cmux-customization - SKILL.md Agent Skill

name: cmux-customization description: "Customize cmux for an end user. Use when changing cmux.json actions, custom commands, workspace layouts, plus-button behavior, surface tab bar buttons, Command Palette entries, Dock controls, sidebar and app settings, shortcuts, notifications, browser routing, examples-library presets, or Ghostty-backed terminal preferences."

cmux Customization

Use this skill for user-facing cmux customization. Keep the user's config intact, prefer schema-backed edits, and validate before reporting completion.

What Can Be Customized

Custom actions: define reusable actions in cmux.json. Actions can appear in Cmd+Shift+P, surface tab bars, shortcuts, and the plus-button right-click menu.
New workspace button: set ui.newWorkspace.action to replace the normal plus-button click, and ui.newWorkspace.contextMenu to control right-click actions. ui.newWorkspace.rightClick is accepted as an alias, but new examples should use contextMenu.
Surface tab bar buttons: set ui.surfaceTabBar.buttons to replace the default tab bar buttons. Include built-in IDs such as cmux.newTerminal, cmux.newBrowser, cmux.splitRight, and cmux.splitDown only when they should stay visible.
Workflows and layouts: use commands with workspace definitions to open a worktree, multiple checkouts, local services, browser previews, or SSH sessions in a deliberate split layout.
Dock controls: create .cmux/dock.json or ~/.config/cmux/dock.json for right-sidebar terminal controls such as logs, test watchers, git TUIs, dev servers, queues, or cmux feed tui --opentui.
Sidebar and app behavior: use cmux-settings for supported settings such as appearance, sidebar display, notification behavior, browser routing, automation, shortcuts, and new-workspace placement.
Workspace metadata: use the cmux CLI or cmux-workspace for workspace names, descriptions, colors, read state, and sidebar metadata updates.
Feed and notifications: use cmux hooks setup for Feed event sources, notification settings for delivery behavior, and notification hooks in cmux.json for filtering or post-processing banners.
Team presets and examples: use project-local .cmux/cmux.json and .cmux/dock.json to share worktree, SSH, review, dev, CI, and docs workspace patterns with a repo.
Import, export, and reset: back up the current config, apply the smallest diff, validate it, and keep a rollback path for user-owned customizations.
Terminal behavior: use Ghostty config for fonts, themes, cursor style, copy-on-select, shell integration, terminal keybindings, and terminal rendering.

Choose the Right Surface

cmux app preferences: use cmux-settings for global ~/.config/cmux/cmux.json settings such as appearance, sidebar, notifications, browser behavior, automation, and shortcuts.
Custom actions, workspace layouts, tab bar buttons, plus-button behavior, and Command Palette entries: edit ~/.config/cmux/cmux.json globally or .cmux/cmux.json in the project. Project-local actions and commands override global entries with the same ID or name.
Dock controls: edit .cmux/dock.json in the project or ~/.config/cmux/dock.json globally. Run cmux docs dock when available.
Terminal rendering and terminal keybindings: use Ghostty config, usually ~/.config/ghostty/config. This includes fonts, cursor style, copy-on-select, shell integration, themes, and terminal keybindings.
Project-specific behavior: prefer .cmux/cmux.json in the project so actions, commands, UI action wiring, and notification hooks travel with the repo. Do not put global app preferences there.

If a request can be handled by Ghostty config, say that and use Ghostty config instead of inventing cmux UI settings.

Examples Library

For reusable patterns such as worktree agents, full-stack dev layouts, SSH devboxes, PR review workspaces, docs workspaces, quick agent tab buttons, and CI watches, read references/examples.md. Load it when the user asks for examples, presets, templates, starter configs, or a known workflow shape.

Workflow

Inspect existing config before editing.

test -f ~/.config/cmux/cmux.json && sed -n '1,220p' ~/.config/cmux/cmux.json
test -f .cmux/cmux.json && sed -n '1,220p' .cmux/cmux.json

Pick global or project-local scope. Ask only when the choice changes behavior meaningfully. Default to project-local for repo-specific commands and global for app preferences.

Before editing, back up the target file when it already exists:

stamp="$(date +%Y%m%d-%H%M%S)"
test -f ~/.config/cmux/cmux.json && cp -p ~/.config/cmux/cmux.json ~/.config/cmux/cmux.json."$stamp".bak
test -f .cmux/cmux.json && cp -p .cmux/cmux.json .cmux/cmux.json."$stamp".bak

Use the applicable path only. Do not create a backup for a missing file.

For app settings and cmux-owned shortcuts, use the settings helper from the installed skill or checkout:

~/.agents/skills/cmux-settings/scripts/cmux-settings list-supported
~/.agents/skills/cmux-settings/scripts/cmux-settings set browser.openTerminalLinksInCmuxBrowser true
~/.agents/skills/cmux-settings/scripts/cmux-settings validate

If the user installed with skills.sh, use ~/.codex/skills/cmux-settings/scripts/cmux-settings instead.

For actions, UI wiring, workspace layouts, notification hooks, and Dock controls, edit JSONC or JSON carefully. Preserve unrelated sections such as vault, rightSidebar, commands, actions, ui, and notifications.
Reload config after successful edits:
```
cmux reload-config
```
Verify the configured entrypoint exists. For shortcuts, read back the binding. For custom actions, confirm the action ID and where it should appear.

Common Patterns

Add a Command Palette action that opens Codex in a new tab. It will appear in Cmd+Shift+P unless palette is false:

{
  "actions": {
    "codex-new-tab": {
      "type": "agent",
      "agent": "codex",
      "title": "Codex",
      "subtitle": "Start Codex in this workspace",
      "target": "newTabInCurrentPane",
      "palette": true
    }
  }
}

Replace the plus-button click and define the plus-button right-click menu. This is the pattern for "bring your own worktree, multiple checkouts, or SSH setup". The workspaceCommand action ID is worktree-agents, and its commandName must match a command named Worktree Agents in the same config:

{
  "actions": {
    "worktree-agents": {
      "type": "workspaceCommand",
      "title": "Worktree Agents",
      "commandName": "Worktree Agents",
      "icon": { "type": "symbol", "name": "folder.badge.plus" }
    }
  },
  "ui": {
    "newWorkspace": {
      "action": "worktree-agents",
      "contextMenu": [
        { "action": "worktree-agents", "title": "Worktree Agents" },
        { "type": "separator" },
        { "action": "cmux.newTerminal", "title": "New Terminal" },
        { "action": "cmux.newBrowser", "title": "New Browser" }
      ]
    }
  },
  "commands": [
    {
      "name": "Worktree Agents",
      "description": "Create a worktree and open agents inside it",
      "workspace": {
        "name": "Worktree Agents",
        "cwd": "../worktrees/my-feature",
        "layout": {
          "direction": "horizontal",
          "children": [
            {
              "pane": {
                "surfaces": [
                  { "type": "terminal", "name": "Codex", "command": "codex" }
                ]
              }
            },
            {
              "pane": {
                "surfaces": [
                  { "type": "terminal", "name": "SSH", "command": "ssh devbox" }
                ]
              }
            }
          ]
        }
      }
    }
  ]
}

Add a project workspace layout:

{
  "commands": [
    {
      "name": "dev",
      "workspace": {
        "name": "Dev",
        "cwd": ".",
        "layout": {
          "direction": "horizontal",
          "children": [
            { "pane": { "surfaces": [{ "type": "terminal", "command": "bun dev" }] } },
            { "pane": { "surfaces": [{ "type": "browser", "url": "http://localhost:3000" }] } }
          ]
        }
      }
    }
  ]
}

Replace surface tab bar buttons:

{
  "ui": {
    "surfaceTabBar": {
      "buttons": [
        "cmux.newTerminal",
        "cmux.newBrowser",
        {
          "action": "codex-new-tab",
          "title": "Codex",
          "icon": { "type": "symbol", "name": "terminal" }
        }
      ]
    }
  }
}

Add project Dock controls:

{
  "controls": [
    {
      "id": "git",
      "title": "Git",
      "command": "lazygit",
      "cwd": ".",
      "height": 300
    },
    {
      "id": "feed",
      "title": "Feed",
      "command": "cmux feed tui --opentui",
      "height": 260
    }
  ]
}

Validation

App settings: run cmux-settings validate.
JSONC shape: keep valid JSONC and avoid duplicate keys.
Dock JSON: parse .cmux/dock.json or ~/.config/cmux/dock.json with a JSON parser before reporting completion.
Runtime reload: run cmux reload-config when the CLI is available.
User-facing action: confirm the action title, shortcut, plus-button behavior, context-menu entry, or tab bar placement the user asked for.

Rules

Do not overwrite whole top-level config sections unless you own the full section.
Do not store secrets directly in actions, commands, or prompts. Use environment variables or the user's secret manager.
Do not use app/runtime sleeps or timing workarounds in generated commands.
Do not add a cmux setting for behavior Ghostty already owns.
Keep labels short enough for menus, buttons, and the Command Palette.