Cmux Custom Sidebar

cmux Custom Sidebar

cmux renders custom sidebars from a small SwiftUI-style file at runtime: no Xcode, no build step, no signing. The file hot-reloads on save, binds to live cmux state (workspaces, tabs, git, PRs, clock), and can run real cmux commands on tap.

The person asking is usually describing a result ("a sidebar that shows my workspaces and lets me jump between them"), not an implementation. Turn that into a clean, native-looking sidebar and make the engineering decisions for them. Do not ask them about SwiftUI, files, or syntax.

Full reference

This skill is the workflow summary. The complete authoring contract (every supported view, modifier, language feature, and data field) is one command away; read it before writing a non-trivial sidebar:

cmux docs sidebars
curl -fsSL https://raw.githubusercontent.com/manaflow-ai/cmux/main/docs/custom-sidebars.md

Workflow

1. Enable the beta (once). Custom sidebars are behind Settings → Beta features → Custom sidebars (customSidebars.beta.enabled). If a written sidebar does not appear in the picker, this flag is the first thing to check.
2. Write a named file. The name becomes the menu label; use short kebab-case:

   ~/.config/cmux/sidebars/<name>.swift
   

   The file is a single SwiftUI-style view expression (no struct, no var body, no imports). A .json variant exists for static layouts; prefer .swift for anything dynamic.
3. Validate and select it:

   cmux sidebar validate <name>   # parse/interpret check with real data shapes
   cmux sidebar select <name>     # switch the sidebar to it
   

   The user can also pick it manually: right-click the sidebar toggle button.
4. Iterate. Saving the file hot-reloads the sidebar in place (cmux sidebar reload forces it). Look at the result, fix what looks off, and verify rows show real data and taps do the right thing before declaring it done.

Authoring rules

• Default to live data. Bind to the workspaces context instead of hard-coding text so the sidebar stays correct on its own.
• Make it interactive by default. Rows that represent something openable should run the matching cmux(...) action on tap. A list that just displays text is rarely what they wanted.
• Prefer Reorderable for workspace-like lists. It gives persisted drag-and-drop reordering for free.
• Keep it native and uncluttered: a title, a divider, then the content.
• Cap long lists (.prefix(20), filter/sort before rendering). The sidebar re-evaluates about once a second; do not render hundreds of rows.
• Stay inside the supported subset. Unsupported syntax is skipped gracefully (never crashes), but choose the closest supported approach rather than shipping a half-blank sidebar.

Quick start

cat > ~/.config/cmux/sidebars/mine.swift <<'SWIFT'
VStack(alignment: .leading, spacing: 8) {
    Text("My sidebar").font(.title3).bold()
    Text(clock.time).font(.caption).foregroundColor(.secondary)
    Divider()
    Reorderable(workspaces, move: "workspace.reorder") { w in
        Button(action: { cmux("workspace.select", workspace_id: w.id) }) {
            HStack {
                Text(w.selected ? "●" : "○").foregroundColor(w.selected ? "#FF8800" : .secondary)
                Text(w.title)
                Spacer()
            }.padding(4)
        }
    }
}
SWIFT
cmux sidebar validate mine && cmux sidebar select mine

Live data context (read-only, refreshes ~1s)

• workspaces: array with id, title, selected, pinned, index, directory, ports + portCount, unread, tabs + tabCount; plus, when present: description, color, branch + dirty, pr / prs ({number, label, url, status, stale, branch}), progress ({value, label}), latestMessage, latestPrompt, latestAt, remote ({target, state, connected}).
• workspaces[i].tabs: id, title, focused, pinned; plus directory, branch + dirty, ports when available.
• clock: {time, hour, minute, second, weekday, epoch}.
• Scalars: workspaceCount, selectedTitle, selectedId, unreadTotal.

Optional fields are omitted when absent; guard with if let b = w.branch { ... } or w.pr != nil ? ... : ....

Actions

A button or .onTapGesture body calls cmux("<method>", param: value), dispatched through the same surface as the cmux CLI. Common methods: workspace.select (workspace_id), surface.focus (surface_id), workspace.reorder (workspace_id + index). openURL("https://...") opens links. Discover the full command surface with cmux docs api.

Supported subset at a glance

Containers: stacks (incl. lazy), Group, List, Section, grids, ViewThatFits, ScrollView, HSplitView (two resizable columns). Content: Text, Label, Image(systemName:), Button (title and label form), Menu, ProgressView, Gauge, Spacer, Divider, shapes, gradients via .background. Modifiers: full typography set, colors as hex strings or tokens, .padding/.frame/layout, .background/.overlay/.mask/.contextMenu with arbitrary nested views, shadows/borders/opacity/effects, .onTapGesture, .help, .disabled. Language: let, user func helpers, for/ForEach, if/else, ternary, string interpolation, arithmetic, array methods (filter/map/sorted/prefix/...), string and number formatting.

Not yet supported (write the natural Swift anyway; it degrades gracefully): @State and input controls (TextField, Toggle, Slider, Picker), custom struct/View definitions, navigation (sheet/popover), AsyncImage. Two-way editing does not work yet; taps that run cmux(...) do.

Troubleshooting

• Sidebar missing from the right-click picker: the beta flag is off, or the file is not directly under ~/.config/cmux/sidebars/.
• Blank or partial render: run cmux sidebar validate <name>; errors show inline in the sidebar with the failing location. A broken save keeps the last working render on screen, so re-save after fixing.
• Rows not tappable: wrap the row in Button(action: { cmux(...) }) { ... } or add .onTapGesture { cmux(...) }.
• Reorder not persisting: use Reorderable(data, move: "workspace.reorder"), not List/.onMove/.draggable.