Fix serialization of OpenAIPromptExecutionSettings with System.Type in ResponseFormat

dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Settings/OpenAIPromptExecutionSettingsTests.cs

@@ -819,6 +819,22 @@ public void PrepareChatHistoryToRequestAsyncAddsDeveloperWhenSystemExists()
         Assert.Equal("Hello", chatHistory[2].Content);
     }
 
+    [Fact]
+    public void ItCanSerializeResponseFormatAsType()
+    {
+        // Arrange
+        var settings = new OpenAIPromptExecutionSettings
+        {
+            ResponseFormat = typeof(MyTestClass)
+        };
+
+        // Act & Assert
+        var json = JsonSerializer.Serialize(settings);
+
+        Assert.NotNull(json);
+        Assert.Contains("response_format", json);
+    }
+
     /// <summary>
     /// Test implementation of OpenAIPromptExecutionSettings that exposes the protected PrepareChatHistoryToRequestAsync method.
     /// </summary>
@@ -851,4 +867,9 @@ private static void AssertExecutionSettings(OpenAIPromptExecutionSettings execut
         Assert.Equal("""{"format":"mp3","voice":"alloy"}""", JsonSerializer.Serialize(executionSettings.Audio));
         Assert.Equal("""["audio","text"]""", JsonSerializer.Serialize(executionSettings.Modalities));
     }
+
+    private sealed class MyTestClass
+    {
+        public int MyProperty { get; set; }
+    }
 }

dotnet/src/Connectors/Connectors.OpenAI/Settings/OpenAIPromptExecutionSettings.cs

@@ -177,6 +177,7 @@ public long? Seed
     /// </remarks>
     [JsonPropertyName("response_format")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    [JsonConverter(typeof(TypeOrFunctionResponseFormatConverter))]
     public object? ResponseFormat
     {
         get => this._responseFormat;

dotnet/src/Connectors/Connectors.OpenAI/Settings/TypeOrFunctionResponseFormatConverter.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Microsoft.SemanticKernel.Connectors.OpenAI;
+
+internal sealed class TypeOrFunctionResponseFormatConverter : JsonConverter<object>
+{
+    public override bool CanConvert(Type typeToConvert)
+    {
+        return typeToConvert == typeof(object);
+    }
+
+    public override object? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        throw new NotImplementedException();
+    }
+
+    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
+    {
+        if (value is Type type)
+        {
+            var responseFormat = OpenAIChatResponseFormatBuilder.GetJsonSchemaResponseFormat(type);
+            JsonSerializer.Serialize(writer, responseFormat, options);
+        }
+        else
+        {
+            JsonSerializer.Serialize(writer, value, value.GetType(), options);
+        }
+    }
+}