The Background Fleet

Caliban can run sub-agents in the background — detached from your current session — and let you monitor, attach to, or stop them at will. A per-workspace supervisor daemon (caliband) owns the fleet and keeps agents alive even after the parent caliban process exits.

Spawning a background agent

From the command line

The quickest way to fire off a background task is the --bg flag:

caliban --bg "refactor the auth module to use the new token type"

This is shorthand for caliban agents spawn --prompt <task>. Caliban auto-starts caliband if it is not already running, then returns immediately with the new agent's id.

From inside a session

The model can request a background sub-agent by setting background: true in an AgentTool call. The parent session receives the id and a note to check back via caliban attach <id>.

The `caliband` daemon

caliband is a separate binary shipped alongside caliban. It runs as a per-workspace daemon: each workspace root gets its own daemon instance. A workspace is usually a single git repository, but since v0.5.0 the supervisor can manage a workspace spanning multiple sources (repos), each with its own worktree isolation.

Socket path (resolution order):

$CALIBAN_DAEMON_RUNTIME_DIR/<hash>.sock if CALIBAN_DAEMON_RUNTIME_DIR is set.
$XDG_RUNTIME_DIR/caliban/<hash>.sock if $XDG_RUNTIME_DIR is set.
$TMPDIR/caliban-daemon/<hash>.sock (fallback; typical on macOS).

The <hash> is a 16-hex-char SHA-256 prefix of the absolute workspace root path, so each workspace gets a stable, unique socket without naming collisions. (For a single-repo workspace the workspace root is the repo root, so the socket is unchanged from earlier releases.)

caliband auto-starts when any caliban agents command or --bg flag needs it. You should rarely need to launch it directly.

Installing caliband

cargo install caliban installs only the caliban binary. To also install the daemon run:

cargo install caliban-supervisor --bin caliband

Both binaries must be on your $PATH for background fleet features to work.

Networked control plane (beta)

By default caliband serves its control plane over the local Unix domain socket described above. Since v0.5.0 it can instead serve that same line-delimited (NDJSON) protocol over TCP, so a remote client (for example prospero) can drive the fleet across the network rather than only from the same host. Enable it by passing --listen <host:port> (or setting CALIBAN_DAEMON_LISTEN) when the daemon starts:

caliband --workspace-root /path/to/workspace \
  --listen 0.0.0.0:7070 \
  --tls-cert cert.pem --tls-key key.pem \
  --token "$CALIBAN_DAEMON_TOKEN"

TCP mode is fail-closed

The networked control plane requires both a bearer token (--token) and TLS (--tls-cert/--tls-key) — since v0.6.0 the daemon refuses to bind a TCP listener that is unauthenticated or plaintext. The default Unix-socket mode is unchanged and needs neither. The TCP transport is still beta.

Agent lifecycle states

State	Meaning
`spawning`	Registered, not yet executing
`running`	Actively processing turns
`idle`	Waiting for input; no compute pending
`killed`	Stopped via `kill`
`done`	Finished successfully
`failed`	Finished with an error
`crashed`	Daemon restarted while agent was active; needs recovery

`caliban agents` subcommands

`caliban agents list`

Print all registered agents and their status.

caliban agents list

`caliban agents spawn`

Spawn a new background agent with an explicit prompt.

caliban agents spawn --prompt "audit all SQL queries for injection risks"
caliban agents spawn --prompt "write tests for crates/caliban-tools-builtin" --label my-test-agent

Options:

Flag	Description
`--prompt <TEXT>`	Initial prompt for the agent (required)
`--label <NAME>`	Human-readable label shown in `list` and logs

`caliban agents attach <id>`

Stream a running agent's transcript live. Press Ctrl+D to detach without stopping the agent.

caliban agents attach a3f8b2c1

`caliban agents logs <id>`

Print the agent's session log (session.json).

caliban agents logs a3f8b2c1

`caliban agents kill <id>`

Terminate an agent (SIGTERM, escalating to SIGKILL after a grace period).

caliban agents kill a3f8b2c1

`caliban agents respawn <id>`

Kill the agent and restart it with the same original spawn spec (same prompt, model, isolation settings).

caliban agents respawn a3f8b2c1

Note that respawn assigns a new id; the old id is removed from the registry.

`caliban agents rm <id>`

Remove an agent from the registry. The agent must be stopped first, unless --force is passed.

caliban agents rm a3f8b2c1
caliban agents rm a3f8b2c1 --force   # remove even if still running

Top-level shorthands

Four common operations have top-level sugar to save typing:

Shorthand	Equivalent
`caliban attach <id>`	`caliban agents attach <id>`
`caliban logs <id>`	`caliban agents logs <id>`
`caliban stop <id>`	`caliban agents kill <id>`
`caliban kill <id>`	`caliban agents kill <id>`
`caliban respawn <id>`	`caliban agents respawn <id>`
`caliban rm <id>`	`caliban agents rm <id>`

`caliban daemon` subcommands

`caliban daemon status`

Print daemon health, PID, uptime, agent count, and the socket path.

caliban daemon status

`caliban daemon stop`

Ask the daemon to shut down gracefully after finishing in-flight requests. Running agents are not automatically killed; stop them first if you want a clean shutdown.

caliban daemon stop

Session storage

Each background agent's transcript is stored as a regular caliban session file at <base>/agents/<id>/session.json. This means all session tooling (compaction, replay, audit) works on background agents out of the box. Attaching to an agent is conceptually the same as resuming its session over the agent's per-agent socket.

Diagram: agent lifecycle

flowchart LR
    A([caliban --bg task]) -->|spawn request| D[caliband daemon]
    D -->|registers| R[(Registry)]
    D -->|starts| W[Agent worker]
    W -->|streams turns| S[(session.json)]
    W -->|per-agent socket| T([caliban attach id])
    W -->|done/failed| R
    T2([caliban agents kill id]) -->|kill request| D
    D -->|SIGTERM→SIGKILL| W

For how background agents use git worktree isolation, see Worktree Isolation.

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Caliban User Guide