diff --git a/ios/app/VoltraModule.swift b/ios/app/VoltraModule.swift
index 219b90d..0625e7c 100644
--- a/ios/app/VoltraModule.swift
+++ b/ios/app/VoltraModule.swift
@@ -2,6 +2,7 @@ import ActivityKit
 import Compression
 import ExpoModulesCore
 import Foundation
+import os
 import WidgetKit
 
 public class VoltraModule: Module {
@@ -10,7 +11,7 @@ public class VoltraModule: Module {
   private let TIMELINE_WARNING_SIZE = 100_000 // 100KB per timeline
   private let liveActivityService = VoltraLiveActivityService()
   private var wasLaunchedInBackground: Bool = false
-  private var monitoredActivityIds: Set<String> = []
+  private let monitoredActivityIds = OSAllocatedUnfairLock<Set<String>>(initialState: [])
 
   enum VoltraErrors: Error {
     case unsupportedOS
@@ -55,7 +56,7 @@ public class VoltraModule: Module {
 
     OnStopObserving {
       VoltraEventBus.shared.unsubscribe()
-      monitoredActivityIds.removeAll()
+      monitoredActivityIds.withLock { $0.removeAll() }
     }
 
     OnCreate {
@@ -608,32 +609,38 @@ private extension VoltraModule {
 
   /// Set up observers for an activity's lifecycle (only once per activity)
   private func monitorActivity(_ activity: Activity<VoltraAttributes>) {
+    guard activity.activityState != .ended else { return }
+
     let activityId = activity.id
+    let activityName = activity.attributes.name
+    let pushEnabled = pushNotificationsEnabled
 
-    // Skip if we're already monitoring this activity
-    guard !monitoredActivityIds.contains(activityId) else { return }
-    monitoredActivityIds.insert(activityId)
+    // Thread-safe check-and-insert
+    let shouldMonitor = monitoredActivityIds.withLock { ids -> Bool in
+      guard !ids.contains(activityId) else { return false }
+      ids.insert(activityId)
+      return true
+    }
+    guard shouldMonitor else { return }
 
-    // Observe lifecycle state changes (active → dismissed → ended)
     Task {
       for await state in activity.activityStateUpdates {
         VoltraEventBus.shared.send(
           .stateChange(
-            activityName: activity.attributes.name,
+            activityName: activityName,
             state: String(describing: state)
           )
         )
       }
     }
 
-    // Observe push token updates if enabled
-    if pushNotificationsEnabled {
+    if pushEnabled {
       Task {
         for await pushTokenData in activity.pushTokenUpdates {
           let pushTokenString = pushTokenData.hexString
           VoltraEventBus.shared.send(
             .tokenReceived(
-              activityName: activity.attributes.name,
+              activityName: activityName,
               pushToken: pushTokenString
             )
           )