It was unfortunately annoying for me since I noticed that my "idle" agents kept on disappearing from herdr's agent pane. I'm pretty sure this is related to my setup since I was unable to find any similar complaints on herdr's issues page.

Getting back, I realized that while I had the problem for months now; herdr exposed a brilliant approach to solve it. To be clear, my desires are not for a sidebar but for a marker to show if my agents are either running, idle or waiting for my input. Once I've the signal, I can go back to the right tmux window and jump to right pane to perform the deed.

Voila, 15 mins of herdr... and another 20ish mins with Claude, I was able to devise a bash script which renames the window pane to append the markers signifying the agent states.

#!/bin/bash
# ○ busy   ● waiting   ⊗ idle
#
# Output is intentionally empty — job is renaming windows, not
# displaying status. Status-right just needs to invoke it on a timer.

SESSION_NAME=$(tmux display-message -p '#S')

AGENTS=$(claude agents --json 2>/dev/null)
[ -z "$AGENTS" ] || [ "$AGENTS" == "null" ] && AGENTS="[]"

PANE_TTYS=$(tmux list-panes -a -t "$SESSION_NAME" -F '#{pane_tty} #{window_id}')

# "window_id status" per agent, resolved via tty
WINDOW_AGENT_LINES=$(while read -r pid status; do
    [ -z "$pid" ] && continue
    tty=$(ps -o tty= -p "$pid" 2>/dev/null | tr -d ' ')
    [ -z "$tty" ] || [ "$tty" == "??" ] && continue
    window_id=$(echo "$PANE_TTYS" | awk -v t="/dev/$tty" '$1==t{print $2}')
    [ -n "$window_id" ] && echo "$window_id $status"
done < <(echo "$AGENTS" | jq -r '.[] | select(.pid != null) | "\(.pid) \(.status // "idle")"'))

while read -r window_id window_name; do
    BASE_NAME=$(echo "$window_name" | sed 's/[●○⊗]*$//')

    STATUS_STR=""
    for s in $(echo "$WINDOW_AGENT_LINES" | awk -v w="$window_id" '$1==w{print $2}'); do
        case "$s" in
            busy) STATUS_STR+="○" ;;
            waiting) STATUS_STR+="●" ;;
            idle) STATUS_STR+="⊗" ;;
        esac
    done

    if [ -n "$STATUS_STR" ] && [ "${BASE_NAME}${STATUS_STR}" != "$window_name" ]; then
        tmux rename-window -t "$window_id" "${BASE_NAME}${STATUS_STR}"
    elif [ -z "$STATUS_STR" ] && [ "$window_name" != "$BASE_NAME" ]; then
        tmux rename-window -t "$window_id" "$BASE_NAME"
    fi
done < <(tmux list-windows -t "$SESSION_NAME" -F '#{window_id} #{window_name}')

tmux.conf setup (both required):

set -g status-interval 5
set -g status-right '#(~/.config/tmux/agent_monitor.sh)'

Please note, that the if already have a status-right; append agent-monitor.sh to it. The script does not produces any output but fires every 5 seconds (configurable) by tmux.

And a tmux reload later, I've the markers I've been waiting for ❤️

Credit to herdr for this inspiring idea; wouldn't have thought of markers-over-sidebars without seeing their approach first.