diff --git a/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Flattening/FlatteningService.cs b/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Flattening/FlatteningService.cs
index 7b45d22a..c99af3e4 100644
--- a/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Flattening/FlatteningService.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Flattening/FlatteningService.cs
@@ -175,6 +175,7 @@ public class FlatteningService
                     CanonicalName = attr.Name,
                     Value = attr.Value,
                     DataType = attr.DataType.ToString(),
+                    ElementDataType = attr.ElementDataType?.ToString(),
                     IsLocked = attr.IsLocked,
                     Description = attr.Description,
                     DataSourceReference = attr.DataSourceReference,
diff --git a/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Flattening/FlatteningServiceTests.cs b/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Flattening/FlatteningServiceTests.cs
index 47e92e8a..7f06d7cc 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Flattening/FlatteningServiceTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Flattening/FlatteningServiceTests.cs
@@ -788,4 +788,67 @@ public class FlatteningServiceTests
         Assert.Equal("ns=2;s=Tank07", result.Value.NativeAlarmSources[0].SourceReference);
         Assert.Equal("Override", result.Value.NativeAlarmSources[0].Source);
     }
+
+    // ── MV-4: ElementDataType carried through flatten/override ────────────
+
+    [Fact]
+    public void Flatten_ListAttribute_ElementDataTypeCarriedToResolvedAttribute()
+    {
+        // A template with a List attribute whose ElementDataType is String.
+        // The flattened result must carry DataType == "List" and ElementDataType == "String".
+        var template = CreateTemplate(1, "Base");
+        template.Attributes.Add(new TemplateAttribute("Tags")
+        {
+            DataType = DataType.List,
+            ElementDataType = DataType.String,
+            Value = null
+        });
+
+        var instance = CreateInstance();
+        var result = _sut.Flatten(
+            instance,
+            [template],
+            new Dictionary<int, IReadOnlyList<TemplateComposition>>(),
+            new Dictionary<int, IReadOnlyList<Template>>(),
+            new Dictionary<int, DataConnection>());
+
+        Assert.True(result.IsSuccess);
+        var attr = result.Value.Attributes.First(a => a.CanonicalName == "Tags");
+        Assert.Equal("List", attr.DataType);
+        Assert.Equal("String", attr.ElementDataType);
+    }
+
+    [Fact]
+    public void Flatten_ListAttributeWithInstanceOverride_ElementDataTypePreservedThroughOverride()
+    {
+        // An instance override replaces the VALUE of a List attribute.
+        // The ElementDataType must survive the override path unchanged.
+        var template = CreateTemplate(1, "Base");
+        template.Attributes.Add(new TemplateAttribute("Tags")
+        {
+            DataType = DataType.List,
+            ElementDataType = DataType.String,
+            Value = "[\"a\",\"b\"]"
+        });
+
+        var instance = CreateInstance();
+        instance.AttributeOverrides.Add(new InstanceAttributeOverride("Tags")
+        {
+            OverrideValue = "[\"x\",\"y\",\"z\"]"
+        });
+
+        var result = _sut.Flatten(
+            instance,
+            [template],
+            new Dictionary<int, IReadOnlyList<TemplateComposition>>(),
+            new Dictionary<int, IReadOnlyList<Template>>(),
+            new Dictionary<int, DataConnection>());
+
+        Assert.True(result.IsSuccess);
+        var attr = result.Value.Attributes.First(a => a.CanonicalName == "Tags");
+        Assert.Equal("[\"x\",\"y\",\"z\"]", attr.Value);      // override applied
+        Assert.Equal("Override", attr.Source);               // came from override path
+        Assert.Equal("List", attr.DataType);                 // type unchanged
+        Assert.Equal("String", attr.ElementDataType);        // element type preserved
+    }
 }