The full checklist: design-rules.md. Use it for reviews.
Copyable XAML: patterns.md.
The Settings page: settings.md.
Commands: command bars, menus, context menus, and shortcuts: commands.md.
Microsoft's own text on one topic, when a rule here is not enough: the Windows app design pages on Microsoft Learn (learn.microsoft.com/windows/apps/design/), for example controls/navigationview or signature-experiences/materials. The WinUI Gallery app shows every control live.
App-wide tokens (colors, radii, motion): call the Skill tool with design-system.
How a feature works and what it says (names, flow, dialogs, errors, accessibility): call the Skill tool with ux-design.
Spacing, alignment, and the checks that catch misalignment: call the Skill tool with layout-grid.
Parts of your own instead of Fluent's: call the Skill tool with design-kit, or design-from-app to match an app the user names.

Fluent principles

Use them as the review lens for every screen (Windows 11 design principles):

Effortless: faster and easier to do what people want, with focus and precision. Cut what does not earn its place.
Calm: quieter, less cluttered, less noisy. Soft materials, rounded corners, motion that informs and never shows off.
Personal: the app fits the person. It follows their theme, accent, text size, and contrast settings.
Familiar: things that look the same work the same. Standard controls, standard shortcuts, standard words.
Complete and coherent: the app feels like part of Windows on every screen, in light, dark, and high contrast, with mouse, keyboard, touch, and pen.

Every screen answers: Where am I? What can I do? Where can I go?

Before you write pages

Write these under ## Design plan in docs/BRIEF.md, or in docs/DESIGN.md when the brief would pass one page:

Archetype: navigation window, single-page tool, tray app, or quick-capture panel. Most personal apps are a navigation window plus one quick-capture entry point. A tray utility is "tray icon + flyout or panels + Settings window": the navigation rules then apply to the Settings window only.
Information architecture: navigation pages, at most two levels.
Command map: every action, where it lives (command bar, context menu, tray menu), and its shortcut.
Empty states: what each page shows before there is data.
Feeling: the one feeling the app aims for (calm, quick, confident). Motion, words, and color all reinforce it.

Entry points first

A Windows utility is used through its small entry points: a global shortcut, the tray icon, Start, a quick panel, drag and drop, the taskbar jump list. Design those first. A to-do app lives in its quick-add panel.

Quick capture: one small window with one input, opened by a global shortcut, closed by Esc or Enter.
Optimistic UI: on Enter, close and show the result at once. Save in the background. Offer Undo, not a progress ring.

Layout

The window: Mica backdrop, ExtendsContentIntoTitleBar with the TitleBar control (app icon, app name, the pane toggle, a search box in the middle when the app has search), and a left NavigationView with Settings at the bottom. No NavigationView when the app has one page.
NavigationView pane modes: Left on a wide window. It collapses to a compact rail and then to a hamburger on its own as the window narrows. Keep that behavior.
Each page: a title in TitleTextBlockStyle, page commands in a CommandBar on the right of the title row, then content.
Content sits on cards (CardBackgroundFillColorDefaultBrush, a 1 epx CardStrokeColorDefaultBrush border, OverlayCornerRadius) over the Mica layer. The window background itself stays Mica.
List and detail: a list column (320 epx) and a detail card, or a ListView that opens a detail page on narrow windows.
Progressive disclosure: an empty page shows one empty state with the first action. Filters appear once there is content.
Spacing from UI/Space.xaml, on a 4 epx grid: 36 epx page margins on the sides, 8 between related controls, 16 between cards in a row, 24 between a title and its content, 32 above a group header. Controls take their height from WinUI (32 epx buttons and boxes), never a fixed height.
The minimum window size keeps every page usable. Test it in snapshots by setting the window to the minimum.

Materials

Mica on the main window. It tints with the desktop wallpaper and goes flat when the window is inactive. That is expected.
Mica Alt (MicaBackdrop Kind="BaseAlt") for windows with tabs in the title bar.
Acrylic belongs to transient surfaces: flyouts, menus, tooltips, and a quick panel. WinUI gives flyouts and menus acrylic on their own. Never put acrylic on a page background.
Content gets solid card fills, never a backdrop of its own.
Never a material on a material: a card on Mica, yes; an acrylic card on Mica, no.

Commands and menus

Page commands live in a CommandBar with DefaultLabelPosition="Right" for up to four primary commands, the rest in the overflow. Icons from Segoe Fluent Icons, every one with a label.
One accent button (AccentButtonStyle) per page or dialog, only for the main completing action: Save, Send, Create. Most pages have none.
Every command also has a KeyboardAccelerator. WinUI shows it in the tooltip and the overflow menu. The self-test checks every command.
Standard shortcuts keep their meaning: Ctrl+N, Ctrl+O, Ctrl+S, Ctrl+Z, Ctrl+Y, Ctrl+C, Ctrl+V, Ctrl+X, Ctrl+A, Ctrl+F, F2 (rename), Delete, Ctrl+W (close), F5 (refresh). Windows has no standard Settings shortcut: Settings opens from the navigation pane.
Disable commands that do not apply; keep them visible.
A MenuBar (File, Edit, View) only for document apps with many commands, like Notepad or Paint. Most apps need none.
Window titles name the content: "Today - Tasks".
Full rules: commands.md.

Windows open calm

Nothing grabs focus when a window or dialog opens: no blinking caret in a text box, no selected text. The one exception is a window whose only job is typing, like a search or quick-add panel: it focuses its field.
WinUI shows focus rectangles only after keyboard use. Never turn them off: keyboard users need them.
A ContentDialog sizes to its content. It scrolls only when the content is taller than the window allows.
System scroll bars. They shrink to a thin line until the pointer comes near. Do not restyle them.

Keyboard first

Every frequent action has a shortcut, and every shortcut shows feedback in the same frame.
Enter is the default action, Esc cancels, Delete removes, F2 renames, arrows move, Tab moves between groups.
Scope keys that would break typing (Delete, arrows, Enter) to the list with KeyboardAccelerator.ScopeOwner, so they never fire in a text box.
New items get focus (Focus(FocusState.Programmatic)) so the user can type at once.
Access keys (AccessKey="N") let people press Alt and a letter. Add them to the command bar in apps used all day.
Teach shortcuts in tooltips, menus, and a list in Settings.

Color, type, dark mode

Text uses TextFillColorPrimaryBrush, TextFillColorSecondaryBrush, TextFillColorTertiaryBrush. Surfaces use the theme's fill brushes. No fixed white, black, or gray in chrome or text.
One accent. Accent text uses AccentTextFillColorPrimaryBrush; text on an accent fill uses TextOnAccentFillColorPrimaryBrush, never a fixed white.
Status color always comes with an icon or a word. InfoBar carries severity for you.
The type ramp, in Segoe UI Variable: Caption 12, Body 14, Body Strong 14 semibold, Subtitle 20, Title 28, Title Large 40, Display 68. Use the built-in styles (BodyTextBlockStyle, TitleTextBlockStyle). Nothing under 12.
Sentence case for every label, button, title, and menu item: "New item", "Open data folder".
Dark mode is its own palette, not inverted light mode. Check both in every snapshot. UI built in C# must get its colors through a XAML style with {ThemeResource} setters: a brush read from Application.Current.Resources in code ignores the panel's theme (csharp-winui, "UI built in code"). High contrast themes replace every brush: theme brushes handle it; hard-coded colors break it.
Icons from Segoe Fluent Icons through FontIcon or SymbolIcon, the same set Windows uses.

Interaction

ContextFlyout with a MenuFlyout on everything a user can act on, with icons on the common commands (Copy, Delete) and the shortcut text.
Hover states come with the controls. Custom controls need PointerOver, Pressed, and Disabled states (design-kit).
Drag and drop in and out where it fits (CanDragItems, AllowDrop), with a real file when dragged to Explorer.
Relaunch restores the window size, the page, and the selection. Closing never asks unless typed work would be lost.
Undo for every destructive or bulk action. A one-click delete shows an InfoBar or a toast with Undo. Confirm with a ContentDialog only when undo is impossible.
Settings is a page in the navigation pane (the gear at the bottom), laid out like Windows Settings: settings.md.
Respect "Animation effects" off (UISettings.AnimationsEnabled) and "Transparency effects" off (Mica falls back on its own). Name icon-only buttons with ToolTipService.ToolTip and AutomationProperties.Name.

When to customize

Custom chrome is fine when it makes the app clearly better at its one job, like a quick-add field or a calendar grid. Write why in a code comment, and match the system: ControlCornerRadius (4) for controls, OverlayCornerRadius (8) for cards and flyouts, Segoe UI Variable, the 4 epx grid.

Review set

Snapshot every surface in light and dark: each page, the tray menu, each flyout and dialog, editors, and Settings (snapshot mode does pages and Settings). Look for a caret nobody asked for, scroll bars with nothing to scroll, empty bands under short content, title case labels, and empty states over data that exists. Capture each window active: snapshot mode brings it to the front first. Mica looks flat on an inactive window; that capture does not count as reviewed.

Done when every rule in design-rules.md holds on the light and dark snapshots of every surface, or has a comment saying why not.

windows-design

Windows design