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herdr Backend

CAO's default backend is tmux. For agentic workloads, CAO also supports herdr -- a terminal-native agent runtime and multiplexer designed specifically for AI coding agents.

The key difference: tmux has no concept of "agent state," so CAO must poll terminal output and match regex patterns to detect when an agent is idle. herdr exposes a Unix socket API that emits real-time status events (working, idle, done, blocked). This eliminates polling entirely and enables instant inbox delivery.

tmux vs herdr

tmux herdr
Status detection Poll capture-pane output + regex matching Native socket events (idle, done, blocked)
Inbox delivery Watchdog polls every 5s, delivers on idle pattern Event-driven: delivers immediately on status change
Session identity Inferred from pane content Tracked natively per pane
Attach to agents tmux attach -t <session> herdr attach or herdr TUI
Maturity Stable, well-tested default Experimental

Choose herdr if you want lower-latency inter-agent messaging and native agent lifecycle tracking. Choose tmux if you want the stable, battle-tested default.

Prerequisites

  1. herdr installed and available on $PATH. See herdr.dev for installation instructions.
  2. herdr server for the configured session. CAO starts it automatically if the session socket is absent, so pre-starting is optional. To start (or attach to) it yourself:
herdr                        # default session name
herdr --session cao          # explicit session name (recommended)

Configuration

Set terminal.backend in ~/.aws/cli-agent-orchestrator/settings.json:

{
  "terminal": {
    "backend": "herdr"
  }
}

Optionally specify the herdr session name (defaults to "cao"):

{
  "terminal": {
    "backend": "herdr",
    "herdr_session": "my-session"
  }
}

See configuration.md for the full schema, the CAO_TERMINAL_BACKEND / CAO_HERDR_SESSION env vars, and the cao config CLI.

Launching

With terminal.backend set in settings.json, start the server the same way as with tmux -- CAO detects the backend and connects to herdr automatically:

cao-server

To select herdr without editing settings.json, pass --terminal:

cao-server --terminal herdr

The --terminal flag (tmux or herdr) overrides terminal.backend from settings.json for that run. The herdr_session name, if set, is still read from settings.json.

Viewing and Attaching

# List active CAO sessions
cao session list

# Attach to the herdr TUI (shows all workspaces and tabs)
herdr --session cao

# Attach to a specific CAO session
cao session attach <session-name>

How It Works

CAO maps its concepts to herdr primitives:

CAO concept herdr primitive
Session (e.g. cao-my-task) Workspace (labeled with session name)
Terminal / window (e.g. conductor-a1b2) Tab within workspace (labeled with window name)

Event-driven inbox delivery

HerdrInboxService connects to the herdr Unix socket at startup and subscribes to pane.agent_status_changed events for each managed pane. When a pane transitions to idle or done, pending inbox messages are delivered immediately.

Startup and reconnect behavior

On server start (or reconnect after socket disconnect):

  1. Startup cleanup -- cross-checks all DB terminal records against live herdr tabs. Removes ghost records left by prior server runs that exited uncleanly.
  2. Reconnect reconcile -- prunes stale pane subscriptions from the in-memory map and re-subscribes only to panes that are still alive.
  3. Lifecycle events -- subscribes to pane.closed and workspace.closed events for real-time cleanup when agents exit or sessions end.

The socket connection uses exponential backoff (1s to 30s) on disconnect.

Switching Back to tmux

Remove terminal from settings.json, or set it explicitly:

{
  "terminal": {
    "backend": "tmux"
  }
}

Restart the CAO server. Existing herdr sessions are unaffected (they remain running in herdr). New sessions will be created in tmux.

Troubleshooting

cao session list shows no sessions after server restart

Ghost DB records from a prior run were cleaned up. This is expected. Restart the server and check the log for:

Startup DB cleanup: removed N ghost terminal(s)

If sessions are genuinely running in herdr, they will be re-discovered on the next cao launch.

Session visible in herdr but not in CAO

The CAO server may be connected to the wrong herdr session. Verify terminal.herdr_session in settings.json matches the session herdr is running under:

# Check which session herdr is using
herdr workspace list                        # default session
herdr --session cao workspace list          # named session

Socket connection errors in CAO logs

herdr must be running before the CAO server starts. If you see socket errors:

  1. Confirm herdr is running: herdr --session cao workspace list
  2. If not running, start it: herdr --session cao
  3. Restart the CAO server: cao-server

The default socket path is derived from the session name. If you use a non-default herdr_session, ensure herdr was started with the matching --session flag.