Reference tmux focus events required
Claude Code's built-in /remote-control (and forge_spawn_session.sh line 97-101 which polls for "remote-control is active") relies on tmux focus events to know the bridge has activated. With focus-events off (tmux default) the banner never appears and the spawn aborts:
Fix lives in ~/.tmux.conf:
Applied to the live server with tmux set-option -g focus-events on. Survives reboots via the conf file.
If you ever see the same failure on a fresh box / new tmux server, check tmux show-options -g focus-events first.