diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
index a2406255..c8bd3afa 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
@@ -431,11 +431,37 @@ public class InstanceActor : ReceiveActor
             return;
         }
 
+        // MV (C1): for a data-sourced List attribute the incoming command.Value is
+        // the canonical JSON array string (ScopeAccessors encodes the script's
+        // List<T> for transport/storage). Writing that string straight to the DCL
+        // would push a String scalar to an array node. Decode it back to a typed
+        // List<T> so the DCL/Variant write produces a real array. A non-empty value
+        // that fails to decode (malformed JSON / bad element) is poison — reject the
+        // write rather than forward garbage to the device (mirrors the static-path
+        // rejection in HandleSetStaticAttributeCore). Scalars are unchanged.
+        object? writeValue = command.Value;
+        if (IsListAttribute(resolved) && !string.IsNullOrWhiteSpace(command.Value))
+        {
+            var decoded = DecodeAttributeValue(resolved, command.Value);
+            if (decoded == null)
+            {
+                _logger.LogWarning(
+                    "SetAttribute rejected — value for data-sourced List attribute '{Attribute}' on instance '{Instance}' is not a valid list",
+                    attributeName, instanceName);
+                caller.Tell(new SetStaticAttributeResponse(
+                    correlationId, instanceName, attributeName, false,
+                    $"Invalid list value for attribute '{attributeName}'", DateTimeOffset.UtcNow));
+                return;
+            }
+
+            writeValue = decoded;
+        }
+
         var writeRequest = new WriteTagRequest(
             correlationId,
             resolved.BoundDataConnectionName!,
             resolved.DataSourceReference!,
-            command.Value,
+            writeValue,
             DateTimeOffset.UtcNow);
 
         // Ask the DCL and pipe the result back to the original caller. The DCL
diff --git a/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/RealOpcUaClientArrayWriteTests.cs b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/RealOpcUaClientArrayWriteTests.cs
index c3babe72..ab888819 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/RealOpcUaClientArrayWriteTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/RealOpcUaClientArrayWriteTests.cs
@@ -1,20 +1,44 @@
+using System.Collections.Generic;
 using Opc.Ua;
 
 namespace ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests.Adapters;
 
 /// <summary>
-/// MV-8: the OPC UA WRITE path (<see cref="RealOpcUaClient.WriteValueAsync"/>)
-/// wraps the outgoing value in <c>new Variant(value)</c> and lets the OPC
-/// Foundation SDK serialize it. For a structured multi-value (List) attribute
-/// the value handed down is a CLR array. These tests assert that the load-bearing
-/// step — wrapping an array in a <see cref="Variant"/> — succeeds without
-/// throwing, which is what the write path relies on (no separate array handling
-/// is required in our code). A full device round-trip needs a live server and is
-/// covered by the live OPC UA browse/read smoke tests.
+/// SCOPE: these tests cover ONLY the SDK-level building block the write path
+/// relies on — that <c>new Variant(collection)</c> wraps a CLR array / list as a
+/// typed array <see cref="Variant"/> (ValueRank = OneDimension) without throwing.
+/// They are NOT an end-to-end test of the runtime write flow: they feed a
+/// hand-built collection straight into <see cref="Variant"/>, bypassing the
+/// InstanceActor decode step that produces that collection.
+///
+/// The END-TO-END flow — a script's canonical JSON list string being DECODED to a
+/// typed <c>List&lt;T&gt;</c> before the <c>WriteTagRequest</c> reaches the DCL
+/// (so OPC UA writes an array node, not a String scalar) — is covered by
+/// <c>InstanceActorTests.InstanceActor_DataSourcedListWrite_SendsTypedArrayToDcl_NotJsonString</c>.
+/// The runtime hands <see cref="RealOpcUaClient.WriteValueAsync"/> a
+/// <c>List&lt;T&gt;</c> (the codec's decode result), which the SDK wraps
+/// identically to a CLR array — see the <c>List&lt;int&gt;</c> case below. A full
+/// device round-trip needs a live server and is covered by the live OPC UA smoke
+/// tests.
 /// </summary>
 [Trait("Category", "Unit")]
 public class RealOpcUaClientArrayWriteTests
 {
+    [Fact]
+    public void Variant_wraps_int_list_as_array_without_throwing()
+    {
+        // The runtime actually hands WriteValueAsync a List<T> (the decode result),
+        // not a raw T[]; assert the SDK wraps it as a typed array all the same.
+        var value = new List<int> { 10, 20, 30 };
+
+        var ex = Record.Exception(() => new Variant(value));
+
+        Assert.Null(ex);
+        var variant = new Variant(value);
+        Assert.Equal(BuiltInType.Int32, variant.TypeInfo.BuiltInType);
+        Assert.Equal(ValueRanks.OneDimension, variant.TypeInfo.ValueRank);
+    }
+
     [Fact]
     public void Variant_wraps_int_array_without_throwing()
     {
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs
index 4c18004b..f93a238e 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTests.cs
@@ -660,6 +660,94 @@ public class InstanceActorTests : TestKit, IDisposable
         Assert.Equal(1450, response.Value);
     }
 
+    /// <summary>
+    /// MV (C1 fix): a WRITE to a data-sourced <c>DataType.List</c> attribute must
+    /// send the DCL a TYPED collection (so OPC UA writes an array node), NOT the
+    /// canonical JSON string the script layer produced. The script path encodes
+    /// <c>List&lt;int&gt;</c> to <c>"[10,20,30]"</c>; HandleSetDataAttribute must
+    /// decode that back to a typed <c>List&lt;int&gt;</c> before building the
+    /// WriteTagRequest. We assert the captured WriteTagRequest.Value is the typed
+    /// list {10,20,30} — never the string "[10,20,30]".
+    /// </summary>
+    [Fact]
+    public void InstanceActor_DataSourcedListWrite_SendsTypedArrayToDcl_NotJsonString()
+    {
+        const string tag = "ns=3;s=Pump.Setpoints";
+        var config = new FlattenedConfiguration
+        {
+            InstanceUniqueName = "Pump-ListWrite",
+            Attributes =
+            [
+                new ResolvedAttribute
+                {
+                    CanonicalName = "Setpoints", Value = null,
+                    DataType = "List", ElementDataType = "Int32",
+                    DataSourceReference = tag, BoundDataConnectionName = "PLC"
+                }
+            ]
+        };
+
+        var dcl = CreateTestProbe();
+        var actor = CreateInstanceActorWithDcl("Pump-ListWrite", config, dcl);
+
+        // Script-style write: ScopeAccessors (AttributeValueCodec.Encode) has
+        // already encoded the script's List<int> to the canonical JSON array string,
+        // which is an array of element STRINGS (not raw JSON numbers).
+        actor.Tell(new SetStaticAttributeCommand(
+            "corr-write", "Pump-ListWrite", "Setpoints", "[\"10\",\"20\",\"30\"]", DateTimeOffset.UtcNow));
+
+        // The DCL must receive a WriteTagRequest carrying a TYPED collection.
+        var write = dcl.ExpectMsg<WriteTagRequest>(TimeSpan.FromSeconds(5));
+        Assert.Equal("PLC", write.ConnectionName);
+        Assert.Equal(tag, write.TagPath);
+        Assert.IsNotType<string>(write.Value);
+        var list = Assert.IsType<List<int>>(write.Value);
+        Assert.Equal(new[] { 10, 20, 30 }, list);
+
+        // Complete the Ask so the actor replies success to the caller.
+        dcl.Reply(new WriteTagResponse("corr-write", true, null, DateTimeOffset.UtcNow));
+        var response = ExpectMsg<SetStaticAttributeResponse>(TimeSpan.FromSeconds(5));
+        Assert.True(response.Success);
+    }
+
+    /// <summary>
+    /// MV (C1 fix): a malformed value written to a data-sourced List attribute
+    /// must be REJECTED before reaching the DCL — Success=false and NO
+    /// WriteTagRequest is forwarded (mirrors the static-path malformed rejection).
+    /// </summary>
+    [Fact]
+    public void InstanceActor_DataSourcedListWrite_Malformed_Rejected_NoDclWrite()
+    {
+        const string tag = "ns=3;s=Pump.Bad";
+        var config = new FlattenedConfiguration
+        {
+            InstanceUniqueName = "Pump-ListWriteBad",
+            Attributes =
+            [
+                new ResolvedAttribute
+                {
+                    CanonicalName = "Setpoints", Value = null,
+                    DataType = "List", ElementDataType = "Int32",
+                    DataSourceReference = tag, BoundDataConnectionName = "PLC"
+                }
+            ]
+        };
+
+        var dcl = CreateTestProbe();
+        var actor = CreateInstanceActorWithDcl("Pump-ListWriteBad", config, dcl);
+
+        // Malformed JSON (unterminated array, non-int element) → reject the write.
+        actor.Tell(new SetStaticAttributeCommand(
+            "corr-bad-write", "Pump-ListWriteBad", "Setpoints", "[\"a\"", DateTimeOffset.UtcNow));
+
+        var response = ExpectMsg<SetStaticAttributeResponse>(TimeSpan.FromSeconds(5));
+        Assert.False(response.Success);
+        Assert.NotNull(response.ErrorMessage);
+
+        // No write must reach the DCL.
+        dcl.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
+    }
+
     private void ExpectNoTerminated(IActorRef actor, TimeSpan within)
     {
         // The actor is Watch()ed; assert no Terminated arrives in the window.