Persistent mode: --input-format stream-json

Do. Keep one process for many turns, reading user messages as stream-json on stdin (Claude Code, Qwen, Amp, Kimi wire mode). Enables mid-turn steering of agent-to-agent messages, approval prompts via Claude's control protocol (control_request can_use_tool), and no MCP restart per turn. Needs AgentMux to set persistent_launch_args for Mux Code.

Spec §6.3.