diff --git a/src/ZB.MOM.WW.ScadaBridge.Commons/Types/AttributeValueCodec.cs b/src/ZB.MOM.WW.ScadaBridge.Commons/Types/AttributeValueCodec.cs
index ecc4cbea..b848b79e 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Commons/Types/AttributeValueCodec.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Commons/Types/AttributeValueCodec.cs
@@ -25,11 +25,10 @@ public static class AttributeValueCodec
             case string s: return s;                       // already canonical
             case IFormattable f: return f.ToString(null, CultureInfo.InvariantCulture);
             case IEnumerable e:
-                var items = e.Cast<object?>()
-                             .Select(x => x is IFormattable xf
-                                 ? xf.ToString(null, CultureInfo.InvariantCulture)
-                                 : x?.ToString());
-                return JsonSerializer.Serialize(items, JsonOpts);
+                // Native-typed JSON: serialize the runtime collection so System.Text.Json emits
+                // numbers/bools unquoted, strings quoted, and DateTime as ISO-8601. Boxed as object
+                // so STJ uses the runtime element type. STJ numbers/dates are culture-invariant.
+                return JsonSerializer.Serialize<object>(e, JsonOpts);
             default: return value.ToString();
         }
     }
@@ -46,18 +45,25 @@ public static class AttributeValueCodec
         if (elementType is null)
             throw new FormatException("List attribute requires an element type.");
 
-        string?[] raw;
-        try { raw = JsonSerializer.Deserialize<string?[]>(value) ?? []; }
+        JsonElement[] raw;
+        try { raw = JsonSerializer.Deserialize<JsonElement[]>(value) ?? []; }
         catch (JsonException ex) { throw new FormatException("Malformed list JSON.", ex); }
 
         var clrType = ElementClrType(elementType.Value);
         var listType = typeof(List<>).MakeGenericType(clrType);
         var result = (IList)Activator.CreateInstance(listType)!;
-        foreach (var item in raw)
-            result.Add(ParseScalar(item, elementType.Value));
+        foreach (var el in raw)
+            result.Add(ParseScalar(JsonElementToString(el), elementType.Value));
         return result;
     }
 
+    private static string? JsonElementToString(JsonElement el) => el.ValueKind switch
+    {
+        JsonValueKind.String => el.GetString(),   // old form, or string-typed lists
+        JsonValueKind.Null => null,               // ParseScalar throws "may not be null"
+        _ => el.GetRawText()                       // number/bool → "10" / "1.5" / "true"
+    };
+
     private static Type ElementClrType(DataType t) => t switch
     {
         DataType.String => typeof(string),
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/AttributeValueCodecTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/AttributeValueCodecTests.cs
index a4aed551..15229611 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/AttributeValueCodecTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/AttributeValueCodecTests.cs
@@ -64,7 +64,7 @@ public class AttributeValueCodecTests
         try
         {
             CultureInfo.CurrentCulture = new CultureInfo("de-DE");
-            Assert.Equal("[\"1.5\",\"2.5\"]",
+            Assert.Equal("[1.5,2.5]",
                 AttributeValueCodec.Encode(new List<double> { 1.5, 2.5 }));
         }
         finally
@@ -73,6 +73,53 @@ public class AttributeValueCodecTests
         }
     }
 
+    [Fact]
+    public void Encode_Int32List_ProducesNativeNumbers() =>
+        Assert.Equal("[10,20,30]",
+            AttributeValueCodec.Encode(new List<int> { 10, 20, 30 }));
+
+    [Fact]
+    public void Encode_BoolList_ProducesNativeBooleans() =>
+        Assert.Equal("[true,false]",
+            AttributeValueCodec.Encode(new List<bool> { true, false }));
+
+    [Fact]
+    public void Encode_StringList_StaysQuoted() =>
+        Assert.Equal("[\"a\",\"b\"]",
+            AttributeValueCodec.Encode(new List<string> { "a", "b" }));
+
+    [Fact]
+    public void Encode_DateTimeList_IsIso8601()
+    {
+        var json = AttributeValueCodec.Encode(
+            new List<DateTime> { new(2026, 6, 16, 0, 0, 0, DateTimeKind.Utc) });
+        Assert.Contains("2026-06-16T00:00:00", json);
+        Assert.DoesNotContain("06/16/2026", json);
+    }
+
+    [Fact]
+    public void Decode_NewNativeIntForm_Parses()
+    {
+        var back = (IList<int>)AttributeValueCodec.Decode("[10,20]", DataType.List, DataType.Int32)!;
+        Assert.Equal(new[] { 10, 20 }, back);
+    }
+
+    [Fact]
+    public void Decode_OldStringIntForm_BackwardCompatible()
+    {
+        var back = (IList<int>)AttributeValueCodec.Decode("[\"10\",\"20\"]", DataType.List, DataType.Int32)!;
+        Assert.Equal(new[] { 10, 20 }, back);
+    }
+
+    [Theory]
+    [InlineData("[true,false]")]
+    [InlineData("[\"True\",\"False\"]")]
+    public void Decode_BoolForms_BothParse(string json)
+    {
+        var back = (IList<bool>)AttributeValueCodec.Decode(json, DataType.List, DataType.Boolean)!;
+        Assert.Equal(new[] { true, false }, back);
+    }
+
     [Fact]
     public void RoundTrip_Int32List()
     {