mobile - SKILL.md Agent Skill

name: mobile description: Use for any mobile app creation task — planning, designing, implementing screens, components, API routes, and delivering. Includes authentication, email, and AI agent capabilities as optional modules.

Mobile App

Stack

App: Expo (React Native) — iOS, Android, Web
API: Cloudflare Workers with Hono + Vite (deployed separately)
Database: Cloudflare D1 (SQLite) via Drizzle ORM
Data Fetching: Hono RPC Client + React Query
Styling: React Native StyleSheet or NativeWind (Tailwind for RN)

Preflight

Ask questions: purpose, target platforms (iOS/Android/both), need templates (MUST confirm — assume YES if unclear), style, screens, features.
If templates wanted, call show_templates with relevant query and type: mobile. This must be its own standalone call.
Form a plan. State assumptions as decisions — the user corrects what's wrong. Include: what's being built, screens needed, navigation structure, API endpoints, visual direction. Prepare a detailed outline covering the app and design guidelines for consistent theming.

Do not start implementation until the user approves or adjusts the plan.

Design Guidelines

Document design direction in design.md inside the project root before writing UI code. Reference it throughout for consistency.

Typography: system fonts for performance, or load custom fonts via expo-font. Clear hierarchy through size/weight: title (28/700), heading (20/600), body (16/400), caption (14/400). Minimum 16px body, 14px captions. Consider Dynamic Type / font scaling accessibility.
Color: semantic colors — primary, secondary, background, surface, text, textSecondary, error. Support light and dark modes from the start using useColorScheme(). Ensure WCAG AA contrast. Define both palettes upfront.
Spacing: consistent scale (4, 8, 12, 16, 24, 32, 48). Generous padding in containers (16–24px horizontal). Adequate spacing between list items (12–16px). Safe area awareness for notches and home indicators.
Touch Targets: minimum 44x44pt (Apple HIG). Adequate spacing between interactive elements. Visual feedback on press (opacity, scale, or color change).
Layout: SafeAreaView for screen containers. Flexbox for all layouts. ScrollView or FlatList for scrollable content. KeyboardAvoidingView for forms. Respect platform conventions (iOS vs Android).
Animation: use react-native-reanimated for complex animations. Keep animations subtle and purposeful (200–300ms). Respect reduced motion preferences. Layout animations for list changes.
Platform: use Platform.select() for platform-specific styles (iOS shadows vs Android elevation). Test on both platforms regularly.
Anti-patterns (will feel broken): tiny touch targets (< 44pt), text smaller than 14px, no dark mode support, ignoring safe areas, web-like designs that don't feel native, overloaded screens with too much content, missing loading and error states.

// Example color palette
const colors = {
  light: {
    primary: "#007AFF",
    background: "#FFFFFF",
    surface: "#F2F2F7",
    text: "#000000",
    textSecondary: "#8E8E93",
  },
  dark: {
    primary: "#0A84FF",
    background: "#000000",
    surface: "#1C1C1E",
    text: "#FFFFFF",
    textSecondary: "#8E8E93",
  },
};

// Example spacing scale
const spacing = { xs: 4, sm: 8, md: 12, lg: 16, xl: 24, xxl: 32 };

// Example typography
const typography = {
  title: { fontSize: 28, fontWeight: "700" },
  heading: { fontSize: 20, fontWeight: "600" },
  body: { fontSize: 16, fontWeight: "400" },
  caption: { fontSize: 14, fontWeight: "400", color: "#666" },
};

Workflow

Run preflight.
Read this project's README.md for structure reference.
Write design.md in the project root with the design direction from preflight (fonts, colors, spacing, style). This file guides all UI code for consistency.
Load any relevant feature reference from ./references/ before implementing specialized capabilities.
Build screens, components, API routes, database schema.
Verify with bun build to ensure no errors.
Test each implemented feature using mb (browser tool) against the Expo web preview. See Testing.
Call deliver with type: mobile-app, project folder path at index 0.

Optional Capabilities

For optional capabilities, consult the matching reference before implementation:

Authentication: references/authentication.md
Email: references/email.md
AI agent flows: references/ai-agent.md

Project Structure

packages/
├── app/                    # Expo mobile app
│   ├── App.tsx             # Root component
│   ├── app.json            # Expo config (scheme, icons, splash)
│   ├── index.ts            # Entry point
│   ├── assets/             # Icons, splash screens
│   └── src/
│       ├── screens/        # Screen components
│       ├── components/     # Reusable components (providers.tsx)
│       ├── navigation/     # Navigation config
│       ├── hooks/          # Custom hooks
│       ├── lib/            # API client, auth client, utilities
│       └── constants/      # Colors, fonts, config
│
└── api/                    # Cloudflare Workers API (deployed separately)
    ├── src/
    │   ├── index.ts        # Hono routes (exports AppType)
    │   ├── auth.ts         # Better Auth config
    │   ├── agent/          # AI agent config + tools
    │   ├── routes/         # Route modules
    │   ├── middleware/      # Auth middleware, etc.
    │   └── database/
    │       ├── schema.ts       # Drizzle schema (re-exports)
    │       ├── auth-schema.ts  # Generated Better Auth tables
    │       └── migrations/     # D1 migrations
    ├── wrangler.json       # Cloudflare config
    ├── vite.config.ts      # Vite + Cloudflare plugin
    └── drizzle.config.ts   # Drizzle Kit config

Development

bun dev              # Start both app and API
bun dev --app        # App only (Expo on :8081)
bun dev --api        # API only (Vite on :8787)
bun build            # Build both (verify no errors)

Preview

The sandbox provides two ways to preview the app:

Web Preview: Automatic browser preview at the sandbox URL (Expo web export)
Expo Go: Users scan the QR code with the Expo Go app on their phone

API Integration

The app uses Hono's type-safe RPC client with React Query for data fetching. The API package is installed as a workspace dependency (@sandbox/api) for type sharing.

Important: The API is deployed separately from the app. The Expo client connects to the API via EXPO_PUBLIC_API_URL (defaults to http://localhost:8787 in dev).

API Routes (packages/api/src/index.ts)

Define routes using method chaining to preserve types:

import { Hono } from "hono";
import { cors } from "hono/cors";

type Bindings = {
  DB: D1Database;
};

const app = new Hono<{ Bindings: Bindings }>()
  .use("*", cors())
  .get("/api/health", (c) => {
    return c.json({ status: "ok", timestamp: Date.now() });
  })
  .get("/api/items", (c) => {
    return c.json({ items: [{ id: "1", name: "Item 1" }] });
  })
  .post("/api/items", async (c) => {
    const body = await c.req.json<{ name: string }>();
    return c.json({ id: crypto.randomUUID(), name: body.name });
  });

export type AppType = typeof app;
export default app;

API Client (packages/app/src/lib/api.ts)

import { hc } from "hono/client";
import type { AppType } from "@sandbox/api";

const API_URL = process.env.EXPO_PUBLIC_API_URL ?? "http://localhost:8787";

export const api = hc<AppType>(API_URL);

Providers (packages/app/src/components/providers.tsx)

import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
import type { ReactNode } from "react";

const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 1000 * 60,
      retry: 2,
    },
  },
});

export function Providers({ children }: { children: ReactNode }) {
  return (
    <QueryClientProvider client={queryClient}>
      {children}
    </QueryClientProvider>
  );
}

Using in Components

import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
import { api } from "../lib/api";

function ItemList() {
  const queryClient = useQueryClient();

  const { data, isLoading } = useQuery({
    queryKey: ["items"],
    queryFn: async () => {
      const res = await api.api.items.$get();
      return res.json();
    },
  });

  const createItem = useMutation({
    mutationFn: async (name: string) => {
      const res = await api.api.items.$post({ json: { name } });
      return res.json();
    },
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: ["items"] });
    },
  });

  if (isLoading) return <ActivityIndicator />;

  return (
    <View>
      {data?.items.map((item) => (
        <Text key={item.id}>{item.name}</Text>
      ))}
      <Button title="Add Item" onPress={() => createItem.mutate("New Item")} />
    </View>
  );
}

Navigation

Install React Navigation:

cd packages/app
bun add @react-navigation/native @react-navigation/native-stack
bun add react-native-screens react-native-safe-area-context

Setup in App.tsx:

import { NavigationContainer } from "@react-navigation/native";
import { createNativeStackNavigator } from "@react-navigation/native-stack";
import { Providers } from "./src/components/providers";

const Stack = createNativeStackNavigator();

export default function App() {
  return (
    <Providers>
      <NavigationContainer>
        <Stack.Navigator>
          <Stack.Screen name="Home" component={HomeScreen} />
          <Stack.Screen name="Details" component={DetailsScreen} />
        </Stack.Navigator>
      </NavigationContainer>
    </Providers>
  );
}

Environment Variables

Create .env in packages/app:

EXPO_PUBLIC_API_URL=http://localhost:8787

For production, set this to your deployed API URL.

The API uses Cloudflare Workers environment bindings configured in wrangler.json (D1, secrets, etc.).

Deployment

API (Cloudflare Workers)

cd packages/api
bun run deploy

App (EAS Build)

For production app store builds:

cd packages/app
eas build --platform ios
eas build --platform android

Testing

Before delivering, verify every implemented feature actually works. Do not deliver untested code.

1. Build Validation

Run bun build from the project root. Fix all errors before proceeding.

2. API Smoke Test

Start the dev servers with bun dev. Use curl or the browser tool to hit API endpoints directly:

curl http://localhost:8787/api/health

3. Browser-Based Flow Testing

Use the mb (mini-browser) tool to exercise the Expo web preview for every user-facing feature:

Authentication: Open the web preview, navigate to sign-up, create an account, sign out, sign back in. Verify protected screens redirect unauthenticated users. Verify session persists across navigation.
Email: Trigger the flow that sends email (e.g. contact form, welcome on sign-up). Verify the API responds successfully and the UI shows the correct success/error state.
AI Agent: Open the chat screen, send a prompt, verify streaming text appears. If tools are configured, trigger a tool call and verify the result renders.
General UI: Navigate between screens. Verify loading states, error states, empty states. Check no console errors.

4. Native-Only Behaviors

Some features (push notifications, secure storage, biometrics) cannot be tested via web preview. For these:

Verify the code compiles (bun build)
Call out explicitly in your delivery notes which features need on-device testing
Recommend the user test via Expo Go on a physical device

Do not mark a feature as done until you have tested it.