diff --git a/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Services/InstanceService.cs b/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Services/InstanceService.cs
index 1c69a847..1f859bca 100644
--- a/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Services/InstanceService.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Services/InstanceService.cs
@@ -172,6 +172,7 @@ public class InstanceService
         if (existingOverride != null)
         {
             existingOverride.OverrideValue = overrideValue;
+            existingOverride.ElementDataType = templateAttr.ElementDataType;
             await _repository.UpdateInstanceAttributeOverrideAsync(existingOverride, cancellationToken);
             await _repository.SaveChangesAsync(cancellationToken);
 
@@ -185,7 +186,8 @@ public class InstanceService
             var newOverride = new InstanceAttributeOverride(attributeName)
             {
                 InstanceId = instanceId,
-                OverrideValue = overrideValue
+                OverrideValue = overrideValue,
+                ElementDataType = templateAttr.ElementDataType
             };
 
             await _repository.AddInstanceAttributeOverrideAsync(newOverride, cancellationToken);
diff --git a/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Services/InstanceServiceTests.cs b/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Services/InstanceServiceTests.cs
index b1cbc66b..b7e0a7fc 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Services/InstanceServiceTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Services/InstanceServiceTests.cs
@@ -269,6 +269,106 @@ public class InstanceServiceTests
         _repoMock.Verify(r => r.AddInstanceConnectionBindingAsync(It.IsAny<InstanceConnectionBinding>(), It.IsAny<CancellationToken>()), Times.Exactly(2));
     }
 
+    // --- NJ-2: ElementDataType stamping on SetAttributeOverrideAsync ---
+
+    [Fact]
+    public async Task SetAttributeOverride_CreatePath_ListAttribute_StampsElementDataType()
+    {
+        var instance = new Instance("Inst1") { Id = 1, TemplateId = 1 };
+        _repoMock.Setup(r => r.GetInstanceByIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(instance);
+        _repoMock.Setup(r => r.GetAttributesByTemplateIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new List<TemplateAttribute>
+            {
+                new("Tags")
+                {
+                    IsLocked = false,
+                    DataType = DataType.List,
+                    ElementDataType = DataType.String
+                }
+            });
+        _repoMock.Setup(r => r.GetOverridesByInstanceIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new List<InstanceAttributeOverride>()); // no existing override → create path
+        _repoMock.Setup(r => r.SaveChangesAsync(It.IsAny<CancellationToken>()))
+            .ReturnsAsync(1);
+
+        InstanceAttributeOverride? captured = null;
+        _repoMock.Setup(r => r.AddInstanceAttributeOverrideAsync(It.IsAny<InstanceAttributeOverride>(), It.IsAny<CancellationToken>()))
+            .Callback<InstanceAttributeOverride, CancellationToken>((o, _) => captured = o);
+
+        var result = await _sut.SetAttributeOverrideAsync(1, "Tags", "[\"a\"]", "admin");
+
+        Assert.True(result.IsSuccess);
+        Assert.NotNull(captured);
+        Assert.Equal(DataType.String, captured.ElementDataType);
+    }
+
+    [Fact]
+    public async Task SetAttributeOverride_UpdatePath_ListAttribute_StampsElementDataType()
+    {
+        var instance = new Instance("Inst1") { Id = 1, TemplateId = 1 };
+        _repoMock.Setup(r => r.GetInstanceByIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(instance);
+        _repoMock.Setup(r => r.GetAttributesByTemplateIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new List<TemplateAttribute>
+            {
+                new("Tags")
+                {
+                    IsLocked = false,
+                    DataType = DataType.List,
+                    ElementDataType = DataType.String
+                }
+            });
+
+        var existingOverride = new InstanceAttributeOverride("Tags") { Id = 42, InstanceId = 1, OverrideValue = "[\"old\"]" };
+        _repoMock.Setup(r => r.GetOverridesByInstanceIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new List<InstanceAttributeOverride> { existingOverride }); // pre-existing → update path
+        _repoMock.Setup(r => r.SaveChangesAsync(It.IsAny<CancellationToken>()))
+            .ReturnsAsync(1);
+
+        InstanceAttributeOverride? captured = null;
+        _repoMock.Setup(r => r.UpdateInstanceAttributeOverrideAsync(It.IsAny<InstanceAttributeOverride>(), It.IsAny<CancellationToken>()))
+            .Callback<InstanceAttributeOverride, CancellationToken>((o, _) => captured = o);
+
+        var result = await _sut.SetAttributeOverrideAsync(1, "Tags", "[\"new\"]", "admin");
+
+        Assert.True(result.IsSuccess);
+        Assert.NotNull(captured);
+        Assert.Equal(DataType.String, captured.ElementDataType);
+    }
+
+    [Fact]
+    public async Task SetAttributeOverride_CreatePath_ScalarAttribute_ElementDataTypeIsNull()
+    {
+        var instance = new Instance("Inst1") { Id = 1, TemplateId = 1 };
+        _repoMock.Setup(r => r.GetInstanceByIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(instance);
+        _repoMock.Setup(r => r.GetAttributesByTemplateIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new List<TemplateAttribute>
+            {
+                new("Threshold")
+                {
+                    IsLocked = false,
+                    DataType = DataType.Float,
+                    ElementDataType = null
+                }
+            });
+        _repoMock.Setup(r => r.GetOverridesByInstanceIdAsync(1, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new List<InstanceAttributeOverride>());
+        _repoMock.Setup(r => r.SaveChangesAsync(It.IsAny<CancellationToken>()))
+            .ReturnsAsync(1);
+
+        InstanceAttributeOverride? captured = null;
+        _repoMock.Setup(r => r.AddInstanceAttributeOverrideAsync(It.IsAny<InstanceAttributeOverride>(), It.IsAny<CancellationToken>()))
+            .Callback<InstanceAttributeOverride, CancellationToken>((o, _) => captured = o);
+
+        var result = await _sut.SetAttributeOverrideAsync(1, "Threshold", "3.14", "admin");
+
+        Assert.True(result.IsSuccess);
+        Assert.NotNull(captured);
+        Assert.Null(captured.ElementDataType);
+    }
+
     [Fact]
     public async Task AssignToArea_AreaInDifferentSite_ReturnsFailure()
     {