claude-engineering-plugin/plugins/compound-engineering/skills/agent-native-architecture/references/mobile-patterns.md

Mobile is a first-class platform for agent-native apps. It has unique constraints and opportunities. This guide covers why mobile matters, iOS storage architecture, checkpoint/resume patterns, and cost-aware design.

<why_mobile>

Why Mobile Matters

Mobile devices offer unique advantages for agent-native apps:

A File System

Agents can work with files naturally, using the same primitives that work everywhere else. The filesystem is the universal interface.

Rich Context

A walled garden you get access to. Health data, location, photos, calendars—context that doesn't exist on desktop or web. This enables deeply personalized agent experiences.

Local Apps

Everyone has their own copy of the app. This opens opportunities that aren't fully realized yet: apps that modify themselves, fork themselves, evolve per-user. App Store policies constrain some of this today, but the foundation is there.

Cross-Device Sync

If you use the file system with iCloud, all devices share the same file system. The agent's work on one device appears on all devices—without you having to build a server.

The Challenge

Agents are long-running. Mobile apps are not.

An agent might need 30 seconds, 5 minutes, or an hour to complete a task. But iOS will background your app after seconds of inactivity, and may kill it entirely to reclaim memory. The user might switch apps, take a call, or lock their phone mid-task.

This means mobile agent apps need:

• Checkpointing — Saving state so work isn't lost
• Resuming — Picking up where you left off after interruption
• Background execution — Using the limited time iOS gives you wisely
• On-device vs. cloud decisions — What runs locally vs. what needs a server </why_mobile>

<ios_storage>

iOS Storage Architecture

Needs validation: This is an approach that works well, but better solutions may exist.

For agent-native iOS apps, use iCloud Drive's Documents folder for your shared workspace. This gives you free, automatic multi-device sync without building a sync layer or running a server.

Why iCloud Documents?

Approach	Cost	Complexity	Offline	Multi-Device
Custom backend + sync	$$$	High	Manual	Yes
CloudKit database	Free tier limits	Medium	Manual	Yes
iCloud Documents	Free (user's storage)	Low	Automatic	Automatic

iCloud Documents:

• Uses user's existing iCloud storage (free 5GB, most users have more)
• Automatic sync across all user's devices
• Works offline, syncs when online
• Files visible in Files.app for transparency
• No server costs, no sync code to maintain

Implementation: iCloud-First with Local Fallback

// Get the iCloud Documents container
func iCloudDocumentsURL() -> URL? {
    FileManager.default.url(forUbiquityContainerIdentifier: nil)?
        .appendingPathComponent("Documents")
}

// Your shared workspace lives in iCloud
class SharedWorkspace {
    let rootURL: URL

    init() {
        // Use iCloud if available, fall back to local
        if let iCloudURL = iCloudDocumentsURL() {
            self.rootURL = iCloudURL
        } else {
            // Fallback to local Documents (user not signed into iCloud)
            self.rootURL = FileManager.default.urls(
                for: .documentDirectory,
                in: .userDomainMask
            ).first!
        }
    }

    // All file operations go through this root
    func researchPath(for bookId: String) -> URL {
        rootURL.appendingPathComponent("Research/\(bookId)")
    }

    func journalPath() -> URL {
        rootURL.appendingPathComponent("Journal")
    }
}

Directory Structure in iCloud

iCloud Drive/
└── YourApp/                          # Your app's container
    └── Documents/                    # Visible in Files.app
        ├── Journal/
        │   ├── user/
        │   │   └── 2025-01-15.md     # Syncs across devices
        │   └── agent/
        │       └── 2025-01-15.md     # Agent observations sync too
        ├── Research/
        │   └── {bookId}/
        │       ├── full_text.txt
        │       └── sources/
        ├── Chats/
        │   └── {conversationId}.json
        └── context.md                # Agent's accumulated knowledge

Handling iCloud File States

iCloud files may not be downloaded locally. Handle this:

func readFile(at url: URL) throws -> String {
    // iCloud may create .icloud placeholder files
    if url.pathExtension == "icloud" {
        // Trigger download
        try FileManager.default.startDownloadingUbiquitousItem(at: url)
        throw FileNotYetAvailableError()
    }

    return try String(contentsOf: url, encoding: .utf8)
}

// For writes, use coordinated file access
func writeFile(_ content: String, to url: URL) throws {
    let coordinator = NSFileCoordinator()
    var error: NSError?

    coordinator.coordinate(
        writingItemAt: url,
        options: .forReplacing,
        error: &error
    ) { newURL in
        try? content.write(to: newURL, atomically: true, encoding: .utf8)
    }

    if let error = error { throw error }
}

What iCloud Enables

1. User starts experiment on iPhone → Agent creates config file
2. User opens app on iPad → Same experiment visible, no sync code needed
3. Agent logs observation on iPhone → Syncs to iPad automatically
4. User edits journal on iPad → iPhone sees the edit

Entitlements Required

Add to your app's entitlements:

<key>com.apple.developer.icloud-container-identifiers</key>
<array>
    <string>iCloud.com.yourcompany.yourapp</string>
</array>
<key>com.apple.developer.icloud-services</key>
<array>
    <string>CloudDocuments</string>
</array>
<key>com.apple.developer.ubiquity-container-identifiers</key>
<array>
    <string>iCloud.com.yourcompany.yourapp</string>
</array>

When NOT to Use iCloud Documents

• Sensitive data - Use Keychain or encrypted local storage instead
• High-frequency writes - iCloud sync has latency; use local + periodic sync
• Large media files - Consider CloudKit Assets or on-demand resources
• Shared between users - iCloud Documents is single-user; use CloudKit for sharing </ios_storage>

<background_execution>

Background Execution & Resumption

Needs validation: These patterns work but better solutions may exist.

Mobile apps can be suspended or terminated at any time. Agents must handle this gracefully.

The Challenge

User starts research agent
     ↓
Agent begins web search
     ↓
User switches to another app
     ↓
iOS suspends your app
     ↓
Agent is mid-execution... what happens?

Checkpoint/Resume Pattern

Save agent state before backgrounding, restore on foreground:

class AgentOrchestrator: ObservableObject {
    @Published var activeSessions: [AgentSession] = []

    // Called when app is about to background
    func handleAppWillBackground() {
        for session in activeSessions {
            saveCheckpoint(session)
            session.transition(to: .backgrounded)
        }
    }

    // Called when app returns to foreground
    func handleAppDidForeground() {
        for session in activeSessions where session.state == .backgrounded {
            if let checkpoint = loadCheckpoint(session.id) {
                resumeFromCheckpoint(session, checkpoint)
            }
        }
    }

    private func saveCheckpoint(_ session: AgentSession) {
        let checkpoint = AgentCheckpoint(
            sessionId: session.id,
            conversationHistory: session.messages,
            pendingToolCalls: session.pendingToolCalls,
            partialResults: session.partialResults,
            timestamp: Date()
        )
        storage.save(checkpoint, for: session.id)
    }

    private func resumeFromCheckpoint(_ session: AgentSession, _ checkpoint: AgentCheckpoint) {
        session.messages = checkpoint.conversationHistory
        session.pendingToolCalls = checkpoint.pendingToolCalls

        // Resume execution if there were pending tool calls
        if !checkpoint.pendingToolCalls.isEmpty {
            session.transition(to: .running)
            Task { await executeNextTool(session) }
        }
    }
}

State Machine for Agent Lifecycle

enum AgentState {
    case idle           // Not running
    case running        // Actively executing
    case waitingForUser // Paused, waiting for user input
    case backgrounded   // App backgrounded, state saved
    case completed      // Finished successfully
    case failed(Error)  // Finished with error
}

class AgentSession: ObservableObject {
    @Published var state: AgentState = .idle

    func transition(to newState: AgentState) {
        let validTransitions: [AgentState: Set<AgentState>] = [
            .idle: [.running],
            .running: [.waitingForUser, .backgrounded, .completed, .failed],
            .waitingForUser: [.running, .backgrounded],
            .backgrounded: [.running, .completed],
        ]

        guard validTransitions[state]?.contains(newState) == true else {
            logger.warning("Invalid transition: \(state) → \(newState)")
            return
        }

        state = newState
    }
}

Background Task Extension (iOS)

Request extra time when backgrounded during critical operations:

class AgentOrchestrator {
    private var backgroundTask: UIBackgroundTaskIdentifier = .invalid

    func handleAppWillBackground() {
        // Request extra time for saving state
        backgroundTask = UIApplication.shared.beginBackgroundTask { [weak self] in
            self?.endBackgroundTask()
        }

        // Save all checkpoints
        Task {
            for session in activeSessions {
                await saveCheckpoint(session)
            }
            endBackgroundTask()
        }
    }

    private func endBackgroundTask() {
        if backgroundTask != .invalid {
            UIApplication.shared.endBackgroundTask(backgroundTask)
            backgroundTask = .invalid
        }
    }
}

User Communication

Let users know what's happening:

struct AgentStatusView: View {
    @ObservedObject var session: AgentSession

    var body: some View {
        switch session.state {
        case .backgrounded:
            Label("Paused (app in background)", systemImage: "pause.circle")
                .foregroundColor(.orange)
        case .running:
            Label("Working...", systemImage: "ellipsis.circle")
                .foregroundColor(.blue)
        case .waitingForUser:
            Label("Waiting for your input", systemImage: "person.circle")
                .foregroundColor(.green)
        // ...
        }
    }
}

</background_execution>

## Permission Handling

Mobile agents may need access to system resources. Handle permission requests gracefully.

Common Permissions

Resource	iOS Permission	Use Case
Photo Library	PHPhotoLibrary	Profile generation from photos
Files	Document picker	Reading user documents
Camera	AVCaptureDevice	Scanning book covers
Location	CLLocationManager	Location-aware recommendations
Network	(automatic)	Web search, API calls

Permission-Aware Tools

Check permissions before executing:

struct PhotoTools {
    static func readPhotos() -> AgentTool {
        tool(
            name: "read_photos",
            description: "Read photos from the user's photo library",
            parameters: [
                "limit": .number("Maximum photos to read"),
                "dateRange": .string("Date range filter").optional()
            ],
            execute: { params, context in
                // Check permission first
                let status = await PHPhotoLibrary.requestAuthorization(for: .readWrite)

                switch status {
                case .authorized, .limited:
                    // Proceed with reading photos
                    let photos = await fetchPhotos(params)
                    return ToolResult(text: "Found \(photos.count) photos", images: photos)

                case .denied, .restricted:
                    return ToolResult(
                        text: "Photo access needed. Please grant permission in Settings → Privacy → Photos.",
                        isError: true
                    )

                case .notDetermined:
                    return ToolResult(
                        text: "Photo permission required. Please try again.",
                        isError: true
                    )

                @unknown default:
                    return ToolResult(text: "Unknown permission status", isError: true)
                }
            }
        )
    }
}

Graceful Degradation

When permissions aren't granted, offer alternatives:

func readPhotos() async -> ToolResult {
    let status = PHPhotoLibrary.authorizationStatus(for: .readWrite)

    switch status {
    case .denied, .restricted:
        // Suggest alternative
        return ToolResult(
            text: """
            I don't have access to your photos. You can either:
            1. Grant access in Settings → Privacy → Photos
            2. Share specific photos directly in our chat

            Would you like me to help with something else instead?
            """,
            isError: false  // Not a hard error, just a limitation
        )
    // ...
    }
}

Permission Request Timing

Don't request permissions until needed:

// BAD: Request all permissions at launch
func applicationDidFinishLaunching() {
    requestPhotoAccess()
    requestCameraAccess()
    requestLocationAccess()
    // User is overwhelmed with permission dialogs
}

// GOOD: Request when the feature is used
tool("analyze_book_cover", async ({ image }) => {
    // Only request camera access when user tries to scan a cover
    let status = await AVCaptureDevice.requestAccess(for: .video)
    if status {
        return await scanCover(image)
    } else {
        return ToolResult(text: "Camera access needed for book scanning")
    }
})

<cost_awareness>

Cost-Aware Design

Mobile users may be on cellular data or concerned about API costs. Design agents to be efficient.

Model Tier Selection

Use the cheapest model that achieves the outcome:

enum ModelTier {
    case fast      // claude-3-haiku: ~$0.25/1M tokens
    case balanced  // claude-3-sonnet: ~$3/1M tokens
    case powerful  // claude-3-opus: ~$15/1M tokens

    var modelId: String {
        switch self {
        case .fast: return "claude-3-haiku-20240307"
        case .balanced: return "claude-3-sonnet-20240229"
        case .powerful: return "claude-3-opus-20240229"
        }
    }
}

// Match model to task complexity
let agentConfigs: [AgentType: ModelTier] = [
    .quickLookup: .fast,        // "What's in my library?"
    .chatAssistant: .balanced,  // General conversation
    .researchAgent: .balanced,  // Web search + synthesis
    .profileGenerator: .powerful, // Complex photo analysis
    .introductionWriter: .balanced,
]

Token Budgets

Limit tokens per agent session:

struct AgentConfig {
    let modelTier: ModelTier
    let maxInputTokens: Int
    let maxOutputTokens: Int
    let maxTurns: Int

    static let research = AgentConfig(
        modelTier: .balanced,
        maxInputTokens: 50_000,
        maxOutputTokens: 4_000,
        maxTurns: 20
    )

    static let quickChat = AgentConfig(
        modelTier: .fast,
        maxInputTokens: 10_000,
        maxOutputTokens: 1_000,
        maxTurns: 5
    )
}

class AgentSession {
    var totalTokensUsed: Int = 0

    func checkBudget() -> Bool {
        if totalTokensUsed > config.maxInputTokens {
            transition(to: .failed(AgentError.budgetExceeded))
            return false
        }
        return true
    }
}

Network-Aware Execution

Defer heavy operations to WiFi:

class NetworkMonitor: ObservableObject {
    @Published var isOnWiFi: Bool = false
    @Published var isExpensive: Bool = false  // Cellular or hotspot

    private let monitor = NWPathMonitor()

    func startMonitoring() {
        monitor.pathUpdateHandler = { [weak self] path in
            DispatchQueue.main.async {
                self?.isOnWiFi = path.usesInterfaceType(.wifi)
                self?.isExpensive = path.isExpensive
            }
        }
        monitor.start(queue: .global())
    }
}

class AgentOrchestrator {
    @ObservedObject var network = NetworkMonitor()

    func startResearchAgent(for book: Book) async {
        if network.isExpensive {
            // Warn user or defer
            let proceed = await showAlert(
                "Research uses data",
                message: "This will use approximately 1-2 MB of cellular data. Continue?"
            )
            if !proceed { return }
        }

        // Proceed with research
        await runAgent(ResearchAgent.create(book: book))
    }
}

Batch API Calls

Combine multiple small requests:

// BAD: Many small API calls
for book in books {
    await agent.chat("Summarize \(book.title)")
}

// GOOD: Batch into one request
let bookList = books.map { $0.title }.joined(separator: ", ")
await agent.chat("Summarize each of these books briefly: \(bookList)")

Caching

Cache expensive operations:

class ResearchCache {
    private var cache: [String: CachedResearch] = [:]

    func getCachedResearch(for bookId: String) -> CachedResearch? {
        guard let cached = cache[bookId] else { return nil }

        // Expire after 24 hours
        if Date().timeIntervalSince(cached.timestamp) > 86400 {
            cache.removeValue(forKey: bookId)
            return nil
        }

        return cached
    }

    func cacheResearch(_ research: Research, for bookId: String) {
        cache[bookId] = CachedResearch(
            research: research,
            timestamp: Date()
        )
    }
}

// In research tool
tool("web_search", async ({ query, bookId }) => {
    // Check cache first
    if let cached = cache.getCachedResearch(for: bookId) {
        return ToolResult(text: cached.research.summary, cached: true)
    }

    // Otherwise, perform search
    let results = await webSearch(query)
    cache.cacheResearch(results, for: bookId)
    return ToolResult(text: results.summary)
})

Cost Visibility

Show users what they're spending:

struct AgentCostView: View {
    @ObservedObject var session: AgentSession

    var body: some View {
        VStack(alignment: .leading) {
            Text("Session Stats")
                .font(.headline)

            HStack {
                Label("\(session.turnCount) turns", systemImage: "arrow.2.squarepath")
                Spacer()
                Label(formatTokens(session.totalTokensUsed), systemImage: "text.word.spacing")
            }

            if let estimatedCost = session.estimatedCost {
                Text("Est. cost: \(estimatedCost, format: .currency(code: "USD"))")
                    .font(.caption)
                    .foregroundColor(.secondary)
            }
        }
    }
}

</cost_awareness>

<offline_handling>

Offline Graceful Degradation

Handle offline scenarios gracefully:

class ConnectivityAwareAgent {
    @ObservedObject var network = NetworkMonitor()

    func executeToolCall(_ toolCall: ToolCall) async -> ToolResult {
        // Check if tool requires network
        let requiresNetwork = ["web_search", "web_fetch", "call_api"]
            .contains(toolCall.name)

        if requiresNetwork && !network.isConnected {
            return ToolResult(
                text: """
                I can't access the internet right now. Here's what I can do offline:
                - Read your library and existing research
                - Answer questions from cached data
                - Write notes and drafts for later

                Would you like me to try something that works offline?
                """,
                isError: false
            )
        }

        return await executeOnline(toolCall)
    }
}

Offline-First Tools

Some tools should work entirely offline:

let offlineTools: Set<String> = [
    "read_file",
    "write_file",
    "list_files",
    "read_library",  // Local database
    "search_local",  // Local search
]

let onlineTools: Set<String> = [
    "web_search",
    "web_fetch",
    "publish_to_cloud",
]

let hybridTools: Set<String> = [
    "publish_to_feed",  // Works offline, syncs later
]

Queued Actions

Queue actions that require connectivity:

class OfflineQueue: ObservableObject {
    @Published var pendingActions: [QueuedAction] = []

    func queue(_ action: QueuedAction) {
        pendingActions.append(action)
        persist()
    }

    func processWhenOnline() {
        network.$isConnected
            .filter { $0 }
            .sink { [weak self] _ in
                self?.processPendingActions()
            }
    }

    private func processPendingActions() {
        for action in pendingActions {
            Task {
                try await execute(action)
                remove(action)
            }
        }
    }
}

</offline_handling>

<battery_awareness>

Battery-Aware Execution

Respect device battery state:

class BatteryMonitor: ObservableObject {
    @Published var batteryLevel: Float = 1.0
    @Published var isCharging: Bool = false
    @Published var isLowPowerMode: Bool = false

    var shouldDeferHeavyWork: Bool {
        return batteryLevel < 0.2 && !isCharging
    }

    func startMonitoring() {
        UIDevice.current.isBatteryMonitoringEnabled = true

        NotificationCenter.default.addObserver(
            forName: UIDevice.batteryLevelDidChangeNotification,
            object: nil,
            queue: .main
        ) { [weak self] _ in
            self?.batteryLevel = UIDevice.current.batteryLevel
        }

        NotificationCenter.default.addObserver(
            forName: NSNotification.Name.NSProcessInfoPowerStateDidChange,
            object: nil,
            queue: .main
        ) { [weak self] _ in
            self?.isLowPowerMode = ProcessInfo.processInfo.isLowPowerModeEnabled
        }
    }
}

class AgentOrchestrator {
    @ObservedObject var battery = BatteryMonitor()

    func startAgent(_ config: AgentConfig) async {
        if battery.shouldDeferHeavyWork && config.isHeavy {
            let proceed = await showAlert(
                "Low Battery",
                message: "This task uses significant battery. Continue or defer until charging?"
            )
            if !proceed { return }
        }

        // Adjust model tier based on battery
        let adjustedConfig = battery.isLowPowerMode
            ? config.withModelTier(.fast)
            : config

        await runAgent(adjustedConfig)
    }
}

</battery_awareness>

<on_device_vs_cloud>

On-Device vs. Cloud

Understanding what runs where in a mobile agent-native app:

Component	On-Device	Cloud
Orchestration	✅
Tool execution	✅ (file ops, photo access, HealthKit)
LLM calls		✅ (Anthropic API)
Checkpoints	✅ (local files)	Optional via iCloud
Long-running agents	Limited by iOS	Possible with server

Implications

Network required for reasoning:

• The app needs network connectivity for LLM calls
• Design tools to degrade gracefully when network is unavailable
• Consider offline caching for common queries

Data stays local:

• File operations happen on device
• Sensitive data never leaves the device unless explicitly synced
• Privacy is preserved by default

Long-running agents: For truly long-running agents (hours), consider a server-side orchestrator that can run indefinitely, with the mobile app as a viewer and input mechanism. </on_device_vs_cloud>

## Mobile Agent-Native Checklist

iOS Storage:

• iCloud Documents as primary storage (or conscious alternative)
• Local Documents fallback when iCloud unavailable
• Handle .icloud placeholder files (trigger download)
• Use NSFileCoordinator for conflict-safe writes

Background Execution:

• Checkpoint/resume implemented for all agent sessions
• State machine for agent lifecycle (idle, running, backgrounded, etc.)
• Background task extension for critical saves (30 second window)
• User-visible status for backgrounded agents

Permissions:

• Permissions requested only when needed, not at launch
• Graceful degradation when permissions denied
• Clear error messages with Settings deep links
• Alternative paths when permissions unavailable

Cost Awareness:

• Model tier matched to task complexity
• Token budgets per session
• Network-aware (defer heavy work to WiFi)
• Caching for expensive operations
• Cost visibility to users

Offline Handling:

• Offline-capable tools identified
• Graceful degradation for online-only features
• Action queue for sync when online
• Clear user communication about offline state

Battery Awareness:

• Battery monitoring for heavy operations
• Low power mode detection
• Defer or downgrade based on battery state