Skills
skills/dagster-io/erk/cmux

cmux

SKILL.md

cmux - Terminal Multiplexer

Overview

cmux is a native macOS terminal application (Swift/AppKit) that provides terminal multiplexing with workspaces, panes, and splits. It uses Ghostty's rendering engine (libghostty) for GPU-accelerated terminal output. The CLI communicates with the running app via a Unix domain socket at /tmp/cmux.sock.

Think of cmux as "tmux reimagined as a native macOS app" -- it has workspaces (like tmux windows), panes (like tmux panes), and surfaces (individual terminal or browser instances within panes), all controllable via a rich CLI.

When to Load the Reference

This skill covers the mental model, common commands, gotchas, and erk integration. Load references/cmux-reference.md when you need:

  • Browser automation syntax (40+ subcommands: navigation, forms, cookies, storage, console, waiting, dialogs, downloads, state)
  • Window management (list-windows, new-window, focus-window, close-window, move-workspace-to-window)
  • Surface management (move-surface, reorder-surface, drag-surface-to-split, surface-health)
  • Panel/Tab commands (list-panels, focus-panel, tab-action, rename-tab)
  • Sidebar metadata (clear-status, list-status, clear-progress, clear-log, list-log, sidebar-state)
  • Full tmux compat (20+ commands: pipe-pane, wait-for, copy-mode, set-hook, bind-key, popup, etc.)
  • Utility/diagnostic (ping, capabilities, claude-hook, set-app-focus)
  • Workflow recipes (browser automation, layout scripting, notification workflows)

Core Concepts

Object Hierarchy

Window (top-level OS window)
  +-- Workspace (vertical tab, like a tmux window)
        +-- Pane (a split region)
              +-- Surface (terminal or browser instance)
  • Window: macOS application window. Multiple windows supported.
  • Workspace: A tab in the sidebar. Contains one or more panes.
  • Pane: A split region within a workspace. Can be split left/right/up/down.
  • Surface: An actual terminal or browser instance inside a pane.

Addressing / Refs

Objects are referenced by UUID (A1B2C3D4-...), short ref (workspace:1, pane:2, surface:3), or numeric index (0, 1). Use --id-format refs|uuids|both to control output format.

Environment Variables

cmux sets these inside managed terminals: CMUX_WORKSPACE_ID (current workspace UUID, used as default for --workspace), CMUX_SURFACE_ID, CMUX_TAB_ID, CMUX_SOCKET_PATH (default: /tmp/cmux.sock).

Socket Communication

Unix domain socket at /tmp/cmux.sock. V2 API uses newline-delimited JSON. CLI wraps socket calls into user-friendly commands.

Quick Command Reference

Global Flags

--socket PATH              Override socket path (default: /tmp/cmux.sock)
--window WINDOW            Target specific window
--password PASSWORD        Socket auth password
--json                     Output as JSON
--id-format refs|uuids|both  Control ID format
--version, -v              Show version
--help, -h                 Show help

Workspace Commands

Command Description
new-workspace [--command <text>] Create workspace (auto-appends \n). Returns OK <uuid>
list-workspaces List all workspaces
select-workspace --workspace <ref> Switch to workspace
rename-workspace [--workspace <ref>] <title> Rename workspace
close-workspace --workspace <ref> Close workspace
current-workspace Print current workspace ID

Pane and Input Commands

Command Description
new-split <left|right|up|down> [--workspace <ref>] Split pane
new-surface [--type <terminal|browser>] [--pane <ref>] Add surface
list-panes [--workspace <ref>] List panes
focus-pane --pane <ref> Focus pane
close-surface [--surface <ref>] Close surface
send [--workspace <ref>] [--surface <ref>] <text> Send text (must include \n)
send-key [--workspace <ref>] [--surface <ref>] <key> Send keystroke
read-screen [--workspace <ref>] [--surface <ref>] [--scrollback] [--lines <n>] Read terminal content

Notification / Status Commands

Command Description
notify --title <text> [--subtitle <text>] [--body <text>] Send notification
set-status <key> <value> [--icon <name>] [--color <hex>] Set sidebar status
set-progress <0.0-1.0> [--label <text>] Set progress bar
log [--level <level>] [--source <name>] <message> Add log entry

Browser Subsystem (Summary)

The browser subsystem has 40+ commands for full web automation. Key categories:

  • Navigation: browser open, browser navigate/goto, browser back/forward/reload
  • Interaction: browser click, browser fill, browser type, browser select, browser press
  • Inspection: browser snapshot [--interactive], browser eval <js>, browser get, browser is
  • Waiting: browser wait --selector|--text|--url-contains|--function [--timeout-ms]
  • --snapshot-after flag: Most interaction commands accept this flag to automatically capture a snapshot after the action

Load references/cmux-reference.md for full browser command syntax.

Key Commands Not in Quick Reference

These are documented in the reference file:

  • wait-for <channel> -- Inter-process synchronization (tmux compat)
  • claude-hook <event> -- Claude Code integration hook
  • pipe-pane -- Pipe pane output to external command
  • popup -- Show popup overlay
  • set-hook / bind-key / unbind-key -- Hook and key binding management

Critical Gotchas

--command auto-appends newline

cmux new-workspace --command 'echo hello' automatically appends \n, so the command executes immediately. Do NOT add your own \n.

send requires explicit \n

Unlike --command, cmux send does NOT auto-append a newline. You must include it:

# WRONG - text is typed but not executed
cmux send --workspace "$WS" "echo hello"

# CORRECT - command executes
cmux send --workspace "$WS" $'echo hello\n'

Output format: OK <uuid>

cmux new-workspace returns OK <uuid>. Extract the workspace ID:

WS=$(cmux new-workspace | awk '{print $2}')

Shell quoting with --command

Use single quotes to prevent outer shell expansion of $():

# CORRECT - subshell runs inside workspace
cmux new-workspace --command 'source "$(erk pr teleport 456 --new-slot --script --sync)"'

# WRONG - subshell expands in current shell
cmux new-workspace --command "source \"$(erk pr teleport 456 --new-slot --script --sync)\""

No --working-directory CLI flag

The CLI does not expose a working directory flag. Workaround:

cmux new-workspace --command 'cd /path/to/project'

Startup delay

There is a ~0.5s internal delay after workspace creation before commands are sent, to allow the shell to initialize. Keep this in mind when chaining commands.

Workspace names are just labels

cmux knows nothing about git branches. Branch names used as workspace titles via rename-workspace are purely cosmetic labels for human navigation.

Erk Integration

Configuration

cmux integration is gated on a global config flag:

# ~/.erk/config.toml
cmux_integration = true

Check/set via CLI:

erk config get cmux_integration
erk config set cmux_integration true

erk exec cmux-open-pr

Creates a cmux workspace that opens a PR with checkout or teleport mode:

erk exec cmux-open-pr --pr 8152
erk exec cmux-open-pr --pr 8152 --branch "my-branch"
erk exec cmux-open-pr --pr 8152 --mode teleport

What it does:

  1. Auto-detects PR head branch via gh pr view --json headRefName (if --branch omitted)
  2. Creates workspace with:
    • checkout mode (default): source "$(erk pr checkout <pr> --script)"
    • teleport mode: source "$(erk pr teleport <pr> --new-slot --script --sync)"
  3. Renames workspace to the branch name
  4. Outputs JSON: {"success": true, "pr_number": N, "branch": "...", "workspace_name": "..."}

TUI Dashboard Commands

When cmux_integration is enabled, the erk dash TUI command palette exposes:

  • cmux checkout (action) -- Creates a cmux workspace for the selected plan's PR
  • copy cmux checkout (copy) -- Copies the cmux open command to clipboard

Both require a plan with pr_number and pr_head_branch.

Common Scripting Patterns

# Create workspace and run command
WS=$(cmux new-workspace --command 'cd ~/code/myproject && make build' | awk '{print $2}')
cmux rename-workspace --workspace "$WS" "build"

# Create workspace, then send commands separately
WS=$(cmux new-workspace | awk '{print $2}')
cmux send --workspace "$WS" $'cd ~/code/myproject\n'
cmux send --workspace "$WS" $'make test\n'

# Read terminal output
cmux read-screen --workspace "$WS" --lines 50
cmux read-screen --workspace "$WS" --scrollback

# Split workspace layout
WS=$(cmux new-workspace --command 'cd ~/code/project' | awk '{print $2}')
cmux new-split right --workspace "$WS"

# JSON output for scripting
cmux --json list-workspaces
cmux select-workspace --workspace workspace:0

Troubleshooting

Issue Cause Fix
Connection refused cmux app not running Launch cmux.app
Socket not found Wrong socket path Check /tmp/cmux.sock exists
Command typed but not executed Missing \n in send Use $'command\n'
Wrong workspace targeted No --workspace flag Use $CMUX_WORKSPACE_ID or explicit ref
--command text expanded too early Double quotes around $() Use single quotes
Weekly Installs
2
Repository
dagster-io/erk
GitHub Stars
74
First Seen
12 days ago
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Installed on
opencode2
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