background-execution - SKILL.md Agent Skill

name: background-execution description: Writes and reviews Swift code for background execution on Apple platforms - BGTaskScheduler (app refresh, processing, continued processing), beginBackgroundTask assertions, background URLSession downloads/uploads, silent and VoIP push, background modes (audio, location, BLE), the SwiftUI .backgroundTask modifier, and macOS schedulers. Use when scheduling deferred work, finishing work after the app backgrounds, transferring files in the background, waking the app via push, or debugging background tasks that never run. license: MIT metadata: author: Anton Novoselov version: "1.0"

Write and review Swift code that runs work while the app is backgrounded or suspended, choosing the right API for the job and respecting the system's constraints so the work actually runs and the app is not terminated.

Review process:

Establish the background execution model (lifecycle, budgets, force-quit, the five principles) using references/fundamentals.md.
Confirm the chosen API matches the work using the decision tree below; reroute if it does not.
Validate BGTaskScheduler usage (registration timing, Info.plist, expiration, setTaskCompleted) using references/bg-task-scheduler.md.
Validate task assertions (beginBackgroundTask/endBackgroundTask, performExpiringActivity) using references/task-assertions.md.
Validate background URLSession config and the relaunch flow using references/background-url-session.md.
Validate silent/VoIP push payloads, headers, and handlers using references/background-push.md.
Validate UIBackgroundModes declarations and location specifics using references/background-modes.md.
Validate background audio (session category, recording while locked, interruptions, route changes, mic permission) using references/background-audio.md.
Validate SwiftUI ScenePhase / .backgroundTask usage using references/swiftui-background-tasks.md.
Validate macOS schedulers (NSBackgroundActivityScheduler, beginActivity, SMAppService) using references/macos-background.md.
Check testing/debugging and constraint handling using references/testing-and-debugging.md.
Catch common mistakes using references/anti-patterns.md.

If doing partial work, load only the relevant reference files.

Task-based routing

Match the user's goal to the read order. Load only what you need.

"Refresh my app's content in the background"

references/bg-task-scheduler.md - BGAppRefreshTask, register/submit/expiration
references/testing-and-debugging.md - simulate the launch, handle backgroundRefreshStatus

"Run long maintenance / ML / cleanup in the background"

references/bg-task-scheduler.md - BGProcessingTask, power/network requirements
references/testing-and-debugging.md - simulate launch and expiration

"Let a user-started export keep running after they leave (iOS 26)"

references/bg-task-scheduler.md - BGContinuedProcessingTask, progress, submission strategy, GPU
references/fundamentals.md - the consent / user-initiated principle

"Finish an in-flight upload / save when the app backgrounds"

references/task-assertions.md - beginBackgroundTask/endBackgroundTask, performExpiringActivity

"Download or upload a large file that survives suspension"

references/background-url-session.md - background config, delegate, the relaunch flow
references/testing-and-debugging.md - force-quit and constraint caveats

"Wake my app from the server"

references/background-push.md - silent push payload/headers/handler, or PushKit for calls
references/background-modes.md - the remote-notification mode

"Keep playing audio in the background"

references/background-audio.md - the audio mode + active session, category/mode/options, interruptions, route changes

"Record / dictate / transcribe with the screen locked"

references/background-audio.md - .playAndRecord, recording while locked, mic permission, AVAudioEngine taps, the production gotchas

"Track location in the background"

references/background-modes.md - the location mode, significant-change and region monitoring

"Do this the SwiftUI way"

references/swiftui-background-tasks.md - ScenePhase, .backgroundTask(.appRefresh/.urlSession)
references/bg-task-scheduler.md - you still schedule with BGTaskScheduler

"Background work on macOS"

references/macos-background.md - NSBackgroundActivityScheduler, beginActivity, SMAppService

"My background task never runs / how do I test it"

references/testing-and-debugging.md - lldb simulate commands, the states that gate execution
references/anti-patterns.md - the usual causes

"I'm hitting bugs or App Review issues"

references/anti-patterns.md - catches with before/after fixes

Decision tree

Which background API should I use?

Does the work require user consent or input mid-flight?
  -> Background runtime is wrong. Bring it to the foreground.

Is work already running and you just need to finish it as the app backgrounds?
  -> beginBackgroundTask / endBackgroundTask (apps)
  -> ProcessInfo.performExpiringActivity (app extensions, watchOS)

Is it a user-initiated long task that should continue with visible progress? (iOS/iPadOS 26)
  -> BGContinuedProcessingTask

Is it a large/long file download or upload that must survive suspension/termination?
  -> background URLSession

Does the server need to tell the app new content exists?
  -> background (silent) push  (apns-push-type: background, content-available: 1)

Is it an incoming call invitation?
  -> PushKit + CallKit (report the call on every push)

Is it periodic content refresh before the user opens the app?
  -> BGAppRefreshTask

Is it long, deferrable maintenance / ML / cleanup?
  -> BGProcessingTask (optionally requiresExternalPower / requiresNetworkConnectivity)

Does the app need to keep playing audio or recording the mic (incl. while locked)?
  -> audio UIBackgroundMode + active AVAudioSession (.playback or .playAndRecord)  [background-audio.md]

Does the app need to keep running for location / BLE / VoIP?
  -> declare the matching UIBackgroundModes capability  [background-modes.md]

macOS, deferrable repeating maintenance?
  -> NSBackgroundActivityScheduler

macOS, keep critical non-UI work from being napped?
  -> ProcessInfo.beginActivity / performActivity

Which BGTask type?

Short content refresh, system picks the time around app usage?
  -> BGAppRefreshTask          (UIBackgroundModes: fetch)

Long maintenance, ideally on charger + network?
  -> BGProcessingTask          (UIBackgroundModes: processing; requiresExternalPower / requiresNetworkConnectivity)

User-tapped long work with progress UI, continues after backgrounding? (iOS 26)
  -> BGContinuedProcessingTask (title/subtitle, progress, submission strategy, optional .gpu)

Long processing of health-research-study data? (iOS 17)
  -> BGHealthResearchTask

Core Instructions

Never treat background execution as guaranteed. It is opportunistic and discretionary; design every feature with a foreground reconciliation path and never put correctness-critical logic solely in a background task.
Never register a BGTaskScheduler handler lazily, in view code, or after launch finishes. Register all handlers in App.init() / application(_:didFinishLaunchingWithOptions:), because the system may launch the app directly into the background. (BGContinuedProcessingTask is the one exception - it registers dynamically on user intent.)
Never register the same task identifier twice - it crashes the app.
Never omit the expiration handler on a BGTask; without one the system silently marks the task complete-and-unsuccessful. Set it, have it cancel work fast, and call setTaskCompleted(success:) exactly once.
Never forget to reschedule. BGTaskScheduler requests are one-shot; submit the next request as the first line of the launch handler.
Put requiresExternalPower / requiresNetworkConnectivity only on BGProcessingTaskRequest, never on app-refresh. Treat earliestBeginDate as a floor, not a schedule.
Never leave a beginBackgroundTask unbalanced. Call endBackgroundTask on both the success path and inside the expiration handler, idempotently (guard on .invalid); an unbalanced assertion kills the app. Call beginBackgroundTask before starting the work, not at the end of the background-transition delegate.
Never trust backgroundTimeRemaining as a real budget - it is DBL_MAX in the foreground and advisory in the background. Plan for ~30 s and rely on the expiration handler.
Use beginBackgroundTask (apps) or performExpiringActivity (extensions, watchOS) only to finish work already running, never to start fresh long work or schedule future work.
Never use completion-handler convenience methods or dataTask on a background URLSession; background sessions require a delegate. Create one session per identifier, store it, and recreate it at relaunch with the identical identifier and config.
Implement the full relaunch flow for background URLSession: store the handler from handleEventsForBackgroundURLSession, recreate the session, and call the stored handler from urlSessionDidFinishEvents(forBackgroundURLSession:) on the main thread. Move the file from didFinishDownloadingTo's location synchronously inside the callback.
For background uploads, use file-backed uploadTask(with:fromFile:) - data and stream uploads fail after the app exits.
Never send a silent push without apns-push-type: background (and apns-priority: 5), and keep the aps dictionary to content-available only. Call the fetch completion handler within ~30 s. Treat silent push as throttled and coalesced, not reliable.
On iOS 13+, report an incoming call to CallKit on every VoIP push, or the system terminates the app and revokes VoIP-push privileges. Never use VoIP pushes for non-call data.
Never set allowsBackgroundLocationUpdates = true without the location background mode - it is a fatal error. Prefer significant-change / region monitoring over the continuous location mode.
Background audio needs both the audio mode and an active AVAudioSession with a background-capable category (.playback for playback, .playAndRecord / .record for recording-while-locked); the mode alone does nothing. Activate the session when audio begins (not at launch) and deactivate with .notifyOthersOnDeactivation. Always handle interruptionNotification (check .shouldResume before resuming) and routeChangeNotification (pause on .oldDeviceUnavailable).
For mic capture, require permission with AVAudioApplication.requestRecordPermission() (iOS 17+) and declare NSMicrophoneUsageDescription - without the key the app crashes, and without permission capture returns silent zeroed samples, not an error. Install AVAudioEngine taps off the main actor (the closure runs on a realtime thread) and match AVAudioFile settings to the input node's format.
Declare UIBackgroundModes sparingly; App Review rejects unjustified modes. Prefer the lighter alternative.
Account for the user: a force-quit suppresses BGTaskScheduler, silent push, and background URLSession relaunch until the next manual launch. Read backgroundRefreshStatus (degrade on .denied, stay silent on .restricted) and back off under Low Power Mode and elevated thermal state.
Do background-relevant setup (task registration, session recreation, dependency wiring) on a path that runs on every launch, not in .onAppear / .task - a background launch may never build the UI.
With scenes enabled, use sceneDidEnterBackground(_:) (not the deprecated applicationDidEnterBackground(_:)); it has ~5 s to return.
On macOS use NSBackgroundActivityScheduler for deferrable maintenance and ProcessInfo.beginActivity/endActivity (balanced, or performActivity) to control App Nap; performExpiringActivity does not exist on macOS.
Test background tasks on a real device with the lldb _simulateLaunchForTaskWithIdentifier: / _simulateExpirationForTaskWithIdentifier: commands - never ship a reference to them. Always test the expiration path.

Output Format

If the user asks for a review, organize findings by file. For each issue:

State the file and relevant line(s).
Name the anti-pattern being replaced.
Show a brief before/after code fix.

Skip files with no issues. End with a prioritized summary of the most impactful changes to make first.

If the user asks you to write or fix background-execution code, make the changes directly instead of returning a findings report.

Example output:

SyncManager.swift

Line 22: BGTaskScheduler handler registered in .onAppear - never runs on a background launch.

// Before
.onAppear {
    BGTaskScheduler.shared.register(forTaskWithIdentifier: id, using: nil) { ... }
}

// After - register in App.init() / didFinishLaunching
init() {
    BGTaskScheduler.shared.register(forTaskWithIdentifier: id, using: nil) { task in
        self.handleRefresh(task: task as! BGAppRefreshTask)
    }
}

Line 48: missing expiration handler and setTaskCompleted - the app is killed at expiry.

// Before
func handleRefresh(task: BGAppRefreshTask) { refresh() }

// After
func handleRefresh(task: BGAppRefreshTask) {
    scheduleNext()
    let op = RefreshOperation()
    task.expirationHandler = { op.cancel() }
    op.completionBlock = { task.setTaskCompleted(success: !op.isCancelled) }
    queue.addOperation(op)
}

Summary

Registration timing (high): move the handler to App.init() or it never fires on background launch.
Completion contract (high): add the expiration handler and call setTaskCompleted exactly once.

End of example.

References

references/fundamentals.md - the execution model: foreground/background/suspended, system budgets, force-quit, user toggles, the five principles, the lifecycle and scene states, the "which API" map.
references/bg-task-scheduler.md - BackgroundTasks: BGAppRefreshTask, BGProcessingTask, BGContinuedProcessingTask (iOS 26), BGHealthResearchTask, registration, Info.plist keys, submit, expiration, errors.
references/task-assertions.md - beginBackgroundTask/endBackgroundTask, backgroundTimeRemaining, ProcessInfo.performExpiringActivity, assertions vs BGTaskScheduler.
references/background-url-session.md - background URLSession config, mandatory delegate, the relaunch / handleEventsForBackgroundURLSession flow, knobs, resume data.
references/background-push.md - silent/background push (payload, headers, handler, throttling), PushKit/VoIP + the CallKit requirement.
references/background-modes.md - the UIBackgroundModes matrix, background location, BLE/accessory/PTT, deprecated background fetch.
references/background-audio.md - background playback and recording: AVAudioSession categories/modes/options, recording while locked, mic permission, interruptions, route changes, AVAudioEngine taps and production gotchas.
references/swiftui-background-tasks.md - ScenePhase, the .backgroundTask scene modifier, async cancellation, scheduling vs handling.
references/macos-background.md - NSBackgroundActivityScheduler, ProcessInfo.beginActivity/App Nap, SMAppService, cross-platform map.
references/testing-and-debugging.md - lldb simulate-launch/expiration, backgroundRefreshStatus, Low Power Mode, thermal state, what gates execution.
references/anti-patterns.md - common mistakes LLMs make when generating background-execution code.