the deterministic, capability-backed command plane. It is not a Unix shell — it has no string-eval, no pipes, no fork. Every command is a typed ShellCommandDescriptor (name, syntax, required capabilities, required execution environments, output schema). Built-in commands are shipped by xema-shell-api; biome-contributed commands are declared through the contribution protocol. The agent- facing capability shell:run@1 takes a structured argv: string[] and always returns JSON — the same path the human shell uses, with --json forced on.