expo

name	expo
description	Guidelines for building mobile applications using Expo, covering UI design, animations, React context patterns, and native device feature permission (Camera, Location, FileUploads)

Always follow these guidelines when building a mobile application using Expo:

Architecture

• iOS 26 exists. Use NativeTabs from expo-router for native tab bars with liquid glass support. Use isLiquidGlassAvailable() from expo-glass-effect to check availability and fall back to classic Tabs with BlurView for older iOS/Android.
• Follow modern React Native patterns and best-practices.
• Put as much of the app in the frontend as possible. The backend should only be responsible for data persistence and making API calls.
• Use React context for state that is shared across multiple components.
• Use React Query (@tanstack/react-query) for server state fetching.
• Use useState for very local state.
• Minimize the number of files. Collapse similar components into a single file.
• If the app is complex and requires functionality that can't be done in a single request, it is okay to stub out the backend and implement the frontend first.
• ALWAYS use native device capabilities (camera, location, contacts, etc.) when the app requires them. NEVER use fake/mock data when real device features are available and appropriate.
• Client-Server Communication: The client (Expo app) interacts with the server (Express app) through a RESTful API. The server is responsible for data storage.
• Server-side Logic: The server handles API requests, database interactions, authentication, and any other server-specific logic. It's built with Express and uses TypeScript.

Routing

This stack uses Expo Router for file-based routing (similar to Next.js Pages Router) for the frontend. The backend uses Express with TypeScript.

• Every file in the app/ directory becomes a route
• Use _layout.tsx files to define shared layouts
• Use (group) directories for layout groups without affecting URLs
• Use [param].tsx for dynamic routes

Example structure:

app/
  _layout.tsx          # Root layout with providers
  index.tsx            # Home route (/)
  (tabs)/
    _layout.tsx        # Tab layout
    index.tsx          # First tab (/)
    settings.tsx       # Settings tab (/settings)
  profile/
    _layout.tsx        # Profile stack layout
    index.tsx          # /profile
    [id].tsx           # /profile/:id

For dynamic parameters: const { id } = useLocalSearchParams() from "expo-router"

State Management

• Use React Query for server state (always use object API)
• Use useState for very local state
• Avoid props drilling - use React context for shared state
• Don't wrap in a context hook - wrap at the root layout level
• React Query provider should be the top level provider
• Use AsyncStorage inside context providers for persistent state

TypeScript Guidance

• TypeScript first: Proper typing with interfaces and type safety
• Explicit Type Annotations for useState: Always use useState<Type[]>([]) not useState([])
• Type Verification: Before using any property or method, verify it exists in the type definition
• Null/Undefined Handling: Use optional chaining (?.) and nullish coalescing (??)
• Complete Object Creation: Include ALL required properties when creating objects
• Style Properties: Use literal values for variables used in styles (e.g., const fontWeight = "bold" as const)

Imports

You can import using @/ to avoid relative paths (e.g., import { Button } from '@/components/Button')

Styling

Use react-native's StyleSheet for styling.

Networking

• Use the generated React Query hooks from @workspace/api-client-react for server data.
  • For any API change, update lib/api-spec/openapi.yaml first, then run pnpm --filter @workspace/api-spec run codegen, then use the generated hooks.
  • Do not write app-local React Query hooks, manual useQuery wrappers, or custom fetch layers for endpoints that already exist in @workspace/api-client-react.
  • Use the shared QueryClient/fetch setup that ships with the monorepo rather than creating app-local networking infrastructure.
  • The generated hooks return data typed as T directly.
  • For mutations or requests that do not yet exist in the generated client, update the OpenAPI spec and regenerate rather than bypassing the shared API client package.
  • Do not hardcode domain URLs or hostnames in frontend code. The deployment domain is injected at build time and varies between development, preview, and production environments. Use process.env.EXPO_PUBLIC_DOMAIN for domain configuration. Routes like /api or /{service} are routed to the appropriate artifact/services as needed by the environment.

App Icon

Generate a custom app icon for the app. Read the mobile-ui skill's app-icon.md reference for guidelines.

Workflow

• The Expo dev server is accessed via $REPLIT_EXPO_DEV_DOMAIN, not through the shared proxy at localhost:80.
• Ports are assigned dynamically. Do not hardcode port numbers.
• Check the configured workflows list for exact workflow names.
• Use the restart_workflow tool to restart workflows — not shell commands.
• The Expo dev server has Hot Module Reloading. Only restart the mobile workflow when you change dependencies or hit a Metro error, not for normal code changes. Restarting unnecessarily wastes time.
• Backend workflows only requires restart when backend changes were made since it can be a time consuming operation.
• After presenting the artifact, call suggestDeploy() so the user knows their app is ready to publish.

React Native Pitfalls

Avoid these common mistakes:

• UUID Generation:

  • Do not use the 'uuid' package — it requires crypto.getRandomValues() which crashes on iOS/Android
  • Use instead: Date.now().toString() + Math.random().toString(36).substr(2, 9)
  • Or use expo-crypto: import * as Crypto from 'expo-crypto'; Crypto.randomUUID() — pin to version 15.0.x (expo-crypto v55+ crashes in Expo Go)

• Import Verification:

  • Check existing template files for correct import paths before using them
  • useBottomTabBarHeight is from '@react-navigation/bottom-tabs', not 'expo-router'
  • When unsure, read the actual files to verify exports exist

• Scrolling Containers — assess whether scrolling is needed:

  • Not everything needs to be scrollable. Fixed layouts (timers, dashboards, single-screen UIs) should use View, not ScrollView
  • FlatList: Add scrollEnabled={data.length > 0} to prevent empty bounce
  • Avoid contentInsetAdjustmentBehavior="automatic" with transparent/large-title headers — it causes over-scrolling
  • ScrollView is for content that exceeds screen height. If content fits, use View

• useEffect Anti-Patterns:

  • Do not sync props to state with useEffect (causes infinite loops)
  • BAD: useEffect(() => setState(prop), [prop]) then useEffect(() => onChange(state), [state])
  • GOOD: Derive values directly from props: const hours = Math.floor(value / 3600)
  • Only call onChange on explicit user actions (onPress, onChangeText), not in useEffect

• Mobile Typography:

  • Maximum font size for display/hero text (clocks, timers, large numbers): 48-64pt (not 96pt)
  • Body text: 14-16pt, Headers: 20-28pt
  • Always test that text fits on a 375pt wide screen (iPhone SE)

• Empty State Design:

  • Use simple text-based empty states, not placeholder images
  • An icon + descriptive text is sufficient

• Expo Router Header Configuration:

  • Do not set headerShown dynamically inside screen components — it causes remounts on every re-render
  • BAD: Setting options dynamically inside screen components
  • GOOD: Configure all header options in _layout.tsx where screens are registered
  • Individual screens should only set dynamic content (headerLeft/headerRight buttons), not headerShown

• Safe Area / Top Padding:

  • Do not use hardcoded top padding (24px, 40px, etc.) — it will be wrong on different devices
  • Use useSafeAreaInsets() from react-native-safe-area-context
  • Example: Use useSafeAreaInsets() hook and apply insets.top to paddingTop
  • This handles iPhone notch, Dynamic Island, and Android status bars automatically
  • Absolutely positioned headers: When a header is positioned absolutely over scrollable content:
    • Header uses: top: insets.top (positions header below notch/Dynamic Island)
    • Content below needs: paddingTop: insets.top + headerContentHeight
    • BAD: paddingTop: 100 (fixed value fails on devices with different insets)
    • GOOD: paddingTop: insets.top + 70 (where 70 = header's internal height)
    • The header height varies by device because insets vary (47px on older iPhones, 59px+ on Dynamic Island)

• Streaming API Responses (OpenAI, etc.):

  • For streaming, read the mobile-ui skill's expo-fetch.md reference
  • Use import { fetch } from 'expo/fetch' which supports getReader() on all platforms

• Keyboard Handling:

  • For detailed keyboard handling patterns (forms, chat, FlatList with inputs), read the mobile-ui skill's keyboard.md reference
  • Use react-native-keyboard-controller for all keyboard handling — it provides better control than React Native's built-in KeyboardAvoidingView and works consistently across iOS and Android
  • Do not use InputAccessoryView — it doesn't render properly in Expo Go
  • Forms with multiple inputs: Use KeyboardAwareScrollViewCompat with bottomOffset and keyboardShouldPersistTaps="handled"
  • Chat/messaging: Use KeyboardAvoidingView from react-native-keyboard-controller with behavior="padding" on both platforms, keyboardDismissMode="interactive" and keyboardShouldPersistTaps="handled" on FlatList
  • keyboardVerticalOffset: 0 for transparent/no header, headerHeight for opaque header
  • Use useSafeAreaInsets() for bottom padding on the input container to avoid home indicator overlap
  • Do not nest KeyboardAvoidingViews — only one should wrap your content
  • For chat UIs: Use inverted FlatList (see the chat apps guidance below) — no additional scroll logic needed
    • Do not try to implement auto-scroll with scrollToEnd() — it has timing bugs
    • Inverted FlatList handles this automatically — newest message always visible

• Chat Apps — state and FlatList patterns:

  • For chat app implementation patterns (stale closures, inverted FlatList, streaming), read the mobile-ui skill's expo-fetch.md reference
  • Key rules: Use inverted FlatList (not scrollToEnd), capture state before async operations, use ListHeaderComponent for typing indicator
  • FlatList Boolean Props — type coercion:
    • FlatList props like scrollEnabled, showsVerticalScrollIndicator expect boolean values
    • Using expressions like someString || anotherValue can pass a string instead of boolean
    • This causes: TypeError: expected dynamic type 'boolean', but had type 'string'
    • Coerce to boolean with !! when using string variables in boolean contexts
    • BAD: const showFooter = isSending || streamingContent; (streamingContent is string)
    • GOOD: const showFooter = isSending || !!streamingContent; (!! converts to boolean)
    • BAD: scrollEnabled={data || someString}
    • GOOD: scrollEnabled={!!data || !!someString}

• RevenueCat compatibility:

  • RevenueCat works in Expo Go and does not require a native build. In Expo Go, the SDK automatically runs in Preview API Mode, replacing native calls with JavaScript mocks so your app loads without errors.
  • RevenueCat works on the web out of the box without any additional configuration.

• Custom ErrorBoundary component:

  • Use reloadAppAsync function from expo in tandem with the ErrorBoundary component to restart the app when the app crashes. import { reloadAppAsync } from 'expo'. Do not use reloadAsync from "expo-updates" for this purpose.
  • The ErrorBoundary is a minimal class component (required by React's error boundary API) with a functional ErrorFallback component for the UI. The consuming component should remain functional.
  • Do not add local state to the ErrorFallback component because it only renders if the app crashes. It should be used as is, unless the user requests it. (Note: dev-mode-only state guarded by __DEV__ is acceptable for debugging features.)
  • The ErrorFallback component uses useColorScheme and useSafeAreaInsets for proper theming and positioning. The ErrorBoundary wrapper uses React's class component error boundary API (getDerivedStateFromError, componentDidCatch).

• react-native-maps:

  • Pin version to exactly 1.18.0 in package.json (e.g., "react-native-maps": "1.18.0") — this is the only version compatible with Expo Go currently. Other versions will cause crashes or compatibility issues.
  • Do not add react-native-maps to the plugins array in app.json — it will crash the app.

• Over use of text:

  • Mobile apps should be designed for touch input, not text input.
  • Most buttons should not have text. They should have icons.
  • If you have to use text, use it sparingly and only for very important information.
  • Applications do not need a title like "Chatbot" or "AI Assistant". They should look like market leading apps.

Gestures

Use PanResponder from 'react-native' for gesture handling.

Safe Area View

When to use SafeAreaView:

1. Built-in tabs or header: Don't add SafeAreaView - they handle insets automatically
2. Custom header: Add SafeAreaView to the header component
3. Removed header: Add SafeAreaView inside a View with background color (not just white space)
4. Pages inside stacks: Don't add SafeAreaView if parent _layout.tsx has header enabled

Games and absolute positioning:

• Account for safe area insets in positioning calculations
• Use useSafeAreaInsets() hook to get inset values
• Apply insets to positioning calculations in game physics
• Avoid using SafeAreaView in game screens - factor insets into game loop instead

Font Loading and SplashScreen

The scaffold ships with @expo-google-fonts/inter pre-installed and already loaded in _layout.tsx with the correct useFonts + SplashScreen font-gating pattern. Available weights: Inter_400Regular, Inter_500Medium, Inter_600SemiBold, Inter_700Bold.

If the user wants a different font, swap the import and useFonts call — keep the same font-gating pattern.

Rules:

• Do NOT remove the font-gating pattern from _layout.tsx
• Handle fontError alongside fontsLoaded — if font loading fails, the app should still render rather than showing a white screen forever
• Tie SplashScreen.hideAsync() to [fontsLoaded, fontError] — do not call it unconditionally in useEffect([], [])
• The if (!fontsLoaded && !fontError) return null must come after the useEffect hook, not before it (hooks cannot be called conditionally)

Library Compatibility

• ONLY use libraries from the Expo Go compatible list
• Do not install additional native libraries unless they're JS only
• Pre-installed: expo-router, @expo/vector-icons, @tanstack/react-query, react-native-reanimated
• Error boundary: Custom ErrorBoundary component (components/ErrorBoundary.tsx)

Insets

Web-only insets for web edge cases (always implement for every app):

• Apply on web only (Platform.OS === "web")
• Always add at least a 67px top inset to top-level header content for the status bar. Additional vertical padding will likely still be required.
• Always add a 34px bottom insets
  • If using a bottom tab bar, instead modify the tab bar height to be 84px (50px + 34px). Do not add paddingBottom to the tab bar.
• Do not add these insets on iOS/Android; native safe areas already handle them
• Before finishing any screen, verify that web platform insets handling is implemented and are correct.

Web Compatibility

Write code that does not crash in React Native Web. Account for platform-specific compatibility.

Polyfilled APIs (work on web without Platform checks):

• expo-secure-store, expo-haptics, react-native-maps, Alert, RefreshControl

Partial Web Support:

• expo-camera (no switch camera, no recordAsync)
• expo-clipboard, expo-file-system, expo-image, expo-notifications, expo-video
• react-native-reanimated: Layout animations don't work on web, native driver not supported

No Web Support (use Platform.OS checks):

• expo-battery, expo-brightness, expo-contacts, expo-device, expo-local-authentication
• expo-location (use web geolocation API), expo-media-library, expo-sensors, expo-sharing

Example workaround:

if (Platform.OS !== 'web') {
  // Native-only code
} else {
  // Web alternative
}

Payments

If prompted to add payments (i.e. subscriptions, in-app purchases, etc.), always use RevenueCat. Do not use Stripe unless the user explicitly requests it.

Testing

• Add testID to interactive elements for testing
• Use mobile screen sizes for automated testing
• Recommended dimensions: height: 720, width: 400

Replit Environment

• User can scan QR code from Replit's URL bar menu to test on their physical device via Expo Go
• Hot module reloading (HMR) is enabled - no need to restart the dev server for code changes

Forbidden Changes

• NEVER change bundle identifiers after initial setup unless user explicitly requests it.
• NEVER downgrade the version of React Native or Expo that is declared in package.json.
• NEVER run npx expo start or npx expo directly in a shell. Use the restart_workflow tool instead — running expo directly will miss environment variables (like PORT) injected into the workflow.

References

Before writing code, identify whether any reference below applies to the task. If it does, read it first.

• references/react_context.md - Use this reference when creating or modifying shared state with React context, provider composition, or context-based hooks.
• references/design_and_aesthetics.md - Use this reference when designing or restyling UI, selecting iconography, or implementing animations and visual polish.
• references/device_features_and_permissions.md - Use this reference when implementing camera, location, notifications, contacts, file uploads, or any permission request/denial flow.
• mobile-ui skill's references/keyboard.md - Use this reference when implementing any keyboard handling — forms with multiple inputs, chat/messaging UIs, FlatList with inputs, or keyboard utilities (dismiss, detect visibility).
• mobile-ui skill's references/sheets.md - Use this reference when implementing modals, sheets, formSheet presentations, auth flows (login/register), wizards, or overlay UI.
• mobile-ui skill's references/tabs.md - Use this reference when implementing tab bars — covers NativeTabs with liquid glass support (SDK 54+) and classic Tabs fallback with detailed code examples.