Background the job without losing it

On iOS, watch scene phase. When the scene leaves the foreground, cancel a long respond if you must, then write the checkpoint. Apple's default on a tool error or a cancel is to revert the transcript. If you want to keep partial work, set transcriptErrorHandlingPolicy to preserveTranscript, then put the transcript into a good state yourself. That policy is in WWDC26 session 242.

import SwiftUI
import BackgroundTasks

@main
struct WorkflowApp: App {
    @Environment(\.scenePhase) private var scenePhase
    @State private var engine = WorkflowEngine()

    var body: some Scene {
        WindowGroup {
            WorkflowView(engine: engine)
        }
        .onChange(of: scenePhase) { _, phase in
            if phase != .active {
                Task { await engine.persistAndPause() }
            }
        }
    }
}

func registerBackgroundRefresh() {
    BGTaskScheduler.shared.register(forTaskWithIdentifier: "dev.edgefde.workflow", using: nil) { task in
        guard let process = task as? BGProcessingTask else { return }
        process.expirationHandler = { process.setTaskCompleted(success: false) }
        Task {
            do {
                _ = try await WorkflowEngine.shared.resumeFromDisk()
                process.setTaskCompleted(success: true)
            } catch {
                process.setTaskCompleted(success: false)
            }
        }
    }
}

A BGProcessingTask is a chance to resume, not a promise. iOS may delay or skip it. The user may have to open the app. Say that to stakeholders. On macOS you have more room to keep a process alive, but you should still write the same checkpoint. Do not build two designs.

If you cancel mid-response and you used preserveTranscript, wait until isResponding is false, then save. If you used the default revert, save the last completed step only. Do not save a half sentence as a finished plan.

Read Make several agents agree first, Talk to tools with MCP, and Stop prompt attacks and leaks if the background resume would call a side-effect tool. That tool still needs a gate.

Next, put these parts in a small engine you can ship.