diff --git a/dotnet/src/Agents/OpenAI/Extensions/OpenAIResponseExtensions.cs b/dotnet/src/Agents/OpenAI/Extensions/OpenAIResponseExtensions.cs
index dc1ec795cae0..d7c27923142d 100644
--- a/dotnet/src/Agents/OpenAI/Extensions/OpenAIResponseExtensions.cs
+++ b/dotnet/src/Agents/OpenAI/Extensions/OpenAIResponseExtensions.cs
@@ -202,7 +202,7 @@ private static ChatMessageContentItemCollection ToChatMessageContentItemCollecti
         {
             if (part is ReasoningSummaryTextPart text)
             {
-                collection.Add(new TextContent(text.Text, innerContent: text));
+                collection.Add(new ReasoningContent(text.Text) { InnerContent = text });
             }
         }
         return collection;
diff --git a/dotnet/src/Agents/UnitTests/Extensions/ResponseItemExtensionsTests.cs b/dotnet/src/Agents/UnitTests/Extensions/ResponseItemExtensionsTests.cs
index d547b27be0fa..2860251dedc2 100644
--- a/dotnet/src/Agents/UnitTests/Extensions/ResponseItemExtensionsTests.cs
+++ b/dotnet/src/Agents/UnitTests/Extensions/ResponseItemExtensionsTests.cs
@@ -110,6 +110,9 @@ public void VerifyToChatMessageContentFromReasoning()
 
         // Assert
         Assert.NotNull(messageContent);
-        Assert.Equal("Foo", messageContent.Content);
+        Assert.Single(messageContent.Items);
+        var reasoningContent = messageContent.Items[0] as ReasoningContent;
+        Assert.NotNull(reasoningContent);
+        Assert.Equal("Foo", reasoningContent.Text);
     }
 }
diff --git a/dotnet/src/Agents/UnitTests/OpenAI/Extensions/OpenAIResponseExtensionsTests.cs b/dotnet/src/Agents/UnitTests/OpenAI/Extensions/OpenAIResponseExtensionsTests.cs
index 8bd86c950e7b..1687a13d88e9 100644
--- a/dotnet/src/Agents/UnitTests/OpenAI/Extensions/OpenAIResponseExtensionsTests.cs
+++ b/dotnet/src/Agents/UnitTests/OpenAI/Extensions/OpenAIResponseExtensionsTests.cs
@@ -155,6 +155,83 @@ public void VerifyToFunctionCallContent()
         Assert.NotNull(content.Arguments);
     }
 
+    /// <summary>
+    /// Verify that ReasoningResponseItem with SummaryParts generates ReasoningContent correctly.
+    /// </summary>
+    [Fact]
+    public void VerifyToChatMessageContentWithReasoningResponseItem()
+    {
+        // Arrange
+        var reasoningResponseItem = this.CreateReasoningResponseItem("Let me think about this step by step...");
+
+        // Act
+        ChatMessageContent? chatMessageContent = reasoningResponseItem.ToChatMessageContent();
+
+        // Assert
+        Assert.NotNull(chatMessageContent);
+        Assert.Equal(AuthorRole.Assistant, chatMessageContent.Role);
+        Assert.Single(chatMessageContent.Items);
+
+        var reasoningContent = chatMessageContent.Items[0] as ReasoningContent;
+        Assert.NotNull(reasoningContent);
+        Assert.Equal("Let me think about this step by step...", reasoningContent.Text);
+    }
+
+    /// <summary>
+    /// Verify that ReasoningResponseItem converts to correct ChatMessageContentItemCollection with ReasoningContent.
+    /// </summary>
+    [Fact]
+    public void VerifyToChatMessageContentItemCollectionWithReasoningResponseItem()
+    {
+        // Arrange
+        var reasoningResponseItem = this.CreateReasoningResponseItem("Analyzing the problem...");
+
+        // Act
+        ChatMessageContentItemCollection collection = reasoningResponseItem.ToChatMessageContentItemCollection();
+
+        // Assert
+        Assert.NotNull(collection);
+        Assert.Single(collection);
+        Assert.IsType<ReasoningContent>(collection[0]);
+
+        var reasoningContent = collection[0] as ReasoningContent;
+        Assert.Equal("Analyzing the problem...", reasoningContent?.Text);
+    }
+
+    /// <summary>
+    /// Verify that OpenAIResponse with both ReasoningResponseItem and MessageResponseItem generates correct content types.
+    /// </summary>
+    [Fact]
+    public void VerifyToChatMessageContentWithMixedResponseItems()
+    {
+        // Arrange
+        var reasoningResponseItem = this.CreateReasoningResponseItem("Thinking about the answer...");
+        var messageResponseItem = ResponseItem.CreateAssistantMessageItem("Here is my response.");
+
+        OpenAIResponse mockResponse = this.CreateMockOpenAIResponse("gpt-4o-mini",
+            [
+                reasoningResponseItem,
+                messageResponseItem
+            ]);
+
+        // Act
+        ChatMessageContent chatMessageContent = mockResponse.ToChatMessageContent();
+
+        // Assert
+        Assert.NotNull(chatMessageContent);
+        Assert.Equal(2, chatMessageContent.Items.Count);
+
+        // First item should be ReasoningContent
+        Assert.IsType<ReasoningContent>(chatMessageContent.Items[0]);
+        var reasoningContent = chatMessageContent.Items[0] as ReasoningContent;
+        Assert.Equal("Thinking about the answer...", reasoningContent?.Text);
+
+        // Second item should be TextContent
+        Assert.IsType<TextContent>(chatMessageContent.Items[1]);
+        var textContent = chatMessageContent.Items[1] as TextContent;
+        Assert.Equal("Here is my response.", textContent?.Text);
+    }
+
     #region private
     private OpenAIResponse CreateMockOpenAIResponse(string model, IEnumerable<ResponseItem> outputItems)
     {
@@ -250,5 +327,112 @@ private OpenAIResponse CreateMockOpenAIResponse(string id, DateTimeOffset create
         throw new InvalidOperationException("Constructor not found.");
     }
 
+    private ReasoningResponseItem CreateReasoningResponseItem(string? reasoningText = null, IReadOnlyList<ReasoningSummaryPart>? summaryParts = null)
+    {
+        Type reasoningResponseItemType = typeof(ReasoningResponseItem);
+        Type reasoningSummaryTextPartType = typeof(ReasoningSummaryTextPart);
+
+        // If reasoningText is provided and summaryParts is not, create summaryParts with the text
+        if (reasoningText != null && summaryParts == null)
+        {
+            // Try to find any public static factory method or constructor that can create ReasoningSummaryTextPart
+            var createTextPartMethod = typeof(ReasoningSummaryPart).GetMethod(
+                "CreateTextPart",
+                BindingFlags.Static | BindingFlags.Public,
+                null,
+                [typeof(string)],
+                null);
+
+            if (createTextPartMethod != null)
+            {
+                var textPart = createTextPartMethod.Invoke(null, [reasoningText]) as ReasoningSummaryTextPart;
+                summaryParts = textPart != null ? new List<ReasoningSummaryPart> { textPart } : new List<ReasoningSummaryPart>();
+            }
+            else
+            {
+                // Try to find constructor - search for all constructors
+                var textPartConstructors = reasoningSummaryTextPartType.GetConstructors(
+                    BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
+
+                ConstructorInfo? textPartConstructor = null;
+                foreach (var ctor in textPartConstructors)
+                {
+                    var parameters = ctor.GetParameters();
+                    if (parameters.Length >= 1 && parameters[0].ParameterType == typeof(string))
+                    {
+                        textPartConstructor = ctor;
+                        break;
+                    }
+                }
+
+                if (textPartConstructor != null)
+                {
+                    var ctorParams = textPartConstructor.GetParameters();
+                    var args = new object?[ctorParams.Length];
+                    args[0] = reasoningText;
+                    // Fill in any additional parameters with null or default values
+                    for (int i = 1; i < ctorParams.Length; i++)
+                    {
+                        args[i] = null;
+                    }
+
+                    var textPart = textPartConstructor.Invoke(args) as ReasoningSummaryTextPart;
+                    summaryParts = textPart != null ? new List<ReasoningSummaryPart> { textPart } : new List<ReasoningSummaryPart>();
+                }
+                else
+                {
+                    throw new InvalidOperationException("Could not find a way to create ReasoningSummaryTextPart.");
+                }
+            }
+        }
+
+        // Convert null summaryParts to empty list for method calls
+        var partsToPass = summaryParts ?? new List<ReasoningSummaryPart>();
+
+        // Try to find a static factory method first
+        var createReasoningItemMethod = typeof(ResponseItem).GetMethod(
+            "CreateReasoningItem",
+            BindingFlags.Static | BindingFlags.Public,
+            null,
+            [typeof(IEnumerable<ReasoningSummaryPart>)],
+            null);
+
+        if (createReasoningItemMethod != null)
+        {
+            return (ReasoningResponseItem)createReasoningItemMethod.Invoke(null, [partsToPass])!;
+        }
+
+        // If no factory method, look for constructors
+        var constructors = reasoningResponseItemType.GetConstructors(
+            BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
+
+        foreach (var ctor in constructors)
+        {
+            var parameters = ctor.GetParameters();
+
+            // Look for constructor that takes IReadOnlyList<ReasoningSummaryPart> or similar
+            if (parameters.Length >= 1)
+            {
+                var firstParamType = parameters[0].ParameterType;
+                if (firstParamType.IsAssignableFrom(typeof(List<ReasoningSummaryPart>)) ||
+                    firstParamType.IsAssignableFrom(typeof(IReadOnlyList<ReasoningSummaryPart>)) ||
+                    firstParamType.IsAssignableFrom(typeof(IEnumerable<ReasoningSummaryPart>)))
+                {
+                    var args = new object?[parameters.Length];
+                    args[0] = partsToPass;
+                    // Fill in any additional parameters with null
+                    for (int i = 1; i < parameters.Length; i++)
+                    {
+                        args[i] = null;
+                    }
+
+                    return (ReasoningResponseItem)ctor.Invoke(args);
+                }
+            }
+        }
+
+        throw new InvalidOperationException("Constructor not found for ReasoningResponseItem.");
+    }
+
     #endregion
 }