diff --git a/src/GenerativeAI.Microsoft/Extensions/MicrosoftExtensions.cs b/src/GenerativeAI.Microsoft/Extensions/MicrosoftExtensions.cs
index 006a351b..42a76afb 100644
--- a/src/GenerativeAI.Microsoft/Extensions/MicrosoftExtensions.cs
+++ b/src/GenerativeAI.Microsoft/Extensions/MicrosoftExtensions.cs
@@ -146,7 +146,7 @@ where p is not null
     /// <returns>A <see cref="Part"/> object representing the given AI content, or null if the content type is unsupported.</returns>
     public static Part? ToPart(this AIContent content, ChatOptions? options = null)
     {
-        return content switch
+        var part = content switch
         {
             TextContent text => new Part { Text = text.Text },
             DataContent image => new Part
@@ -175,6 +175,15 @@ where p is not null
             },
             _ => null,
         };
+
+        if (part != null && content.AdditionalProperties != null &&
+            content.AdditionalProperties.TryGetValue(AdditionalPropertiesKeys.ThoughtSignature, out var signature) &&
+            signature is string signatureString)
+        {
+            part.ThoughtSignature = signatureString;
+        }
+
+        return part;
     }
 
     /// <summary>
@@ -590,13 +599,25 @@ public static IList<AIContent> ToAiContents(this List<Part>? parts, ChatOptions?
         {
             if (part.Text is not null)
             {
-                (contents ??= new()).Add(new TextContent(part.Text));
+                var textContent = new TextContent(part.Text);
+                if (part.ThoughtSignature != null)
+                {
+                    textContent.AdditionalProperties ??= new AdditionalPropertiesDictionary();
+                    textContent.AdditionalProperties[AdditionalPropertiesKeys.ThoughtSignature] = part.ThoughtSignature;
+                }
+                (contents ??= new()).Add(textContent);
             }
 
             if (part.FunctionCall is not null)
             {
-                (contents ??= new()).Add(new FunctionCallContent(part.FunctionCall.Name, part.FunctionCall.Name,
-                    ConvertFunctionCallArg(part.FunctionCall.Args, part.FunctionCall.Name, options)));
+                var functionCallContent = new FunctionCallContent(part.FunctionCall.Name, part.FunctionCall.Name,
+                    ConvertFunctionCallArg(part.FunctionCall.Args, part.FunctionCall.Name, options));
+                if (part.ThoughtSignature != null)
+                {
+                    functionCallContent.AdditionalProperties ??= new AdditionalPropertiesDictionary();
+                    functionCallContent.AdditionalProperties[AdditionalPropertiesKeys.ThoughtSignature] = part.ThoughtSignature;
+                }
+                (contents ??= new()).Add(functionCallContent);
             }
 
             if (part.FunctionResponse is not null)
diff --git a/tests/GenerativeAI.Microsoft.Tests/MicrosoftExtension_Tests.cs b/tests/GenerativeAI.Microsoft.Tests/MicrosoftExtension_Tests.cs
index b18a3eef..3c137c18 100644
--- a/tests/GenerativeAI.Microsoft.Tests/MicrosoftExtension_Tests.cs
+++ b/tests/GenerativeAI.Microsoft.Tests/MicrosoftExtension_Tests.cs
@@ -310,6 +310,102 @@ public void ToAiContents_BogusData_HandlesVariousParts()
         // Add more specific assertions based on your Bogus rules if needed
     }
 
+    [Fact]
+    public void ToAiContents_WithFunctionCallPart_AndThoughtSignature_ReturnsFunctionCallContentWithSignature()
+    {
+        // Arrange
+        var thoughtSignature = "some_signature";
+        var parts = new List<Part> { new Part
+        {
+            FunctionCall = new FunctionCall { Name = "myFunction", Args = JsonNode.Parse(" { \"arg1\": \"value1\", \"arg2\": \"value2\" }") },
+            ThoughtSignature = thoughtSignature
+        } };
+
+        // Act
+        var result = parts.ToAiContents();
+
+        // Assert
+        result.ShouldNotBeNull();
+        result.Count.ShouldBe(1);
+        result.FirstOrDefault().ShouldBeOfType<FunctionCallContent>();
+        var functionCallContent = (FunctionCallContent)result.FirstOrDefault();
+        functionCallContent.Name.ShouldBe("myFunction");
+        functionCallContent.AdditionalProperties.ShouldNotBeNull();
+        functionCallContent.AdditionalProperties.ContainsKey(AdditionalPropertiesKeys.ThoughtSignature).ShouldBeTrue();
+        functionCallContent.AdditionalProperties[AdditionalPropertiesKeys.ThoughtSignature].ShouldBe(thoughtSignature);
+    }
+
+    [Fact]
+    public void ToPart_WithFunctionCallContent_AndThoughtSignature_ReturnsPartWithSignature()
+    {
+        // Arrange
+        var thoughtSignature = "some_signature";
+        var content = new FunctionCallContent("myFunction", "name")
+        {
+            Arguments = new Dictionary<string, object?>()
+            {
+                {"arg1", "value1"}
+            }
+        };
+        content.AdditionalProperties = new AdditionalPropertiesDictionary
+        {
+            { AdditionalPropertiesKeys.ThoughtSignature, thoughtSignature }
+        };
+
+        // Act
+        var part = content.ToPart();
+
+        // Assert
+        part.ShouldNotBeNull();
+        part.FunctionCall.ShouldNotBeNull();
+        part.ThoughtSignature.ShouldBe(thoughtSignature);
+    }
+
+    [Fact]
+    public void ToAiContents_WithTextPart_AndThoughtSignature_ReturnsTextContentWithSignature()
+    {
+        // Arrange
+        var thoughtSignature = "some_signature";
+        var parts = new List<Part> { new Part
+        {
+            Text = "Hello world",
+            ThoughtSignature = thoughtSignature
+        } };
+
+        // Act
+        var result = parts.ToAiContents();
+
+        // Assert
+        result.ShouldNotBeNull();
+        result.Count.ShouldBe(1);
+        result.FirstOrDefault().ShouldBeOfType<TextContent>();
+        var textContent = (TextContent)result.FirstOrDefault();
+        textContent.Text.ShouldBe("Hello world");
+        textContent.AdditionalProperties.ShouldNotBeNull();
+        textContent.AdditionalProperties.ContainsKey(AdditionalPropertiesKeys.ThoughtSignature).ShouldBeTrue();
+        textContent.AdditionalProperties[AdditionalPropertiesKeys.ThoughtSignature].ShouldBe(thoughtSignature);
+    }
+
+    [Fact]
+    public void ToPart_WithTextContent_AndThoughtSignature_ReturnsPartWithSignature()
+    {
+        // Arrange
+        var thoughtSignature = "some_signature";
+        var content = new TextContent("Hello world");
+        content.AdditionalProperties = new AdditionalPropertiesDictionary
+        {
+            { AdditionalPropertiesKeys.ThoughtSignature, thoughtSignature }
+        };
+
+        // Act
+        var part = content.ToPart();
+
+        // Assert
+        part.ShouldNotBeNull();
+        part.Text.ShouldBe("Hello world");
+        part.ThoughtSignature.ShouldBe(thoughtSignature);
+    }
+
     #endregion
 
     #region ToGenerateContentRequest