diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests/WonderwareHistorianClientTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests/WonderwareHistorianClientTests.cs
index 0689f0dd..503d6b41 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests/WonderwareHistorianClientTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests/WonderwareHistorianClientTests.cs
@@ -361,6 +361,73 @@ public sealed class WonderwareHistorianClientTests
         outcomes[0].ShouldBe(HistorianWriteOutcome.Ack);
     }
 
+    /// <summary>
+    ///     When <c>PerEventStatus</c> is present but its length does not equal the batch size,
+    ///     the client must ignore it and fall back to the legacy <c>PerEventOk</c> path to
+    ///     avoid mis-indexing into the status array. Here a 2-event batch receives
+    ///     <c>PerEventStatus=[1]</c> (length 1) but <c>PerEventOk=[true, false]</c>; the
+    ///     outcomes must reflect the PerEventOk values ([Ack, RetryPlease]), not the status
+    ///     byte (which would have produced [RetryPlease] had it been used).
+    /// </summary>
+    [Fact]
+    public async Task WriteBatchAsync_PerEventStatusLengthMismatch_FallsBackToPerEventOk()
+    {
+        await using var server = new FakeSidecarServer(Secret)
+        {
+            OnWriteAlarmEvents = _ => new WriteAlarmEventsReply
+            {
+                Success = true,
+                PerEventStatus = [1],          // length 1 ≠ batch count 2 → must be ignored
+                PerEventOk = [true, false],    // legacy fallback: true→Ack, false→RetryPlease
+            },
+        };
+        await server.StartAsync();
+
+        await using var client = TcpClientFor(server);
+        var batch = new[]
+        {
+            new AlarmHistorianEvent("ev-1", "Tank/HiHi", "HiHi", "LimitAlarm", AlarmSeverity.High, "Activated",    "msg", "u", null, DateTime.UtcNow),
+            new AlarmHistorianEvent("ev-2", "Tank/HiHi", "HiHi", "LimitAlarm", AlarmSeverity.High, "Acknowledged", "msg", "u", null, DateTime.UtcNow),
+        };
+
+        var outcomes = await client.WriteBatchAsync(batch, CancellationToken.None);
+
+        outcomes.Count.ShouldBe(2);
+        outcomes[0].ShouldBe(HistorianWriteOutcome.Ack);         // PerEventOk[0] = true
+        outcomes[1].ShouldBe(HistorianWriteOutcome.RetryPlease); // PerEventOk[1] = false
+    }
+
+    /// <summary>
+    ///     Status byte 1 (the only value that is neither 0 nor 2) must map to
+    ///     <see cref="HistorianWriteOutcome.RetryPlease"/> via the default arm of the
+    ///     <c>PerEventStatus</c> switch. A single-event batch with <c>PerEventStatus=[1]</c>
+    ///     (length matches batch) must yield <c>[RetryPlease]</c>.
+    /// </summary>
+    [Fact]
+    public async Task WriteBatchAsync_PerEventStatusRetry_MapsToRetryPlease()
+    {
+        await using var server = new FakeSidecarServer(Secret)
+        {
+            OnWriteAlarmEvents = _ => new WriteAlarmEventsReply
+            {
+                Success = true,
+                PerEventStatus = [1], // status 1 → RetryPlease
+            },
+        };
+        await server.StartAsync();
+
+        await using var client = TcpClientFor(server);
+        var batch = new[]
+        {
+            new AlarmHistorianEvent("ev-retry", "Tank/HiHi", "HiHi", "LimitAlarm", AlarmSeverity.High, "Activated", "msg", "u", null, DateTime.UtcNow),
+        };
+
+        var outcomes = await client.WriteBatchAsync(batch, CancellationToken.None);
+
+        outcomes.Count.ShouldBe(1);
+        outcomes[0].ShouldBe(HistorianWriteOutcome.RetryPlease);
+    }
+
     /// <summary>
     ///     Rolling-deploy back-compat: an older sidecar that sends an empty PerEventStatus but a
     ///     populated PerEventOk must still classify via the legacy bool path (false→RetryPlease).