diff --git a/README.md b/README.md
index 433d1c5..ee570ce 100644
--- a/README.md
+++ b/README.md
@@ -81,6 +81,16 @@ Preferences expose Sparkle’s built-in toggles for “Automatically check” an
 
 > Build settings include `INFOPLIST_KEY_SUFeedURL` and `INFOPLIST_KEY_SUPublicEDKey`. Make sure to fill both before shipping a build so Sparkle knows where to fetch updates and how to verify them.
 
+#### Debugging Sparkle updates
+
+Launch the shipped app via CLI with `SPARKLE_VERBOSE_LOGGING=1` to mirror Sparkle’s activity in stdout/stderr:
+
+```bash
+SPARKLE_VERBOSE_LOGGING=1 /Applications/iKeyMon.app/Contents/MacOS/iKeyMon
+```
+
+Sparkle’s installer helper runs in a separate `SUPipedBinary` process. If the installer fails, collect additional details with `log show --process SUPipedBinary --last 5m`.
+
 ### Automated release push
 
 If you want `git push origin master` to build/sign/notarize/upload automatically, enable the provided pre-push hook:
diff --git a/Sources/ViewModels/SparkleUpdater.swift b/Sources/ViewModels/SparkleUpdater.swift
index 48342eb..6ab6269 100644
--- a/Sources/ViewModels/SparkleUpdater.swift
+++ b/Sources/ViewModels/SparkleUpdater.swift
@@ -1,13 +1,19 @@
 import Sparkle
 import Foundation
+import OSLog
 
 @MainActor
 final class SparkleUpdater: NSObject, ObservableObject {
     let controller: SPUStandardUpdaterController
+    private let logger = Logger(subsystem: "net.24unix.iKeyMon", category: "Sparkle")
+    private let verboseLogging: Bool
 
     override init() {
+        self.verboseLogging = ProcessInfo.processInfo.environment["SPARKLE_VERBOSE_LOGGING"] == "1"
         self.controller = SPUStandardUpdaterController(startingUpdater: true, updaterDelegate: nil, userDriverDelegate: nil)
         super.init()
+        controller.updater.delegate = self
+        log("Sparkle updater initialized (verbose=\(verboseLogging)).")
     }
 
     var automaticallyChecksForUpdates: Bool {
@@ -21,6 +27,65 @@ final class SparkleUpdater: NSObject, ObservableObject {
     }
 
     func checkForUpdates() {
+        log("Manual check for updates triggered.")
         controller.checkForUpdates(nil)
     }
+
+    private func log(_ message: String) {
+        logger.log("\(message, privacy: .public)")
+        if verboseLogging {
+            print("[Sparkle] \(message)")
+        }
+    }
+
+    private func logError(_ message: String) {
+        logger.error("\(message, privacy: .public)")
+        if verboseLogging {
+            fputs("[Sparkle][error] \(message)\n", stderr)
+        }
+    }
+
+    private func describe(update item: SUAppcastItem) -> String {
+        let short = item.displayVersionString ?? item.versionString ?? "unknown"
+        let build = item.versionString ?? "?"
+        return "\(short) (build \(build))"
+    }
+}
+
+extension SparkleUpdater: SPUUpdaterDelegate {
+    func updater(_ updater: SPUUpdater, didFinishLoading appcast: SUAppcast) {
+        log("Loaded Sparkle appcast containing \(appcast.items.count) item(s).")
+    }
+
+    func updater(_ updater: SPUUpdater, didFindValidUpdate item: SUAppcastItem) {
+        log("Found valid update \(describe(update: item))")
+    }
+
+    func updaterDidNotFindUpdate(_ updater: SPUUpdater) {
+        log("No updates available.")
+    }
+
+    func updater(_ updater: SPUUpdater, willDownloadUpdate item: SUAppcastItem, with request: NSMutableURLRequest) {
+        log("Downloading \(describe(update: item)) from \(request.url?.absoluteString ?? "unknown URL")")
+    }
+
+    func updater(_ updater: SPUUpdater, didDownloadUpdate item: SUAppcastItem) {
+        log("Finished downloading \(describe(update: item))")
+    }
+
+    func updater(_ updater: SPUUpdater, failedToDownloadUpdate item: SUAppcastItem, error: Error) {
+        logError("Failed to download \(describe(update: item)): \(error.localizedDescription)")
+    }
+
+    func userDidCancelDownload(_ updater: SPUUpdater) {
+        log("User cancelled Sparkle download.")
+    }
+
+    func updater(_ updater: SPUUpdater, willInstallUpdate item: SUAppcastItem) {
+        log("Will install update \(describe(update: item))")
+    }
+
+    func updater(_ updater: SPUUpdater, didAbortWithError error: Error) {
+        logError("Sparkle aborted: \(error.localizedDescription)")
+    }
 }