diff --git a/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/TemplateCommands.cs b/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/TemplateCommands.cs
index d34dd51a..875d3758 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/TemplateCommands.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/TemplateCommands.cs
@@ -8,8 +8,8 @@ public record DeleteTemplateCommand(int TemplateId);
 public record ValidateTemplateCommand(int TemplateId);
 
 // Template member operations
-public record AddTemplateAttributeCommand(int TemplateId, string Name, string DataType, string? Value, string? Description, string? DataSourceReference, bool IsLocked);
-public record UpdateTemplateAttributeCommand(int AttributeId, string Name, string DataType, string? Value, string? Description, string? DataSourceReference, bool IsLocked);
+public record AddTemplateAttributeCommand(int TemplateId, string Name, string DataType, string? Value, string? Description, string? DataSourceReference, bool IsLocked, string? ElementDataType = null);
+public record UpdateTemplateAttributeCommand(int AttributeId, string Name, string DataType, string? Value, string? Description, string? DataSourceReference, bool IsLocked, string? ElementDataType = null);
 public record DeleteTemplateAttributeCommand(int AttributeId);
 public record AddTemplateAlarmCommand(int TemplateId, string Name, string TriggerType, int PriorityLevel, string? Description, string? TriggerConfiguration, bool IsLocked);
 public record UpdateTemplateAlarmCommand(int AlarmId, string Name, string TriggerType, int PriorityLevel, string? Description, string? TriggerConfiguration, bool IsLocked);
diff --git a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
index 8456c69e..a99a0729 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs
@@ -1442,9 +1442,13 @@ public class ManagementActor : ReceiveActor
     private static async Task<object?> HandleAddAttribute(IServiceProvider sp, AddTemplateAttributeCommand cmd, string user)
     {
         var svc = sp.GetRequiredService<TemplateService>();
+        var dataType = Enum.Parse<Commons.Types.Enums.DataType>(cmd.DataType, ignoreCase: true);
+        var elementType = ParseElementDataType(cmd.ElementDataType);
+        ValidateAttributeTypes(cmd.Name, dataType, elementType, cmd.Value);
         var attr = new TemplateAttribute(cmd.Name)
         {
-            DataType = Enum.Parse<ZB.MOM.WW.ScadaBridge.Commons.Types.Enums.DataType>(cmd.DataType, ignoreCase: true),
+            DataType = dataType,
+            ElementDataType = elementType,
             Value = cmd.Value,
             Description = cmd.Description,
             DataSourceReference = cmd.DataSourceReference,
@@ -1457,9 +1461,13 @@ public class ManagementActor : ReceiveActor
     private static async Task<object?> HandleUpdateAttribute(IServiceProvider sp, UpdateTemplateAttributeCommand cmd, string user)
     {
         var svc = sp.GetRequiredService<TemplateService>();
+        var dataType = Enum.Parse<Commons.Types.Enums.DataType>(cmd.DataType, ignoreCase: true);
+        var elementType = ParseElementDataType(cmd.ElementDataType);
+        ValidateAttributeTypes(cmd.Name, dataType, elementType, cmd.Value);
         var attr = new TemplateAttribute(cmd.Name)
         {
-            DataType = Enum.Parse<ZB.MOM.WW.ScadaBridge.Commons.Types.Enums.DataType>(cmd.DataType, ignoreCase: true),
+            DataType = dataType,
+            ElementDataType = elementType,
             Value = cmd.Value,
             Description = cmd.Description,
             DataSourceReference = cmd.DataSourceReference,
@@ -1469,6 +1477,57 @@ public class ManagementActor : ReceiveActor
         return result.IsSuccess ? result.Value : throw new ManagementCommandException(result.Error);
     }
 
+    /// <summary>
+    /// Parses an optional element data type token. Returns null when the token is
+    /// empty/whitespace; throws <see cref="ManagementCommandException"/> on an
+    /// unrecognised type name.
+    /// </summary>
+    private static Commons.Types.Enums.DataType? ParseElementDataType(string? elementDataType)
+    {
+        if (string.IsNullOrWhiteSpace(elementDataType)) return null;
+        if (!Enum.TryParse<Commons.Types.Enums.DataType>(elementDataType, ignoreCase: true, out var parsed))
+            throw new ManagementCommandException($"Unrecognised element type '{elementDataType}'.");
+        return parsed;
+    }
+
+    /// <summary>
+    /// Validates the (DataType, ElementDataType, Value) triple shared by the add
+    /// and update attribute handlers. Throws <see cref="ManagementCommandException"/>
+    /// on any violation:
+    /// <list type="bullet">
+    /// <item>List attributes require a valid scalar element type.</item>
+    /// <item>Scalar attributes may not carry an element type.</item>
+    /// <item>A List default value must decode against the declared element type.</item>
+    /// </list>
+    /// </summary>
+    private static void ValidateAttributeTypes(
+        string name, Commons.Types.Enums.DataType dataType, Commons.Types.Enums.DataType? elementType, string? value)
+    {
+        if (dataType == Commons.Types.Enums.DataType.List)
+        {
+            if (elementType is null || !Commons.Types.AttributeValueCodec.IsValidElementType(elementType.Value))
+                throw new ManagementCommandException(
+                    $"List attribute '{name}' requires a valid element type (String, Int32, Float, Double, Boolean, DateTime).");
+
+            if (!string.IsNullOrWhiteSpace(value))
+            {
+                try
+                {
+                    Commons.Types.AttributeValueCodec.Decode(value, Commons.Types.Enums.DataType.List, elementType);
+                }
+                catch (FormatException ex)
+                {
+                    throw new ManagementCommandException(
+                        $"List attribute '{name}' has an invalid list value: {ex.Message}");
+                }
+            }
+        }
+        else if (elementType is not null)
+        {
+            throw new ManagementCommandException("Element type is only valid on List attributes.");
+        }
+    }
+
     private static async Task<object?> HandleDeleteAttribute(IServiceProvider sp, DeleteTemplateAttributeCommand cmd, string user)
     {
         var svc = sp.GetRequiredService<TemplateService>();
diff --git a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/ManagementActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/ManagementActorTests.cs
index 770fa3dc..da5eea7b 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/ManagementActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/ManagementActorTests.cs
@@ -387,6 +387,140 @@ public class ManagementActorTests : TestKit, IDisposable
         Assert.Contains("Designer", response.Message);
     }
 
+    // ========================================================================
+    // MV-10: ElementDataType accept + validate on attribute add/update
+    // ========================================================================
+
+    [Fact]
+    public void AddListAttribute_WithStringElementType_PersistsBothColumns()
+    {
+        // A template exists with no attributes; AddAttributeAsync will save the
+        // entity built by the handler. Capture it to assert the persisted shape.
+        var template = new Template("T1") { Id = 1 };
+        _templateRepo.GetTemplateByIdAsync(1, Arg.Any<CancellationToken>()).Returns(template);
+        _templateRepo.GetAllTemplatesAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<Template> { template });
+
+        TemplateAttribute? saved = null;
+        _templateRepo
+            .When(r => r.AddTemplateAttributeAsync(Arg.Any<TemplateAttribute>(), Arg.Any<CancellationToken>()))
+            .Do(ci => saved = ci.Arg<TemplateAttribute>());
+        _templateRepo.SaveChangesAsync(Arg.Any<CancellationToken>()).Returns(1);
+        _services.AddScoped<TemplateService>();
+
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new AddTemplateAttributeCommand(1, "Tags", "List", "[\"a\",\"b\"]", null, null, false, "String"),
+            "Designer");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.Equal(envelope.CorrelationId, response.CorrelationId);
+        Assert.NotNull(saved);
+        Assert.Equal(Commons.Types.Enums.DataType.List, saved!.DataType);
+        Assert.Equal(Commons.Types.Enums.DataType.String, saved.ElementDataType);
+        Assert.Equal("[\"a\",\"b\"]", saved.Value);
+    }
+
+    [Fact]
+    public void AddListAttribute_WithNoElementType_ReturnsManagementError()
+    {
+        _services.AddScoped<TemplateService>();
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new AddTemplateAttributeCommand(1, "Tags", "List", null, null, null, false, null),
+            "Designer");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementError>(TimeSpan.FromSeconds(5));
+        Assert.Equal("COMMAND_FAILED", response.ErrorCode);
+        Assert.Contains("requires a valid element type", response.Error);
+    }
+
+    [Fact]
+    public void AddListAttribute_WithBinaryElementType_ReturnsManagementError()
+    {
+        _services.AddScoped<TemplateService>();
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new AddTemplateAttributeCommand(1, "Tags", "List", null, null, null, false, "Binary"),
+            "Designer");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementError>(TimeSpan.FromSeconds(5));
+        Assert.Equal("COMMAND_FAILED", response.ErrorCode);
+        Assert.Contains("requires a valid element type", response.Error);
+    }
+
+    [Fact]
+    public void AddScalarAttribute_WithElementType_ReturnsManagementError()
+    {
+        _services.AddScoped<TemplateService>();
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new AddTemplateAttributeCommand(1, "Count", "Int32", null, null, null, false, "String"),
+            "Designer");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementError>(TimeSpan.FromSeconds(5));
+        Assert.Equal("COMMAND_FAILED", response.ErrorCode);
+        Assert.Contains("only valid on List attributes", response.Error);
+    }
+
+    [Fact]
+    public void AddListAttribute_WithMalformedDefault_ReturnsManagementError()
+    {
+        _services.AddScoped<TemplateService>();
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new AddTemplateAttributeCommand(1, "Tags", "List", "[\"a\"", null, null, false, "String"),
+            "Designer");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementError>(TimeSpan.FromSeconds(5));
+        Assert.Equal("COMMAND_FAILED", response.ErrorCode);
+        Assert.Contains("invalid list value", response.Error);
+    }
+
+    [Fact]
+    public void AddListAttribute_WithTypeMismatchedDefault_ReturnsManagementError()
+    {
+        _services.AddScoped<TemplateService>();
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new AddTemplateAttributeCommand(1, "Counts", "List", "[\"x\"]", null, null, false, "Int32"),
+            "Designer");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementError>(TimeSpan.FromSeconds(5));
+        Assert.Equal("COMMAND_FAILED", response.ErrorCode);
+        Assert.Contains("invalid list value", response.Error);
+    }
+
+    [Fact]
+    public void UpdateScalarAttribute_WithElementType_ReturnsManagementError()
+    {
+        // Update path runs the same pre-service validation: a scalar attribute
+        // may not carry an element type, regardless of repository state.
+        _services.AddScoped<TemplateService>();
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new UpdateTemplateAttributeCommand(5, "Count", "Int32", null, null, null, false, "String"),
+            "Designer");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementError>(TimeSpan.FromSeconds(5));
+        Assert.Equal("COMMAND_FAILED", response.ErrorCode);
+        Assert.Contains("only valid on List attributes", response.Error);
+    }
+
     [Fact]
     public void UpdateApiKey_WithDesignRole_ReturnsUnauthorized()
     {