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 runs real cmux commands on tap.

The person asking is describing a result ("a sidebar that shows my workspaces and lets me jump between them"), not an implementation. Make the engineering decisions for them; do not ask them about SwiftUI, files, or syntax.

This skill is the workflow summary. Read the complete authoring contract (every supported view, modifier, language feature, and data field) 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): Settings > Beta features > Custom sidebars (customSidebars.beta.enabled). If a written sidebar does not appear in the picker, check this first.
2. Write a named file at ~/.config/cmux/sidebars/<name>.swift. The name becomes the menu label; use short kebab-case. 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:

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

   The user can also right-click the sidebar toggle button to pick it.
4. Iterate. Saving hot-reloads in place (cmux sidebar reload forces it). Verify rows show real data and taps do the right thing before declaring it done.

Authoring rules

• Bind to the workspaces context instead of hard-coding text, so the sidebar stays correct on its own.
• Rows that represent something openable run the matching cmux(...) action on tap. A list that only displays text is rarely what they wanted.
• Use 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.
• Stay inside the supported subset. Unsupported syntax is skipped gracefully rather than crashing, but choose the closest supported approach instead of 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: id, title, selected, pinned, index, directory, ports + portCount, unread, status (todo/working/needs-attention/review/done), tabs + tabCount; when present also 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 CLI. Common methods: workspace.select (workspace_id), surface.focus (surface_id), workspace.reorder (workspace_id + index). openURL("https://...") opens links. Full command surface: cmux docs api.

Supported subset

Containers: stacks (including 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

• 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.