Makes iPhone Duo apps work with VoiceOver, Dynamic Type, Reduce Motion, Increase Contrast, hardware keyboards, Switch Control and Voice Control on both displays. Use it when VoiceOver reads two panes row by row or loses its place after the phone folds or unfolds, when text truncates or clips on the 5.4-inch outer display at large text sizes, when a hinge effect or resize animation ignores Reduce Motion, when colours fail contrast, when keyboard shortcuts or focus don't work, or when you add accessibility labels, identifiers or audits. Covers accessibilityElement(children: .contain), accessibilitySortPriority, AccessibilityFocusState with a delayed move after a size-class change, AnyLayout and ViewThatFits reflow with isAccessibilitySize, ScaledMetric, accessibilityReduceMotion with onHingeChange, colorSchemeContrast, keyboardShortcut, accessibilityInputLabels, modifiers before .position, Accessibility Inspector, performAccessibilityAudit and XCUIDevice voiceOverService UI tests on the iPhone Duo simulator.

Reviews screenshots, simulator captures and Figma frames for iPhone Duo, Apple's foldable, against a 15-point checklist (letterboxing, state lost on fold and unfold, sizing from UIScreen.main, text or buttons in the fold via reservedRegions .division and .occlusion, odd grid columns, vertical toolbars with symbols and titles, tabletop and book layouts with ArrangementView overlay, camera preview rotation, outer display feature parity, multiple windows with UIWindowScene.ActivationAction, transitions, 44 pt touch targets, hardware keyboard, drag and drop) and writes findings with severity, rule, where and fix. Use it when someone asks to critique, audit or review a design, mockup, concept or Dribbble shot for the foldable, the outer display or the inner display, or asks what breaks when the phone is closed, open, partly folded like a book, propped on a table, in Split View or under Picture in Picture.

Puts controls where hands can reach them on iPhone Duo, Apple's foldable iPhone: one-handed on the closed phone, two hands on the open display, the book grip, and hands-free on a table. Use it when you decide where a primary action, a floating button, player controls or a custom control strip goes; when buttons sit at the top centre of the open display, on the fold or out of thumb reach; when you check thumb zones or reachability; when you design for left-handed people or right-to-left languages; or when touch targets are small. Covers why bars move to the side edge, toolbarVerticalEdge and verticalBarEdge (read them; right to left changes the name, not the side), visibilityPriority for bottom-bar actions, a ThumbCorner modifier for floating controls, an in-app hand preference, ArrangementView with the overlay style to put controls in the bottom half, reservedRegions(kind: .division) to keep targets off the fold, and 44 pt hit regions with frame(minWidth:minHeight:) and contentShape on the label.

Adds Foundation Models to an iPhone Duo app: the on-device model (SystemLanguageModel) and the Private Cloud Compute server model (PrivateCloudComputeLanguageModel) behind one LanguageModelSession API, an AI side pane beside the content when the phone is open or in book pose (split ArrangementView) and a one-line summary when it is closed or in tabletop, streaming (streamResponse with @Generable partial snapshots) into a pane that keeps going through fold and unfold, availability checks and non-AI fallbacks, quota and error handling (LanguageModelError), tool calling, image prompts (Attachment), reasoning levels (ContextOptions) and privacy. Use when someone adds summaries, an assistant, chat, tagging or image understanding to an app that runs on iPhone Duo, or when an AI feature restarts on a fold, sits in the fold or doesn't fit the outer display.

Core layout rules for iPhone Duo, Apple's foldable. Use when content lands in the fold, a screen breaks on the inner or outer display, or you write SwiftUI or UIKit layout for a foldable. Covers size classes instead of UIScreen.main, idiom or orientation, asymmetric safe areas and margins, reserved regions (ReservedRegion, reservedRegions(kind:options:), .division, .occlusion, .includeInactive), ArrangementView and UIArrangementViewController (split and overlay styles), an even custom split (EvenSplit) for two halves that must always show, even grid columns, and keeping controls off the fold.

Moves an existing iOS app that already ships onto iPhone Duo, Apple's foldable, step by step: from an app built with Xcode 26 (iOS 26 SDK) or Xcode 27.0 to a build with Xcode 27.1 and the iOS 27.1 SDK. Use when an app shows up centred with empty space around it on the open iPhone Duo, when a UIKit app built with the new SDK crashes at launch because it has no scene life cycle (no UIApplicationSceneManifest, no configurationForConnecting, UIWindow(frame:) in didFinishLaunching), when replacing UIScreen.main, device orientation, userInterfaceIdiom, regular width treated as iPad, hard-coded device sizes or UIRequiresFullScreen, when toolbars and tab bars turn vertical after the rebuild, when moving CI off an older Xcode, or when planning for the April 2027 App Store rules (iOS 27 SDK or later, iOS 15 or later).

Picks and builds layout patterns for iPhone Duo, Apple's foldable iPhone, in SwiftUI - list-detail, supporting pane, two-page, dual view, extended canvas, feed reflow with even columns, tabs to sidebar, and displacement off the fold. Use it when you design or fix a screen for the outer display and the inner display, when content or buttons land in the fold, or when choosing between NavigationSplitView, inspector, ArrangementView (split and overlay styles), TabView with sidebarAdaptable and defaultTabBarPlacement, and reserved regions (division, occlusion, includeInactive).

Keeps an iPhone Duo app smooth when the phone folds and unfolds. One fold changes the app's layout size in several steps (a full-width view went 382 → 466 → 951 → 867 pt on open), and every body that reads the size runs at each step; at 120 Hz a frame lasts 8.3 ms. Measures what each fold costs the main thread on the iPhone Duo simulator with duo-fold-cost (Instruments' Hangs and os_signpost instruments while duo-pose folds the phone, with a saved run to compare against), finds the cause, and fixes it: no sorting, formatting or decoding in body, the size read once as a small value, identity kept across poses, lazy rows, follow-up work after the fold settles. Splits hitch and hang time by pose from real users with iOS 27 MetricKit and StateReporting. Says what the simulator can't measure (frame rate, hitches, the SwiftUI instrument) and what to check on a device. Use when a Duo app freezes, stutters or drops frames on fold or unfold, before a release, or to keep a fold from getting slower.

Screenshots an iPhone Duo app in every pose (closed, open, book, tabletop) on the iPhone Duo simulator by pressing Device Hub's pose buttons through the macOS Accessibility API, then writes summary.md and summary.json to review with the duo-design-review checklist. Use when asked to check, test or screenshot an app in each pose, fold or hinge state, to capture outer and inner display frames for a design review or the App Store, to change the simulator's pose from the command line or a script (simctl and devicectl have no pose command), to run /duo-poses, or when a review is missing book or tabletop frames. Ships duo-pose (status, list, set) and duo-pose-matrix, with a manual fallback when the user has not given Accessibility permission, and checks that no UI tests share the simulator before it changes the pose.

Builds screens that two people use at the same time on one iPhone Duo, Apple's foldable iPhone - two-player games, teaching, showing a customer, kid and parent. Use when two people share the inner display or the outer display, sit side by side or face to face across a phone lying flat, or when you need one half per person (SharedSplit, an EvenSplit with seating, not ArrangementView), per-half named coordinate spaces and contentShape, DragGesture minimumDistance with onTapGesture, input gated per seat, one-sided destructive controls, sensoryFeedback per half, persistentSystemOverlays and defersSystemGestures during play, rotationEffect to turn a half for face-to-face use, accessibility per half, and XCUITest identifiers namespaced by seat.

The iOS 27 SwiftUI changes that matter on iPhone Duo, Apple's foldable, where every fold, unfold and rotation resizes the app. Use it when building with Xcode 27.1 breaks a view (@State macro, ContentBuilder, TN3211), when a grid or list that reflows between the outer and inner display needs drag to reorder or swipe actions, or when you adopt the new tab, toolbar, dialog or AsyncImage APIs in a Duo app. Covers the @State macro through a fold and its source breaks, reorderable() and reorderContainer(for:isEnabled:move:) in a 2 or 4 column grid and across the fold, swipeActionsContainer() outside List, Tab(role: .prominent) at the foot of the vertical tab bar, toolbarMinimizationBehavior(_:for:) (the talk's toolbarMinimizeBehavior is not in the SDK), visibilityPriority, ToolbarOverflowMenu, topBarPinnedTrailing, alert and confirmationDialog with item:, AsyncImage HTTP caching and asyncImageURLSession(_:), and resizable iPhone apps. Points to Apple's swiftui-whats-new-27 Xcode skill for the rest.

Writes and runs tests for iPhone Duo apps, Apple's foldable with an outer display and an inner display, on the Mac and on the iPhone Duo simulator in Xcode 27.1. Use when adding logic tests (swiftc script, or Swift Testing with @Test and #expect), UI tests with XCTest and XCUIAutomation, launch arguments such as -uiTesting, -resetState and jump-to-screen switches with a TestSupport type, test markers with accessibilityIdentifier and accessibilityValue, when a tap or drag in a UI test doesn't reach a SwiftUI gesture (coordinate(withNormalizedOffset:), press(forDuration:thenDragTo:)), when XCUIDevice rotation or a fold can't be scripted, or when running xcodebuild test on the existing iPhone Duo simulator with -destination id=<UDID>, -parallel-testing-enabled NO, -only-testing, -resultBundlePath, -collect-test-diagnostics and xcresulttool.

Creates a complete SwiftUI camera studio for iPhone Duo, Apple's foldable: photos and videos with the rear camera and a self-timer, and, while the phone is open, the person being filmed sees themselves on the outer display (CameraCaptureAccessory) with the countdown or the recording time. Closed or open and flat, the controls float along the bottom or trailing edge, off the fold; folded like a book, the preview takes the left page and the controls the right; in tabletop the preview sits above the fold and the controls below it. Use when someone wants to start a new camera, photo booth, self-portrait or filming app from scratch, or runs /duo-new recipe-camera-studio. Covers AVFoundation capture on an actor (photo and movie outputs, rotation coordinators, a second mirrored preview layer), a stand-in camera that makes the simulator usable (it has no camera), an in-app gallery with ShareLink and no file dates, usage strings, a privacy manifest with Mac checks, and UI tests on the iPhone Duo simulator.

Creates a complete SwiftUI personal finance dashboard for iPhone Duo, Apple's foldable: balance and recent spending in one column on the closed outer display; balance, a Swift Charts spending chart and the transaction list side by side on the open inner display; an even 8-column grid with the fold as its middle gutter (DuoColumns and PaneFrames from duo-design-system); and list-detail for a transaction that survives fold and unfold. Use when someone wants to start a new finance, budgeting, banking, expenses or any dashboard app from scratch (a new Xcode project), or runs /duo-new recipe-finance-dashboard. For an existing app, use duo-design-system (grid) and duo-patterns (list-detail) instead. Covers FoldPanes reading the .division reserved region with includeInactive, one NavigationStack whose path comes from the selection, a delayed pop on unfold that keeps the margins right, money as whole cents, a Foundation-only model tested on the Mac, and UI tests that pass in any pose on the iPhone Duo simulator.

Creates a complete SwiftUI shop for iPhone Duo, Apple's foldable, that sells digital packs and a subscription with StoreKit 2: a product grid with even columns that reflows from 2 per row on the closed outer display to 4 on the open inner display with the fold as its middle gutter, product detail beside the grid when open, a basket that checks out one App Store purchase at a time, and a two-pane paywall and checkout (what you get on one side, price and button on the other; one column with the button pinned when closed). Use when someone wants to start a new shop, catalog, in-app purchase or paywall app from scratch, or runs /duo-new recipe-shop. For an existing app, use duo-patterns (feed reflow, list-detail) instead. Covers a local StoreKit configuration file (no network, no real products), PurchaseAction, Transaction.updates and currentEntitlements, restore, Ask to Buy, SKTestSession in UI tests, a Foundation-only model tested on the Mac, and UI tests that pass in any pose on the iPhone Duo simulator.