Cmux

cmux

cmux is a command-line client for the cmux.app on this machine. Each command opens the app's Unix socket and changes the live window. There is no dry-run mode. Written against cmux 0.64.22; run cmux version to compare.

The four nouns

| Noun | What it is | |---|---| | window | A macOS window. Holds workspaces. | | workspace | A tab-like group in a window. Holds panes. | | pane | A split area in a workspace. Holds surfaces. | | surface | One tab in a pane. A terminal, a browser, a simulator, or an agent session. |

The surface is where text goes in and out. Almost every command needs one, and takes it from a flag or from the environment.

You are the caller

You run inside a cmux surface. Your environment has CMUX_WORKSPACE_ID and CMUX_SURFACE_ID, and every targeting flag defaults to them. So cmux send "ls" with no --surface types into your own terminal.

Start with:

cmux identify --json

caller.surface_ref in the output is you. focused.surface_ref is what the user looks at. Send text to your own surface only when the user asks for that.

Handles

A window, workspace, pane, or surface flag accepts three forms:

• ref — workspace:30, surface:217. Printed by every list command.
• uuid — A1B2C3D4-E5F6-4789-ABCD-0123456789AB. Stable and global.
• index — 0, 1, 2. The position in the list.

A ref and an index both look like a number, and they point at different things: the first workspace can print as workspace:1 at index 0. Use refs, and copy them from a listing you just ran. Add --id-format uuids or --id-format both when you need UUIDs.

Orient first

cmux tree            # this window: workspaces, panes, surfaces, titles, ttys
cmux tree --all      # every window
cmux identify --json # who you are

cmux tree shows the whole app in one screen. Read it before you target anything.

Two ways in

CLI verbs — cmux new-workspace, cmux send, cmux browser click. Use these to create things. A CLI verb is the only path that honours creation options.

cmux rpc '' — the raw socket. Use it to read and change things that already exist. It returns JSON on stdout, so it parses cleanly.

cmux rpc workspace.list
cmux rpc surface.list '{"workspace_id":""}'
cmux rpc surface.read_text '{"surface_id":""}'
cmux rpc workspace.rename '{"workspace_id":"","title":"Done"}'

Creation is the split that matters. surface.create over rpc takes no url, and workspace creation over rpc drops the title, cwd, description, and group. So:

cmux new-workspace --name "Tests" --cwd . --description "owner/repo#1" --focus false
cmux new-surface --type browser --url "file:///tmp/board.html" --workspace  --focus false

Errors arrive on stderr with a non-zero exit, not as a JSON error object:

Error: invalid_params: Surface is not a terminal
method_not_found: Unknown method

Look it up

The binary is the source of truth, and it ships its own docs.

cmux --help              # every command
cmux  --help    # exact flags and examples for one command
cmux docs                # doc topics
cmux docs api            # CLI contract and socket API URLs
cmux capabilities        # every v2 socket method, as JSON

cmux --help lists more than 200 commands. Do not guess a flag. Read cmux --help first. To find an rpc method:

cmux capabilities | jq -r '.methods[]' | grep workspace

Most commands take --json for machine-readable output.

Stay out of the user's way

The user is sitting in this app while you work. Pass --focus false on creation so their selection stays put. Focus something only when the user asks to see it.

Recipes

Run a command in a new workspace

cmux new-workspace --name "Tests" --cwd . --command "npm test" --focus false

--command sends the text and Enter.

Send input to another surface

cmux send --surface surface:78 "npm run build\n"
cmux send-key --surface surface:78 ctrl+c

send types the text and stops. End the string with \n to submit it.

Read what a surface shows

cmux read-screen --surface surface:78
cmux read-screen --surface surface:78 --scrollback --lines 200

read-screen returns a snapshot of this moment. It does not wait. It fails on a browser surface with invalid_params: Surface is not a terminal.

Boot an agent in another surface

An agent TUI batches an incoming paste. An Enter that races the paste gets swallowed, and the prompt sits typed but unsent. So type, confirm on screen, then press Enter as a separate call.

S=surface:78
cmux send --surface $S "claude\n"

# 1. Wait for the TUI. Poll for its prompt marker: ⏵⏵ ❯ "for shortcuts"
until cmux read-screen --surface $S | grep -qE '⏵⏵|❯|for shortcuts'; do sleep 0.2; done

# 2. Type the prompt with no Enter.
cmux send --surface $S "Fix the failing test in src/auth.ts"

# 3. Confirm it landed. Match a short head slice — the input box wraps the rest.
until cmux read-screen --surface $S | grep -qF "Fix the failing test"; do sleep 0.1; done

# 4. Now submit.
cmux send-key --surface $S enter

Budgets that hold in practice: the TUI is ready in about 2 seconds, so time out at 20. The typed text appears in well under a second, so time out at 5. Submit anyway on timeout, and tell the user it was not confirmed.

A dedicated agent surface is the other option, with its own UI instead of a TUI:

cmux new-surface --type agent-session --provider claude --working-directory . --focus false

--provider accepts codex, claude, or opencode, and defaults to codex.

Change the layout

cmux new-split right --surface surface:78
cmux move-surface --surface surface:217 --pane pane:38 --focus true
cmux split-off --surface surface:217 right
cmux rename-tab --surface surface:217 "Build"

Wait for a signal

cmux events --category agent --name agent.hook.Stop --limit 1

cmux events prints newline-delimited JSON and blocks. The first frame is an ack with resume data. Use --limit to exit after n frames, --no-heartbeat to drop the 15-second keepalive, and --cursor-file with --reconnect to resume after a restart. agent.hook.Stop arrives only after cmux hooks setup installs the agent hooks.

To hand off between two of your own commands:

cmux wait-for build-done --timeout 120   # blocks
cmux wait-for -S build-done              # releases it

Report progress to the user

cmux notify --title "Tests passed" --body "412 tests, 0 failures"
cmux set-status build ok --icon checkmark --color '#22c55e'
cmux set-progress 0.6 --label "Migrating"
cmux log --level info "Step 3 of 5 complete"

Browser surfaces

A cmux surface can be a real browser that you drive like Playwright:

cmux browser open https://example.com
cmux browser snapshot --interactive
cmux browser click "button.submit"
cmux browser screenshot --out /tmp/shot.png

Run cmux browser --help for the full set, and cmux docs browser for the upstream reference.

Guardrails

• The checklist belongs to the user. Leave cmux todo items and

cmux workspace status alone. Change them only when the user asks you to manage that surface. Keep your own plan in your own task tracking.

• Back up the config before you edit it. Copy ~/.config/cmux/cmux.json to a

timestamped .bak file first. Then run cmux config doctor and cmux reload-config.

• Terminal look belongs to Ghostty. Font, theme, cursor, scrollback, opacity,

and blur live in ~/.config/ghostty/config. App behaviour, sidebar, notifications, and browser settings live in cmux.json. cmux reload-config reloads both with no restart.

• Ask before you close. close-workspace, close-surface, and

close-window destroy running work, and there is no undo.

When a command fails

cmux ping prints PONG when the app is up. Anything else means the app is down, or CMUX_SOCKET_PATH points at the wrong socket. cmux --help, cmux version, and cmux docs work with no socket.

The socket also gates who may connect. cmux capabilities reports the mode as access_mode. The default is cmuxOnly, which accepts only processes that cmux itself spawned — so the same script that works in a cmux terminal is refused from an outside shell, a cron job, or a detached daemon. automation.socketControlMode in cmux.json changes this.

Writing a script that drives cmux

Read [references/automation-patterns.md](references/automation-patterns.md) before you write a loop, a watcher, or anything that creates surfaces in bulk. It covers settling after a mutation, identifying what you just created, browser tabs that go stale, and the workspace-group trap.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: will-ness-ai
• Source: will-ness-ai/skills
• License: MIT

Install and usage instructions live in the source repository linked above.