diff --git a/src/ZB.MOM.WW.MxGateway.Worker.Tests/Probes/ItemHandleReuseProbeTests.cs b/src/ZB.MOM.WW.MxGateway.Worker.Tests/Probes/ItemHandleReuseProbeTests.cs
new file mode 100644
index 0000000..6557b3f
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker.Tests/Probes/ItemHandleReuseProbeTests.cs
@@ -0,0 +1,160 @@
+// ItemHandleReuseProbeTests.cs
+//
+// Probe for issue #166: does MXAccess hand out a new item handle number when the
+// same item is removed and re-added on one LMXProxyServer registration? The
+// AlarmAckWriter's Unconfirmed mitigation (drop the item, re-add it, wait on the
+// fresh handle) relies on the number changing, because OnWriteComplete carries
+// only the (server, item) handle pair.
+//
+// The probe only adds, advises and removes items. It never writes, so it does
+// not acknowledge the alarm or create Historian rows.
+//
+// HOW TO RUN (dev rig with MXAccess installed and the DEV Galaxy running):
+//   1. Remove (or set to null) the Skip parameter on the [Fact] below.
+//   2. dotnet test src/ZB.MOM.WW.MxGateway.Worker.Tests/ZB.MOM.WW.MxGateway.Worker.Tests.csproj
+//        -p:Platform=x86 --filter FullyQualifiedName~ItemHandleReuseProbeTests
+//        --logger "console;verbosity=detailed"
+//   3. Read the logged handles. Results are recorded in docs/AlarmProbeFindings.md.
+
+using System;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using System.Threading;
+using ArchestrA.MxAccess;
+using ZB.MOM.WW.MxGateway.Worker.MxAccess;
+using Xunit.Abstractions;
+
+namespace ZB.MOM.WW.MxGateway.Worker.Tests.Probes;
+
+/// <summary>
+///     Live probe that logs the item handles MXAccess returns when the same
+///     <c>AckMsg</c> item is removed and re-added, to decide whether a late
+///     <c>OnWriteComplete</c> for a dropped handle could match a re-added one.
+/// </summary>
+public sealed class ItemHandleReuseProbeTests
+{
+    private const string AckItem = "TestMachine_001.TestAlarm001.AckMsg";
+    private const string OtherItem = "TestMachine_001.TestAlarm001.Acked";
+    private const int Rounds = 5;
+
+    private readonly ITestOutputHelper output;
+    private readonly List<string> log = new List<string>();
+
+    /// <summary>Initializes a new instance of the <see cref="ItemHandleReuseProbeTests"/> class.</summary>
+    /// <param name="output">Test output helper for logging.</param>
+    public ItemHandleReuseProbeTests(ITestOutputHelper output)
+    {
+        this.output = output;
+    }
+
+    /// <summary>Adds, removes and re-adds the AckMsg item on a fresh registration and logs every handle.</summary>
+    [Fact(Skip = "Live dev-rig probe (issue #166) — flip Skip=null on a host with MXAccess and the DEV Galaxy running.")]
+    public void RemoveThenReAdd_LogsItemHandles()
+    {
+        Exception? threadException = null;
+        Thread thread = new Thread(() =>
+        {
+            try { RunProbe(); }
+            catch (Exception ex) { threadException = ex; }
+        });
+        thread.SetApartmentState(ApartmentState.STA);
+        thread.Start();
+        thread.Join();
+
+        foreach (string line in log)
+        {
+            output.WriteLine(line);
+        }
+
+        if (threadException is not null)
+        {
+            throw new InvalidOperationException("Probe failed: " + threadException.Message, threadException);
+        }
+    }
+
+    private void RunProbe()
+    {
+        LMXProxyServerClass com = new LMXProxyServerClass();
+        try
+        {
+            IMxAccessServer mx = new MxAccessComServer(com);
+            int server = mx.Register("MxGatewayProbe166");
+            Log($"Register -> server handle {server}");
+
+            // A: the AlarmAckWriter drop path (Advise, then UnAdvise + RemoveItem).
+            for (int i = 0; i < Rounds; i++)
+            {
+                int h = mx.AddItem(server, AckItem);
+                mx.Advise(server, h);
+                Log($"A{i}: AddItem+Advise {AckItem} -> {h}");
+                mx.UnAdvise(server, h);
+                mx.RemoveItem(server, h);
+                Log($"A{i}: UnAdvise+RemoveItem {h}");
+            }
+
+            // B: RemoveItem without Advise.
+            for (int i = 0; i < Rounds; i++)
+            {
+                int h = mx.AddItem(server, AckItem);
+                Log($"B{i}: AddItem {AckItem} -> {h}");
+                mx.RemoveItem(server, h);
+                Log($"B{i}: RemoveItem {h}");
+            }
+
+            // C: interleave another item to see whether a freed number is handed to any item.
+            int c1 = mx.AddItem(server, AckItem);
+            Log($"C: AddItem {AckItem} -> {c1}");
+            mx.RemoveItem(server, c1);
+            Log($"C: RemoveItem {c1}");
+            int c2 = mx.AddItem(server, OtherItem);
+            Log($"C: AddItem {OtherItem} -> {c2}");
+            int c3 = mx.AddItem(server, AckItem);
+            Log($"C: AddItem {AckItem} -> {c3}");
+            mx.RemoveItem(server, c2);
+            mx.RemoveItem(server, c3);
+            Log($"C: RemoveItem {c2}, {c3}");
+
+            // D: the same item added twice while the first is still held.
+            int d1 = mx.AddItem(server, AckItem);
+            int d2 = mx.AddItem(server, AckItem);
+            Log($"D: AddItem {AckItem} twice while held -> {d1}, {d2}");
+            mx.RemoveItem(server, d1);
+            mx.RemoveItem(server, d2);
+
+            mx.Unregister(server);
+            Log($"Unregister {server}");
+
+            // E: the TearDownServer path: re-register on the same COM object and add again.
+            int server2 = mx.Register("MxGatewayProbe166");
+            int e1 = mx.AddItem(server2, AckItem);
+            Log($"E: re-Register -> server handle {server2}; AddItem {AckItem} -> {e1}");
+            mx.RemoveItem(server2, e1);
+            mx.Unregister(server2);
+        }
+        finally
+        {
+            Marshal.FinalReleaseComObject(com);
+        }
+
+        // F: a fresh COM object, as AlarmAckWriter creates after a teardown.
+        LMXProxyServerClass com2 = new LMXProxyServerClass();
+        try
+        {
+            IMxAccessServer mx2 = new MxAccessComServer(com2);
+            int server3 = mx2.Register("MxGatewayProbe166");
+            int f1 = mx2.AddItem(server3, AckItem);
+            Log($"F: new COM object Register -> server handle {server3}; AddItem {AckItem} -> {f1}");
+            mx2.RemoveItem(server3, f1);
+            mx2.Unregister(server3);
+        }
+        finally
+        {
+            Marshal.FinalReleaseComObject(com2);
+        }
+    }
+
+    private void Log(string line)
+    {
+        log.Add(DateTime.Now.ToString("HH:mm:ss.fff") + " " + line);
+    }
+}