Add MessageBaseConverter for polymorphic serialization of IMessageBase

app/MindWork AI Studio/Provider/BaseProvider.cs

@@ -40,7 +40,7 @@ public abstract class BaseProvider : IProvider, ISecretId
     protected static readonly JsonSerializerOptions JSON_SERIALIZER_OPTIONS = new()
     {
         PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
-        Converters = { new AnnotationConverter() },
+        Converters = { new AnnotationConverter(), new MessageBaseConverter() },
         AllowTrailingCommas = false
     };
 

app/MindWork AI Studio/Provider/MessageBaseConverter.cs

@@ -0,0 +1,29 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace AIStudio.Provider;
+
+/// <summary>
+/// Custom JSON converter for the IMessageBase interface to handle polymorphic serialization.
+/// </summary>
+/// <remarks>
+/// This converter ensures that when serializing IMessageBase objects, all properties
+/// of the concrete implementation (e.g., TextMessage) are serialized, not just the
+/// properties defined in the IMessageBase interface.
+/// </remarks>
+public sealed class MessageBaseConverter : JsonConverter<IMessageBase>
+{
+    public override IMessageBase Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        // Deserialization is not needed for request objects, as messages are only serialized
+        // when sending requests to LLM providers.
+        throw new NotImplementedException("Deserializing IMessageBase is not supported. This converter is only used for serializing request messages.");
+    }
+
+    public override void Write(Utf8JsonWriter writer, IMessageBase value, JsonSerializerOptions options)
+    {
+        // Serialize the actual concrete type (e.g., TextMessage) instead of just the IMessageBase interface.
+        // This ensures all properties of the concrete type are included in the JSON output.
+        JsonSerializer.Serialize(writer, value, value.GetType(), options);
+    }
+}