diff --git a/CHANGELOG.md b/CHANGELOG.md
index a094f1f2..9e45f22b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,15 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Fixed
+- **OperationLockServiceTests** — replaced flaky `Barrier` + `Thread.Sleep`
+  thread-safety test with deterministic `CountdownEvent` + `ManualResetEventSlim`
+  synchronization; asserts exactly 1 acquisition instead of `>= 1`.
+
+### Changed
+- **README.md** — added missing tech stack entries: Microsoft.Extensions.DependencyInjection,
+  H.NotifyIcon.Wpf, NSubstitute.
+
 ## [0.48.34] - 2026-05-19
 
 ### Fixed
diff --git a/README.md b/README.md
index 34453abd..89304308 100644
--- a/README.md
+++ b/README.md
@@ -452,10 +452,12 @@ and elevation state in a format ready to paste into a bug report.
 
 - .NET 9 (WPF, C# 13)
 - CommunityToolkit.Mvvm for MVVM plumbing
+- Microsoft.Extensions.DependencyInjection for IoC
 - ModernWpfUI for the modern title bar
 - LiveCharts2 for the real-time latency chart
+- H.NotifyIcon.Wpf for system tray integration
 - Serilog for structured logging
-- xUnit and FlaUI for unit, integration, and UI-automation tests
+- xUnit, NSubstitute, and FlaUI for unit, integration, and UI-automation tests
 
 ## Privacy
 
diff --git a/SysManager/SysManager.Tests/OperationLockServiceTests.cs b/SysManager/SysManager.Tests/OperationLockServiceTests.cs
index 86f0d872..e11f4db8 100644
--- a/SysManager/SysManager.Tests/OperationLockServiceTests.cs
+++ b/SysManager/SysManager.Tests/OperationLockServiceTests.cs
@@ -93,20 +93,28 @@ public void GetActiveOperationName_ReturnsNull_WhenNotLocked()
     public async Task TryAcquire_IsThreadSafe()
     {
         int successCount = 0;
-        var barrier = new Barrier(10);
+        var ready = new CountdownEvent(10);
+        var go = new ManualResetEventSlim(false);
+        var allTried = new CountdownEvent(10);
 
         var tasks = Enumerable.Range(0, 10).Select(_ => Task.Run(() =>
         {
-            barrier.SignalAndWait();
-            using var handle = Sut.TryAcquire(OperationCategory.Network, "Race");
+            ready.Signal();
+            go.Wait();
+            var handle = Sut.TryAcquire(OperationCategory.SystemModification, "Race");
             if (handle != null)
                 Interlocked.Increment(ref successCount);
-            Thread.Sleep(50);
+            // Wait until all threads have attempted acquisition before releasing
+            allTried.Signal();
+            allTried.Wait();
+            handle?.Dispose();
         })).ToArray();
 
+        ready.Wait();
+        go.Set();
         await Task.WhenAll(tasks);
 
-        // Only one thread should have acquired the lock at a time
-        Assert.True(successCount >= 1);
+        // Exactly one thread should have acquired the lock
+        Assert.Equal(1, successCount);
     }
 }