diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Session/UpstreamValueMapping.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Session/UpstreamValueMapping.cs
index 3e69b552..c00d5c92 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Session/UpstreamValueMapping.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Session/UpstreamValueMapping.cs
@@ -8,7 +8,12 @@ public static class UpstreamValueMapping
     /// <summary>Materializes scalar and one-dimensional array values without flattening matrices.</summary>
     /// <param name="value">The upstream value.</param>
     /// <returns>The independent CLR payload.</returns>
-    public static object? ToClr(Variant value) => Detach(value.AsBoxedObject(Variant.BoxingBehavior.Legacy));
+    public static object? ToClr(Variant value)
+    {
+        if (value.TypeInfo.IsMatrix)
+            throw new NotSupportedException("Matrix payloads are not supported by the OPC UA driver.");
+        return Detach(value.AsBoxedObject(Variant.BoxingBehavior.Legacy));
+    }
 
     private static object? Detach(object? value) => value switch
     {
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs
index c705200e..da89da04 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs
@@ -548,7 +548,7 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
             // G-2b: resolve any secret:-prefixed Password / UserCertificatePassword through the
             // shared secret store ONCE, here in the async connect flow, just before the credentials
             // are consumed. _options stays the raw config (with secret: refs) — only this
-            // connect-scoped local carries plaintext, and only for the duration of the connect.
+            // session identity carries the resolved credential while activation/signing needs it.
             // Resolving on every InitializeAsync (a full reconnect calls it) picks up rotations.
             // Both credential consumers — the Username password below and the Certificate password
             // in BuildCertificateIdentity — flow through BuildUserIdentity(resolvedOptions), so a
@@ -1474,11 +1474,7 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
                     // Preserve the upstream StatusCode verbatim — including Bad codes per
                     // §8's cascading-quality rule. Also preserve SourceTimestamp so downstream
                     // clients can detect stale upstream data.
-                    results[r] = new DataValueSnapshot(
-                        Value: UpstreamValueMapping.ToClr(dv.WrappedValue),
-                        StatusCode: dv.StatusCode.Code,
-                        SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp.ToDateTime(),
-                        ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? now : dv.ServerTimestamp.ToDateTime());
+                    results[r] = ToSnapshot(dv, now);
                 }
                 RecordDeviceAnswered(generation, now);
             }
@@ -2104,6 +2100,19 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
         PublishNotification(handle, RefOf(item), dv);
     }
 
+    private static DataValueSnapshot ToSnapshot(DataValue value, DateTime now)
+    {
+        // Matrix payloads have no driver-domain contract. SDK 2's Legacy boxing materializes
+        // them into CLR matrices, so refuse that unsupported shape before boxing. 1D arrays
+        // retain their deployed pass-through behavior, including scalar-shaped authored tags.
+        var unsupportedShape = value.WrappedValue.TypeInfo.IsMatrix;
+        return new DataValueSnapshot(
+            Value: unsupportedShape ? null : UpstreamValueMapping.ToClr(value.WrappedValue),
+            StatusCode: unsupportedShape ? StatusCodes.BadTypeMismatch.Code : value.StatusCode.Code,
+            SourceTimestampUtc: value.SourceTimestamp == DateTimeUtc.MinValue ? null : value.SourceTimestamp.ToDateTime(),
+            ServerTimestampUtc: value.ServerTimestamp == DateTimeUtc.MinValue ? now : value.ServerTimestamp.ToDateTime());
+    }
+
     private static string RefOf(MonitoredItem item) => (item.Handle as string) ?? item.DisplayName ?? string.Empty;
 
     private void PublishNotification(OpcUaSubscriptionHandle handle, string fullRef, DataValue dv)
@@ -2113,11 +2122,7 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
         // a delivered notification is liveness evidence — the subscription path never calls ReadAsync, so
         // without this a busy subscription-only driver's LastSuccessfulRead stayed at its connect time.
         RecordLivenessEvidence();
-        var snapshot = new DataValueSnapshot(
-            Value: UpstreamValueMapping.ToClr(dv.WrappedValue),
-            StatusCode: dv.StatusCode.Code,
-            SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp.ToDateTime(),
-            ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? DateTime.UtcNow : dv.ServerTimestamp.ToDateTime());
+        var snapshot = ToSnapshot(dv, DateTime.UtcNow);
         // Review of 3b88056f: the replay check and the publish are one critical section shared with the recovery and
         // trailing reads, so an older value can never be delivered after a newer one (see _deliveryLock).
         lock (_deliveryLock)