diff --git a/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessWriteCompletionCacheTests.cs b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessWriteCompletionCacheTests.cs
new file mode 100644
index 0000000..670bc5f
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessWriteCompletionCacheTests.cs
@@ -0,0 +1,145 @@
+using System;
+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;
+
+/// <summary>
+///     Unit tests for <see cref="MxAccessWriteCompletionCache"/>. The cache is
+///     consumed by the write command executor's bounded pump-wait so a
+///     WriteSecured/WriteSecured2 reply can carry the correlated
+///     OnWriteComplete outcome; its version-baseline contract is exercised in
+///     isolation here before the STA / COM plumbing gets layered on top.
+/// </summary>
+public sealed class MxAccessWriteCompletionCacheTests
+{
+    /// <summary>Verifies that Record bumps the version per key and keys stay isolated.</summary>
+    [Fact]
+    public void Record_IncrementsVersionPerKey()
+    {
+        MxAccessWriteCompletionCache cache = new();
+
+        Assert.Equal(0UL, cache.CurrentVersion(7, 21));
+
+        cache.Record(7, 21, BuildStatuses(detail: 100));
+        Assert.Equal(1UL, cache.CurrentVersion(7, 21));
+
+        cache.Record(7, 21, BuildStatuses(detail: 200));
+        Assert.Equal(2UL, cache.CurrentVersion(7, 21));
+
+        cache.Record(7, 22, BuildStatuses(detail: 300));
+        Assert.Equal(1UL, cache.CurrentVersion(7, 22));
+        Assert.Equal(2UL, cache.CurrentVersion(7, 21));
+    }
+
+    /// <summary>Verifies that a completion newer than the baseline is returned with its status rows.</summary>
+    [Fact]
+    public void TryWaitForCompletion_WhenCompletionNewerThanBaseline_ReturnsStatuses()
+    {
+        MxAccessWriteCompletionCache cache = new();
+        cache.Record(7, 21, BuildStatuses(detail: 4321));
+
+        bool found = cache.TryWaitForCompletion(
+            7,
+            21,
+            sinceVersion: 0UL,
+            deadlineUtc: DateTime.UtcNow.AddSeconds(5),
+            pumpStep: static () => { },
+            out RepeatedField<MxStatusProxy> statuses);
+
+        Assert.True(found);
+        MxStatusProxy row = Assert.Single(statuses);
+        Assert.Equal(4321, row.Detail);
+        Assert.Equal(MxStatusCategory.Ok, row.Category);
+    }
+
+    /// <summary>
+    ///     Verifies that a completion recorded before the baseline was captured is
+    ///     never misattributed to the waiting write: only a strictly newer version
+    ///     satisfies the wait, so a stale row times the wait out.
+    /// </summary>
+    [Fact]
+    public void TryWaitForCompletion_WhenOnlyStaleCompletion_TimesOut()
+    {
+        MxAccessWriteCompletionCache cache = new();
+        cache.Record(7, 21, BuildStatuses(detail: 4321));
+        ulong baseline = cache.CurrentVersion(7, 21);
+
+        bool found = cache.TryWaitForCompletion(
+            7,
+            21,
+            sinceVersion: baseline,
+            deadlineUtc: DateTime.UtcNow.AddMilliseconds(50),
+            pumpStep: static () => { },
+            out RepeatedField<MxStatusProxy> statuses);
+
+        Assert.False(found);
+        Assert.Empty(statuses);
+    }
+
+    /// <summary>
+    ///     Verifies the pump loop is what lets a completion land: the completion is
+    ///     recorded from inside a later pump step (standing in for the STA
+    ///     dispatching the OnWriteComplete message) and the wait then succeeds.
+    /// </summary>
+    [Fact]
+    public void TryWaitForCompletion_InvokesPumpStepEachIteration()
+    {
+        MxAccessWriteCompletionCache cache = new();
+        int pumpCalls = 0;
+
+        bool found = cache.TryWaitForCompletion(
+            7,
+            21,
+            sinceVersion: 0UL,
+            deadlineUtc: DateTime.UtcNow.AddSeconds(5),
+            pumpStep: () =>
+            {
+                pumpCalls++;
+                if (pumpCalls == 2)
+                {
+                    cache.Record(7, 21, BuildStatuses(detail: 55));
+                }
+            },
+            out RepeatedField<MxStatusProxy> statuses);
+
+        Assert.True(found);
+        Assert.True(pumpCalls >= 2);
+        Assert.Equal(55, Assert.Single(statuses).Detail);
+    }
+
+    /// <summary>Verifies that Record stores an independent clone of the caller's rows.</summary>
+    [Fact]
+    public void Record_ClonesStatuses()
+    {
+        MxAccessWriteCompletionCache cache = new();
+        RepeatedField<MxStatusProxy> callerRows = BuildStatuses(detail: 77);
+
+        cache.Record(7, 21, callerRows);
+        callerRows[0].Detail = 999;
+        callerRows.Add(new MxStatusProxy());
+
+        Assert.True(cache.TryWaitForCompletion(
+            7,
+            21,
+            sinceVersion: 0UL,
+            deadlineUtc: DateTime.UtcNow.AddSeconds(5),
+            pumpStep: static () => { },
+            out RepeatedField<MxStatusProxy> statuses));
+        Assert.Equal(77, Assert.Single(statuses).Detail);
+    }
+
+    private static RepeatedField<MxStatusProxy> BuildStatuses(int detail)
+    {
+        return new RepeatedField<MxStatusProxy>
+        {
+            new MxStatusProxy
+            {
+                Success = 1,
+                Category = MxStatusCategory.Ok,
+                Detail = detail,
+            },
+        };
+    }
+}
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessWriteCompletionCache.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessWriteCompletionCache.cs
new file mode 100644
index 0000000..1bb5e35
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessWriteCompletionCache.cs
@@ -0,0 +1,152 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using Google.Protobuf.Collections;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+
+namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;
+
+/// <summary>
+///     Per-session cache of the most recent <c>OnWriteComplete</c> status rows
+///     for each (server handle, item handle) pair. Written by the MXAccess
+///     event sink as completion callbacks arrive; read by the write command
+///     executor so a WriteSecured/WriteSecured2 reply can carry the correlated
+///     MXAccess outcome instead of proving command acceptance only.
+/// </summary>
+/// <remarks>
+///     Same threading posture as <see cref="MxAccessValueCache"/>: writers and
+///     readers run on the worker's STA thread (COM dispatches events on the
+///     apartment thread; commands also execute on the STA), so no internal
+///     locking is required. A single sync root keeps it nominally thread-safe
+///     for tests that drive it from a non-STA thread.
+/// </remarks>
+public sealed class MxAccessWriteCompletionCache
+{
+    private readonly Dictionary<long, CompletionEntry> entries = new();
+    private readonly object syncRoot = new();
+
+    /// <summary>Records the status rows of a fresh OnWriteComplete callback for the given handle pair.</summary>
+    /// <param name="serverHandle">MXAccess server handle.</param>
+    /// <param name="itemHandle">MXAccess item handle.</param>
+    /// <param name="statuses">Status rows from the mapped OnWriteComplete event; cloned before storing.</param>
+    public void Record(
+        int serverHandle,
+        int itemHandle,
+        RepeatedField<MxStatusProxy> statuses)
+    {
+        if (statuses is null)
+        {
+            throw new ArgumentNullException(nameof(statuses));
+        }
+
+        lock (syncRoot)
+        {
+            long key = CreateItemKey(serverHandle, itemHandle);
+            ulong version = entries.TryGetValue(key, out CompletionEntry existing)
+                ? existing.Version + 1
+                : 1UL;
+            entries[key] = new CompletionEntry(version, statuses.Clone());
+        }
+    }
+
+    /// <summary>Returns the current completion version for a handle pair, or 0 if none was recorded.</summary>
+    /// <param name="serverHandle">MXAccess server handle.</param>
+    /// <param name="itemHandle">MXAccess item handle.</param>
+    /// <returns>The current completion version, or 0 if no completion was recorded.</returns>
+    public ulong CurrentVersion(
+        int serverHandle,
+        int itemHandle)
+    {
+        lock (syncRoot)
+        {
+            return entries.TryGetValue(CreateItemKey(serverHandle, itemHandle), out CompletionEntry existing)
+                ? existing.Version
+                : 0UL;
+        }
+    }
+
+    /// <summary>
+    ///     Polls for a completion newer than <paramref name="sinceVersion"/> until it
+    ///     arrives or the deadline elapses, calling <paramref name="pumpStep"/> on every
+    ///     poll iteration so the worker's STA can dispatch the inbound MXAccess
+    ///     OnWriteComplete message. Same loop shape as
+    ///     <see cref="MxAccessValueCache.TryWaitForUpdate"/>.
+    /// </summary>
+    /// <param name="serverHandle">MXAccess server handle.</param>
+    /// <param name="itemHandle">MXAccess item handle.</param>
+    /// <param name="sinceVersion">Version snapshot captured before the write COM call.</param>
+    /// <param name="deadlineUtc">Absolute UTC deadline.</param>
+    /// <param name="pumpStep">Action that pumps any pending Windows messages.</param>
+    /// <param name="statuses">The recorded status rows if a completion arrived before the deadline; empty otherwise.</param>
+    /// <param name="pollIntervalMs">How long to sleep between pump cycles. Default 5 ms.</param>
+    /// <returns><see langword="true"/> if a completion newer than <paramref name="sinceVersion"/> arrived before the deadline; otherwise <see langword="false"/>.</returns>
+    public bool TryWaitForCompletion(
+        int serverHandle,
+        int itemHandle,
+        ulong sinceVersion,
+        DateTime deadlineUtc,
+        Action pumpStep,
+        out RepeatedField<MxStatusProxy> statuses,
+        int pollIntervalMs = 5)
+    {
+        if (pumpStep is null)
+        {
+            throw new ArgumentNullException(nameof(pumpStep));
+        }
+
+        while (true)
+        {
+            pumpStep();
+
+            lock (syncRoot)
+            {
+                if (entries.TryGetValue(CreateItemKey(serverHandle, itemHandle), out CompletionEntry entry)
+                    && entry.Version > sinceVersion)
+                {
+                    statuses = entry.Statuses;
+                    return true;
+                }
+            }
+
+            if (DateTime.UtcNow >= deadlineUtc)
+            {
+                statuses = new RepeatedField<MxStatusProxy>();
+                return false;
+            }
+
+            Thread.Sleep(pollIntervalMs);
+        }
+    }
+
+    private static long CreateItemKey(
+        int serverHandle,
+        int itemHandle)
+    {
+        return ((long)serverHandle << 32) | (uint)itemHandle;
+    }
+
+    /// <summary>
+    ///     Snapshot of the most recent OnWriteComplete status rows for a handle
+    ///     pair. <see cref="Version"/> increments by one on every
+    ///     <see cref="Record"/> call so the write executor can detect "a new
+    ///     completion arrived since I captured my baseline".
+    /// </summary>
+    /// <remarks>
+    ///     Plain readonly struct (not a record) so this compiles under the
+    ///     worker's net48 target, which lacks <c>IsExternalInit</c>.
+    /// </remarks>
+    private readonly struct CompletionEntry
+    {
+        public CompletionEntry(
+            ulong version,
+            RepeatedField<MxStatusProxy> statuses)
+        {
+            Version = version;
+            Statuses = statuses;
+        }
+
+        public ulong Version { get; }
+
+        public RepeatedField<MxStatusProxy> Statuses { get; }
+    }
+}