diff --git a/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/AlarmAckWriterTests.cs b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/AlarmAckWriterTests.cs
new file mode 100644
index 0000000..ce400c4
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/AlarmAckWriterTests.cs
@@ -0,0 +1,184 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.InteropServices;
+using Google.Protobuf.Collections;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+using ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+namespace ZB.MOM.WW.MxGateway.Worker.Tests.MxAccess;
+
+public sealed class AlarmAckWriterTests
+{
+    private static AlarmAckOperator Alice() =>
+        new AlarmAckOperator { UserName = "alice", Domain = "CORP", Password = "pw-secret" };
+
+    private static RepeatedField<MxStatusProxy> Rows(MxStatusCategory category, int detail) =>
+        new RepeatedField<MxStatusProxy> { new MxStatusProxy { Success = category == MxStatusCategory.Ok ? 1 : 0, Category = category, Detail = detail } };
+
+    [Fact]
+    public void Acknowledge_AuthenticatesThenWritesCommentWithOperatorUserId()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        MxAccessWriteCompletionCache cache = new MxAccessWriteCompletionCache();
+        server.OnWrite = (handle, item) => cache.Record(handle, item, Rows(MxStatusCategory.Ok, 0));
+        using AlarmAckWriter writer = new AlarmAckWriter(server, cache, () => { }, TimeSpan.FromSeconds(1), "MxGateway");
+
+        AlarmAckResult result = writer.Acknowledge("T.A.AckMsg", "checked", Alice());
+
+        Assert.Equal(AlarmAckOutcome.Applied, result.Outcome);
+        Assert.Equal(0, result.NativeStatus);
+        Assert.Equal(new[] { "Register:MxGateway", "AuthenticateUser:CORP\\alice", "AddItem:T.A.AckMsg", "Advise", "Write:checked:5" }, server.Operations);
+        Assert.Equal("pw-secret", server.LastPassword);
+    }
+
+    [Fact]
+    public void Acknowledge_WhenWriteRejected_ReturnsDetailAndStatuses()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        MxAccessWriteCompletionCache cache = new MxAccessWriteCompletionCache();
+        server.OnWrite = (handle, item) => cache.Record(handle, item, Rows(MxStatusCategory.SecurityError, 1008));
+        using AlarmAckWriter writer = new AlarmAckWriter(server, cache, () => { }, TimeSpan.FromSeconds(1), "MxGateway");
+
+        AlarmAckResult result = writer.Acknowledge("T.A.AckMsg", "checked", Alice());
+
+        Assert.Equal(AlarmAckOutcome.WriteRejected, result.Outcome);
+        Assert.Equal(1008, result.NativeStatus);
+        Assert.Single(result.Statuses);
+    }
+
+    [Fact]
+    public void Acknowledge_WhenNoCompletion_ReturnsUnconfirmed()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        using AlarmAckWriter writer = new AlarmAckWriter(server, new MxAccessWriteCompletionCache(), () => { }, TimeSpan.FromMilliseconds(50), "MxGateway");
+
+        AlarmAckResult result = writer.Acknowledge("T.A.AckMsg", "checked", Alice());
+
+        Assert.Equal(AlarmAckOutcome.Unconfirmed, result.Outcome);
+        Assert.Equal(0, result.NativeStatus);
+    }
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void Acknowledge_WhenAuthenticationFails_DoesNotWriteOrLeakPassword(bool throwComException)
+    {
+        RecordingAckServer server = new RecordingAckServer
+        {
+            AuthenticateResult = 0,
+            AuthenticateException = throwComException ? new COMException("bad credentials", unchecked((int)0x80070005)) : null,
+        };
+        using AlarmAckWriter writer = new AlarmAckWriter(server, new MxAccessWriteCompletionCache(), () => { }, TimeSpan.FromMilliseconds(50), "MxGateway");
+
+        AlarmAckResult result = writer.Acknowledge("T.A.AckMsg", "checked", Alice());
+
+        Assert.Equal(AlarmAckOutcome.AuthenticationFailed, result.Outcome);
+        Assert.NotEqual(0, result.NativeStatus);
+        Assert.DoesNotContain(server.Operations, op => op.StartsWith("Write", StringComparison.Ordinal));
+        Assert.DoesNotContain("pw-secret", result.Diagnostic, StringComparison.Ordinal);
+        Assert.DoesNotContain(server.Operations, op => op.Contains("pw-secret"));
+    }
+
+    [Fact]
+    public void Acknowledge_Twice_ReusesItemHandle()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        using AlarmAckWriter writer = new AlarmAckWriter(server, new MxAccessWriteCompletionCache(), () => { }, TimeSpan.FromMilliseconds(10), "MxGateway");
+
+        writer.Acknowledge("T.A.AckMsg", "one", Alice());
+        writer.Acknowledge("T.A.AckMsg", "two", Alice());
+
+        Assert.Equal(1, server.Operations.Count(op => op.StartsWith("AddItem", StringComparison.Ordinal)));
+        Assert.Equal(1, server.Operations.Count(op => op == "Advise"));
+    }
+
+    [Fact]
+    public void AlarmAckOperator_ToString_OmitsPassword()
+    {
+        Assert.DoesNotContain("pw-secret", Alice().ToString(), StringComparison.Ordinal);
+    }
+}
+
+internal sealed class RecordingAckServer : IMxAccessServer
+{
+    private int nextItemHandle = 100;
+
+    public List<string> Operations { get; } = new();
+
+    public int AuthenticateResult { get; set; }
+
+    public Exception? AuthenticateException { get; set; }
+
+    public string? LastPassword { get; private set; }
+
+    public Action<int, int>? OnWrite { get; set; }
+
+    public int Register(string clientName)
+    {
+        Operations.Add("Register:" + clientName);
+        return 7;
+    }
+
+    public void Unregister(int serverHandle) => Operations.Add("Unregister");
+
+    public int AddItem(int serverHandle, string itemDefinition)
+    {
+        Operations.Add("AddItem:" + itemDefinition);
+        return nextItemHandle++;
+    }
+
+    public int AddItem2(int serverHandle, string itemDefinition, string itemContext) => AddItem(serverHandle, itemDefinition);
+
+    public void RemoveItem(int serverHandle, int itemHandle) => Operations.Add("RemoveItem");
+
+    public void Advise(int serverHandle, int itemHandle) => Operations.Add("Advise");
+
+    public void UnAdvise(int serverHandle, int itemHandle) => Operations.Add("UnAdvise");
+
+    public void AdviseSupervisory(int serverHandle, int itemHandle)
+    {
+    }
+
+    public void Write(int serverHandle, int itemHandle, object? value, int userId)
+    {
+        Operations.Add("Write:" + value + ":" + userId);
+        OnWrite?.Invoke(serverHandle, itemHandle);
+    }
+
+    public void Write2(int serverHandle, int itemHandle, object? value, object? timestamp, int userId)
+    {
+    }
+
+    public void WriteSecured(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value)
+    {
+    }
+
+    public void WriteSecured2(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value, object? timestamp)
+    {
+    }
+
+    public object Suspend(int serverHandle, int itemHandle) => new object();
+
+    public object Activate(int serverHandle, int itemHandle) => new object();
+
+    public int AuthenticateUser(int serverHandle, string verifyUser, string verifyUserPassword)
+    {
+        Operations.Add("AuthenticateUser:" + verifyUser);
+        LastPassword = verifyUserPassword;
+        if (AuthenticateException is not null)
+        {
+            throw AuthenticateException;
+        }
+
+        return AuthenticateResult;
+    }
+
+    public int ArchestrAUserToId(int serverHandle, string userIdGuid) => 0;
+
+    public int AddBufferedItem(int serverHandle, string itemDefinition, string itemContext) => AddItem(serverHandle, itemDefinition);
+
+    public void SetBufferedUpdateInterval(int serverHandle, int updateIntervalMilliseconds)
+    {
+    }
+}
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckOperator.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckOperator.cs
new file mode 100644
index 0000000..5caee25
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckOperator.cs
@@ -0,0 +1,24 @@
+namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+/// <summary>
+///     Operator identity an alarm acknowledgement authenticates as. The password
+///     is a raw credential: it is passed to MXAccess <c>AuthenticateUser</c> only
+///     and must never be logged or echoed.
+/// </summary>
+public sealed class AlarmAckOperator
+{
+    /// <summary>Gets or sets the operator user name.</summary>
+    public string UserName { get; set; } = string.Empty;
+
+    /// <summary>Gets or sets the optional domain for OS-authenticated galaxies.</summary>
+    public string Domain { get; set; } = string.Empty;
+
+    /// <summary>Gets or sets the operator password (credential-sensitive).</summary>
+    public string Password { get; set; } = string.Empty;
+
+    /// <summary>Gets the user string passed to <c>AuthenticateUser</c>: <c>Domain\UserName</c> when a domain is set.</summary>
+    public string VerifyUser => string.IsNullOrEmpty(Domain) ? UserName : Domain + "\\" + UserName;
+
+    /// <inheritdoc />
+    public override string ToString() => "AlarmAckOperator(" + VerifyUser + ")";
+}
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckOutcome.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckOutcome.cs
new file mode 100644
index 0000000..9f4309a
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckOutcome.cs
@@ -0,0 +1,18 @@
+namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+/// <summary>Outcome of an alarm acknowledgement attempt.</summary>
+public enum AlarmAckOutcome
+{
+    /// <summary>The AckMsg write completed with an OK status.</summary>
+    Applied,
+    /// <summary>The write was sent but no OnWriteComplete arrived before the timeout.</summary>
+    Unconfirmed,
+    /// <summary>The alarm is not in the consumer's current view.</summary>
+    AlarmNotFound,
+    /// <summary>The alarm exists but has no MXAccess ack attribute (non-Galaxy provider or no ack subtag).</summary>
+    NotAckable,
+    /// <summary>AuthenticateUser rejected the operator credentials.</summary>
+    AuthenticationFailed,
+    /// <summary>The AckMsg write completed with a non-OK status.</summary>
+    WriteRejected,
+}
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckResult.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckResult.cs
new file mode 100644
index 0000000..8f27c82
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckResult.cs
@@ -0,0 +1,49 @@
+using System.Collections.Generic;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+
+namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+/// <summary>Result of an alarm acknowledgement.</summary>
+public sealed class AlarmAckResult
+{
+    private static readonly IReadOnlyList<MxStatusProxy> NoStatuses = new MxStatusProxy[0];
+
+    private AlarmAckResult(AlarmAckOutcome outcome, int nativeStatus, string diagnostic, IReadOnlyList<MxStatusProxy>? statuses)
+    {
+        Outcome = outcome;
+        NativeStatus = nativeStatus;
+        Diagnostic = diagnostic;
+        Statuses = statuses ?? NoStatuses;
+    }
+
+    /// <summary>Gets the outcome.</summary>
+    public AlarmAckOutcome Outcome { get; }
+
+    /// <summary>Gets the native status: 0 for applied/unconfirmed, else the HRESULT or status detail.</summary>
+    public int NativeStatus { get; }
+
+    /// <summary>Gets a diagnostic message. Never contains credentials.</summary>
+    public string Diagnostic { get; }
+
+    /// <summary>Gets the OnWriteComplete statuses, empty when none was observed.</summary>
+    public IReadOnlyList<MxStatusProxy> Statuses { get; }
+
+    /// <summary>Creates a result.</summary>
+    /// <param name="outcome">The outcome.</param>
+    /// <param name="nativeStatus">The native status.</param>
+    /// <param name="diagnostic">A credential-free diagnostic.</param>
+    /// <param name="statuses">Observed write statuses.</param>
+    /// <returns>The result.</returns>
+    public static AlarmAckResult Create(
+        AlarmAckOutcome outcome,
+        int nativeStatus,
+        string diagnostic,
+        IReadOnlyList<MxStatusProxy>? statuses = null) =>
+        new AlarmAckResult(outcome, nativeStatus, diagnostic ?? string.Empty, statuses);
+
+    /// <summary>Creates the result for an unresolved ack target.</summary>
+    /// <param name="target">The unresolved target.</param>
+    /// <returns>The result.</returns>
+    public static AlarmAckResult FromTarget(AlarmAckTarget target) =>
+        new AlarmAckResult(target.Failure, -1, target.Reason, null);
+}
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckTarget.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckTarget.cs
new file mode 100644
index 0000000..89d28f5
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckTarget.cs
@@ -0,0 +1,42 @@
+namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+/// <summary>The MXAccess item an acknowledgement writes, or why none exists.</summary>
+public sealed class AlarmAckTarget
+{
+    private AlarmAckTarget(string itemAddress, AlarmAckOutcome failure, string reason)
+    {
+        ItemAddress = itemAddress;
+        Failure = failure;
+        Reason = reason;
+    }
+
+    /// <summary>Gets a value indicating whether an ack item was resolved.</summary>
+    public bool Found => ItemAddress.Length > 0;
+
+    /// <summary>Gets the ack item address (e.g. <c>TestMachine_001.TestAlarm001.AckMsg</c>), empty when not found.</summary>
+    public string ItemAddress { get; }
+
+    /// <summary>Gets the failure outcome when <see cref="Found"/> is false.</summary>
+    public AlarmAckOutcome Failure { get; }
+
+    /// <summary>Gets a human-readable reason when <see cref="Found"/> is false.</summary>
+    public string Reason { get; }
+
+    /// <summary>Creates a resolved target.</summary>
+    /// <param name="itemAddress">The ack item address.</param>
+    /// <returns>The target.</returns>
+    public static AlarmAckTarget For(string itemAddress) =>
+        new AlarmAckTarget(itemAddress ?? string.Empty, AlarmAckOutcome.Applied, string.Empty);
+
+    /// <summary>Creates a not-found result.</summary>
+    /// <param name="reason">Why the alarm was not found.</param>
+    /// <returns>The target.</returns>
+    public static AlarmAckTarget NotFound(string reason) =>
+        new AlarmAckTarget(string.Empty, AlarmAckOutcome.AlarmNotFound, reason);
+
+    /// <summary>Creates a not-ackable result.</summary>
+    /// <param name="reason">Why the alarm cannot be acknowledged over MXAccess.</param>
+    /// <returns>The target.</returns>
+    public static AlarmAckTarget NotAckable(string reason) =>
+        new AlarmAckTarget(string.Empty, AlarmAckOutcome.NotAckable, reason);
+}
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckWriter.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckWriter.cs
new file mode 100644
index 0000000..87a20b5
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/AlarmAckWriter.cs
@@ -0,0 +1,232 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using ArchestrA.MxAccess;
+using Google.Protobuf.Collections;
+using ZB.MOM.WW.MxGateway.Contracts;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+using ZB.MOM.WW.MxGateway.Worker.Conversion;
+
+namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+/// <summary>
+///     Acknowledges alarms the way the AVEVA runtime records operator identity:
+///     AuthenticateUser as the operator, then write the comment to the alarm's
+///     <c>AckMsg</c> attribute with that user id. Owns its own
+///     <c>LMXProxyServerClass</c> registered under the configured client name,
+///     which the Historian records as the acknowledgement's OperatorNode.
+///     Apartment-threaded: every call must run on the worker STA. The password
+///     is passed to MXAccess only; it never reaches diagnostics.
+/// </summary>
+public sealed class AlarmAckWriter : IAlarmAckWriter
+{
+    /// <summary>The ArchestrA alarm-primitive attribute whose write acknowledges the alarm.</summary>
+    public const string AckCommentAttribute = "AckMsg";
+
+    /// <summary>Default MXAccess client name when none is configured.</summary>
+    public const string DefaultClientName = "MxGateway";
+
+    private readonly IMxAccessComObjectFactory? factory;
+    private readonly MxAccessWriteCompletionCache completionCache;
+    private readonly Action pumpStep;
+    private readonly TimeSpan completionTimeout;
+    private readonly string clientName;
+    private readonly MxStatusProxyConverter statusConverter = new MxStatusProxyConverter();
+    private readonly Dictionary<string, int> itemHandles = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase);
+    private object? comObject;
+    private LMXProxyServerClass? comEventSource;
+    private IMxAccessServer? server;
+    private int serverHandle;
+    private bool disposed;
+
+    /// <summary>Creates a writer that registers its own MXAccess server on first use.</summary>
+    /// <param name="factory">Creates the <c>LMXProxyServerClass</c>.</param>
+    /// <param name="pumpStep">Drains the STA message queue (OnWriteComplete is delivered by pumping).</param>
+    /// <param name="completionTimeout">Bounded wait for OnWriteComplete.</param>
+    /// <param name="clientName">MXAccess client name (Historian OperatorNode).</param>
+    public AlarmAckWriter(
+        IMxAccessComObjectFactory factory,
+        Action pumpStep,
+        TimeSpan completionTimeout,
+        string? clientName)
+    {
+        this.factory = factory ?? throw new ArgumentNullException(nameof(factory));
+        this.pumpStep = pumpStep ?? throw new ArgumentNullException(nameof(pumpStep));
+        this.completionTimeout = completionTimeout;
+        this.clientName = string.IsNullOrWhiteSpace(clientName) ? DefaultClientName : clientName!;
+        completionCache = new MxAccessWriteCompletionCache();
+    }
+
+    /// <summary>Test seam: uses an already-constructed server and completion cache.</summary>
+    internal AlarmAckWriter(
+        IMxAccessServer server,
+        MxAccessWriteCompletionCache completionCache,
+        Action pumpStep,
+        TimeSpan completionTimeout,
+        string clientName)
+    {
+        this.server = server ?? throw new ArgumentNullException(nameof(server));
+        this.completionCache = completionCache ?? throw new ArgumentNullException(nameof(completionCache));
+        this.pumpStep = pumpStep ?? throw new ArgumentNullException(nameof(pumpStep));
+        this.completionTimeout = completionTimeout;
+        this.clientName = clientName;
+        serverHandle = server.Register(clientName);
+    }
+
+    /// <summary>Resolves the configured client name from the launcher environment.</summary>
+    /// <returns>The client name, or <see cref="DefaultClientName"/>.</returns>
+    public static string ResolveClientName()
+    {
+        string? value = Environment.GetEnvironmentVariable(WorkerLaunchEnvironment.AlarmAckClientName);
+        return string.IsNullOrWhiteSpace(value) ? DefaultClientName : value!.Trim();
+    }
+
+    /// <inheritdoc />
+    public AlarmAckResult Acknowledge(string itemAddress, string comment, AlarmAckOperator ackOperator)
+    {
+        if (disposed) throw new ObjectDisposedException(nameof(AlarmAckWriter));
+        if (string.IsNullOrWhiteSpace(itemAddress)) throw new ArgumentException("Ack item address is required.", nameof(itemAddress));
+        if (ackOperator is null) throw new ArgumentNullException(nameof(ackOperator));
+
+        IMxAccessServer mx = EnsureRegistered();
+
+        int userId;
+        try
+        {
+            userId = mx.AuthenticateUser(serverHandle, ackOperator.VerifyUser, ackOperator.Password ?? string.Empty);
+        }
+        catch (COMException ex)
+        {
+            return AlarmAckResult.Create(
+                AlarmAckOutcome.AuthenticationFailed,
+                ex.HResult,
+                $"Operator authentication failed for '{ackOperator.VerifyUser}' (HRESULT 0x{ex.HResult:X8}).");
+        }
+
+        if (userId == 0)
+        {
+            return AlarmAckResult.Create(
+                AlarmAckOutcome.AuthenticationFailed,
+                -1,
+                $"Operator authentication failed for '{ackOperator.VerifyUser}' (no user id returned).");
+        }
+
+        if (!itemHandles.TryGetValue(itemAddress, out int itemHandle))
+        {
+            itemHandle = mx.AddItem(serverHandle, itemAddress);
+            // MXAccess rejects writes to added-but-unadvised items with E_INVALIDARG.
+            mx.Advise(serverHandle, itemHandle);
+            itemHandles[itemAddress] = itemHandle;
+        }
+
+        pumpStep();
+        ulong baseline = completionCache.CurrentVersion(serverHandle, itemHandle);
+        mx.Write(serverHandle, itemHandle, comment ?? string.Empty, userId);
+
+        if (!completionCache.TryWaitForCompletion(
+                serverHandle,
+                itemHandle,
+                baseline,
+                DateTime.UtcNow + completionTimeout,
+                pumpStep,
+                out RepeatedField<MxStatusProxy> statuses))
+        {
+            return AlarmAckResult.Create(
+                AlarmAckOutcome.Unconfirmed,
+                0,
+                $"AckMsg write to '{itemAddress}' sent; OnWriteComplete not observed within {completionTimeout.TotalMilliseconds:F0} ms.");
+        }
+
+        foreach (MxStatusProxy status in statuses)
+        {
+            if (status.Category != MxStatusCategory.Ok)
+            {
+                return AlarmAckResult.Create(
+                    AlarmAckOutcome.WriteRejected,
+                    status.Detail != 0 ? status.Detail : -1,
+                    $"AckMsg write to '{itemAddress}' rejected: category {status.Category}, detail {status.Detail}.",
+                    statuses);
+            }
+        }
+
+        return AlarmAckResult.Create(AlarmAckOutcome.Applied, 0, string.Empty, statuses);
+    }
+
+    /// <inheritdoc />
+    public void Dispose()
+    {
+        if (disposed) return;
+        disposed = true;
+
+        if (comEventSource is not null)
+        {
+            try { comEventSource.OnWriteComplete -= OnWriteComplete; } catch { /* best effort */ }
+        }
+
+        IMxAccessServer? mx = server;
+        if (mx is not null)
+        {
+            foreach (int itemHandle in itemHandles.Values)
+            {
+                try { mx.UnAdvise(serverHandle, itemHandle); } catch { /* best effort */ }
+                try { mx.RemoveItem(serverHandle, itemHandle); } catch { /* best effort */ }
+            }
+
+            try { mx.Unregister(serverHandle); } catch { /* best effort */ }
+        }
+
+        itemHandles.Clear();
+        server = null;
+        comEventSource = null;
+        object? toRelease = comObject;
+        comObject = null;
+        if (toRelease is not null && Marshal.IsComObject(toRelease))
+        {
+            try { Marshal.FinalReleaseComObject(toRelease); } catch { /* best effort */ }
+        }
+    }
+
+    private IMxAccessServer EnsureRegistered()
+    {
+        if (server is not null) return server;
+
+        object created = factory!.Create() ?? throw new InvalidOperationException("MXAccess COM factory returned null.");
+        try
+        {
+            comObject = created;
+            if (created is LMXProxyServerClass comServer)
+            {
+                comEventSource = comServer;
+                comServer.OnWriteComplete += OnWriteComplete;
+            }
+
+            IMxAccessServer mx = new MxAccessComServer(created);
+            serverHandle = mx.Register(clientName);
+            server = mx;
+            return mx;
+        }
+        catch
+        {
+            if (comEventSource is not null)
+            {
+                try { comEventSource.OnWriteComplete -= OnWriteComplete; } catch { /* best effort */ }
+                comEventSource = null;
+            }
+
+            if (Marshal.IsComObject(created))
+            {
+                try { Marshal.FinalReleaseComObject(created); } catch { /* best effort */ }
+            }
+
+            comObject = null;
+            throw;
+        }
+    }
+
+    private void OnWriteComplete(int hLMXServerHandle, int phItemHandle, ref MXSTATUS_PROXY[] pVars)
+    {
+        RepeatedField<MxStatusProxy> rows = new RepeatedField<MxStatusProxy>();
+        rows.AddRange(statusConverter.ConvertMany(pVars));
+        completionCache.Record(hLMXServerHandle, phItemHandle, rows);
+    }
+}
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/IAlarmAckWriter.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/IAlarmAckWriter.cs
new file mode 100644
index 0000000..b5877be
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/IAlarmAckWriter.cs
@@ -0,0 +1,17 @@
+using System;
+
+namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+/// <summary>
+///     Acknowledges an alarm by writing its MXAccess ack item (<c>AckMsg</c>)
+///     after authenticating as the operator. Must be called on the worker STA.
+/// </summary>
+public interface IAlarmAckWriter : IDisposable
+{
+    /// <summary>Authenticates <paramref name="ackOperator"/> and writes <paramref name="comment"/> to <paramref name="itemAddress"/>.</summary>
+    /// <param name="itemAddress">The ack item, e.g. <c>Obj.Alarm.AckMsg</c>.</param>
+    /// <param name="comment">The acknowledgement comment.</param>
+    /// <param name="ackOperator">The operator to authenticate as.</param>
+    /// <returns>The outcome.</returns>
+    AlarmAckResult Acknowledge(string itemAddress, string comment, AlarmAckOperator ackOperator);
+}