Cmux

cmux Control

When to Use

• Use when you need to inspect, create, close, or rearrange cmux panes, surfaces, or workspaces.
• Use when you need to send input to or monitor agents running inside cmux.

cmux is a native macOS terminal app for running multiple AI coding agents in parallel. It exposes a CLI (cmux) and a Unix-socket JSON-RPC API (/tmp/cmux.sock) for full topology and browser control.

Core Concepts

• Window — top-level macOS cmux window
• Workspace — sidebar tab within a window (one git branch / project context)
• Pane — split region inside a workspace
• Surface — tab inside a pane (terminal or browser)

Handles default to short refs (workspace:2, pane:1, surface:7); UUIDs accepted as input. Add --id-format uuids|both for UUID output.

Ref syntax — get this right or fail silently

• Always use PREFIXED refs (pane:38, surface:46). A bare number is treated as an INDEX, not an ID — --surface 46 means "the surface at index 46" (usually nonexistent → silent failure), NOT surface:46.
• read-screen and capture-pane have NO --pane flag — they target --workspace or --surface only. Passing --pane errors, and a bare/missing target falls back to your OWN surface (you'll read your own footer and draw wrong conclusions). To read a pane: resolve it to a surface FIRST with cmux list-pane-surfaces --pane pane:N, then cmux read-screen --surface surface:N.
• Never append 2>/dev/null to cmux commands. Errors go to stderr with exit code 1; suppressing them blinds you to your own ref/flag mistakes (the #1 cause of "(no output)").

Detect cmux in a Shell

[ -S "${CMUX_SOCKET_PATH:-/tmp/cmux.sock}" ] || exit 0   # bail if not in cmux
[ -n "${CMUX_WORKSPACE_ID:-}" ] && echo "inside cmux surface"

Injected env vars in every cmux-spawned terminal: CMUX_WORKSPACE_ID, CMUX_SURFACE_ID, CMUX_SOCKET_PATH, CMUX_PORT. Always anchor automation to CMUX_WORKSPACE_ID — the visually focused workspace may not be the agent's caller workspace.

Fast Start — Topology

cmux identify --json                              # who am I (window/workspace/pane/surface)
cmux tree                                         # full hierarchy
cmux list-workspaces --json
cmux list-panes --workspace "$CMUX_WORKSPACE_ID"
cmux list-surfaces --workspace "$CMUX_WORKSPACE_ID"

cmux new-workspace --name "feature-x" --cwd /path/to/repo
cmux new-pane --workspace "$CMUX_WORKSPACE_ID" --type terminal --direction right --focus false
cmux new-pane --workspace "$CMUX_WORKSPACE_ID" --type browser  --direction right --url http://localhost:3000
cmux move-surface --surface surface:7 --pane pane:2 --focus false
cmux split-off --surface surface:7 right
cmux reorder-surface --surface surface:7 --before surface:3
cmux close-surface --surface surface:7

Polling Pi Agents in Panes — Keep Sleeps Short

When launching a Pi Agent inside a cmux pane and polling for output, use short sleep intervals (2–5s). Pi is fast and minimal, and the user runs it on Opus 4.8 Fast via OpenRouter, which streams tokens extremely quickly. Do NOT use sleep 15 unless genuinely needed (a big build/refactor) — most of the time sleep 2–sleep 5 is more than enough.

After every agent check, send the user a one-line status update: what the agent is doing and whether it is on track. Keep it extremely concise.

Claude Code cmux note: after Claude finishes, it may prefill a predicted next user message; that draft is Claude, not the user speaking.

Send Input

Command names: there is NO send-surface / send-key-surface. Target a specific surface with the --surface flag on send / send-key (same commands as the focused terminal). send-panel / send-key-panel exist ONLY for panels (--panel), not surfaces.

cmux send "echo hi\n"                                       # focused terminal
cmux send-key "ctrl+c"                                       # enter|tab|esc|backspace|arrows|ctrl+x|shift+tab
cmux send --surface surface:7 "npm run build"               # specific surface (NOT send-surface)
cmux send-key --surface surface:7 enter                     # specific surface (NOT send-key-surface)

Notifications & Sidebar Metadata

cmux notify --title "Done" --body "tests passed"
cmux set-status build "compiling" --icon hammer --color "#ff9500"
cmux set-progress 0.5 --label "Building..."
cmux log --level success "All 42 tests passed"               # info|progress|success|warning|error
cmux trigger-flash --workspace "$CMUX_WORKSPACE_ID"          # blue-ring attention cue
cmux sidebar-state --json                                    # dump all sidebar metadata

Browser Automation (WKWebView)

Workflow: open → wait → snapshot → act → re-snapshot.

S=$(cmux --json browser open https://example.com | jq -r .result.surface_ref)
cmux browser "$S" wait --load-state complete --timeout-ms 15000
cmux browser "$S" snapshot --interactive                     # returns elements as e1, e2, ...
cmux browser "$S" fill e1 "<email-address>"
cmux browser "$S" click e2 --snapshot-after

# Navigation / inspection
cmux browser "$S" goto URL | back | forward | reload
cmux browser "$S" get url | get title | get text body | get value "#email" | get count ".row"
cmux browser "$S" eval 'return document.title'

# Waits
cmux browser "$S" wait --selector "#ready" --timeout-ms 10000
cmux browser "$S" wait --url-contains "/dashboard" --timeout-ms 10000

# Session
cmux browser "$S" cookies get | cookies set --name foo --value bar
cmux browser "$S" state save /tmp/auth.json | state load /tmp/auth.json

# Diagnostics
cmux browser "$S" console list | errors list | screenshot

Not supported by WKWebView (return not_supported): viewport emulation, geolocation/offline emulation, trace recording, network route interception, raw input injection.

Markdown Viewer

cmux markdown open plan.md --direction right                 # live-watching renderer
cmux open file.pdf                                           # auto-routes to right viewer

cmux markdown open flags: --workspace, --surface, --window, --direction <right|down|left|up>, --focus <true|false>. There is NO --pane flag — passing it errors. To target a pane, pass --surface <existing-md-surface-in-that-pane>.

Reuse the existing right markdown pane (don't spawn strays)

Default behavior of markdown open is to create a new pane every time, even with --direction right. To keep all docs as tabs in ONE right pane, follow this exactly:

# 1. Find the right pane and its surfaces (anchor to THIS workspace)
cmux list-panes --workspace "$CMUX_WORKSPACE_ID"
cmux list-pane-surfaces --pane pane:10        # the right/helper pane

# 2. Open targeting an existing markdown surface IN that pane (reuses pane, adds tab)
cmux markdown open /abs/path/file.md --surface surface:12 --focus false

# 3. If it STILL spawned a new pane (it can), move the new surface in + verify
cmux move-surface --surface surface:NEW --pane pane:10 --focus false
cmux list-panes --workspace "$CMUX_WORKSPACE_ID"   # confirm stray pane is gone

Swapping the file in the single right pane (close-FIRST, then open)

To replace the doc shown in your one right markdown pane, the ONLY reliable order is close the previous surface FIRST, then markdown open the new file fresh — never move an existing viewer, never open-then-close.

# 1. close the previous right markdown surface (right side goes empty)
cmux list-panes --workspace "$CMUX_WORKSPACE_ID"
cmux close-surface --surface surface:PREV
# 2. THEN open the new file fresh
cmux markdown open /abs/path/new.md --direction right --focus false

ORDER MATTERS: close-previous BEFORE open-new. Opening first then closing the old one, or move-surface-ing an existing viewer, leaves the right pane BLANK.

Hard-won lessons (avoid the trial-and-error)

• Surface refs are global, not per-workspace. A ref like surface:126 from an earlier markdown open may live in a different window/workspace. Always re-list (list-panes / list-pane-surfaces) before reusing a ref — never assume a ref from a previous turn is still in the right pane.
• move-surface-ing a markdown viewer often leaves it BLANK. The moved surface keeps type=markdown and surface-health looks fine, but renders nothing. Fix: close-surface it and cmux markdown open <path> fresh, then move the fresh surface if needed. Don't waste time on refresh-surfaces — it usually won't fix a moved-then-blank viewer.
• You cannot screenshot or read-screen a markdown surface (Surface is not a terminal / browser screenshot is WKWebView-only). To verify a markdown viewer rendered, ask the user or open the file in a browser surface instead. Don't burn turns trying to capture it.
• cmux list-surfaces does not exist. Use cmux list-pane-surfaces [--pane ...].