fix: address methods that return void or task in those cases return null

src/AnthropicClient/Models/ToolCall.cs

@@ -1,5 +1,6 @@
 using System.Reflection;
 using System.Text.Json;
+using System.Threading.Tasks;
 
 using AnthropicClient.Json;
 
@@ -65,7 +66,9 @@ public class ToolCall
 
         const string resultPropertyName = "Result";
         var resultProperty = task.GetType().GetProperty(resultPropertyName);
-        result = resultProperty is not null ? (T)resultProperty.GetValue(task) : default;
+        var isVoidTaskResult = resultProperty.PropertyType.FullName.Contains("VoidTaskResult");
+
+        result = resultProperty is not null && isVoidTaskResult is false ? (T)resultProperty.GetValue(task) : default;
       }
       else
       {

src/AnthropicClient/Models/ToolCallResult.cs

@@ -6,7 +6,7 @@ namespace AnthropicClient.Models;
 public class ToolCallResult<T>
 {
   /// <summary>
-  /// The value of the tool call result.
+  /// The value of the tool call result. Can be null if the call failed, the call was successful but the return type is void or Task, or the call was successful but the return value is null
   /// </summary>
   public T? Value { get; }
 

tests/AnthropicClient.Tests/Unit/Models/ToolCallTests.cs

@@ -221,6 +221,40 @@ public class ToolCallTests : SerializationTest
     result.Value.Should().Be("John");
     result.Error.Should().BeNull();
   }
+
+  [Fact]
+  public async Task InvokeAsync_WhenCalledAndToolReturnsVoid_ItShouldReturnSuccessResult()
+  {
+    var func = (int i) => { };
+    var anthropicFunction = new AnthropicFunction(func.Method, func.Target);
+    var tool = new Tool("tool", "description", anthropicFunction);
+
+    var input = new Dictionary<string, object?> { { "i", 42 } };
+    var toolCall = new ToolCall(tool, new ToolUseContent { Input = input });
+
+    var result = await toolCall.InvokeAsync();
+
+    result.IsSuccess.Should().BeTrue();
+    result.Value.Should().BeNull();
+    result.Error.Should().BeNull();
+  }
+
+  [Fact]
+  public async Task InvokeAsync_WhenCalledAndToolReturnsTask_ItShouldReturnSuccessResult()
+  {
+    var func = async (int i) => await Task.CompletedTask;
+    var anthropicFunction = new AnthropicFunction(func.Method, func.Target);
+    var tool = new Tool("tool", "description", anthropicFunction);
+
+    var input = new Dictionary<string, object?> { { "i", 42 } };
+    var toolCall = new ToolCall(tool, new ToolUseContent { Input = input });
+
+    var result = await toolCall.InvokeAsync();
+
+    result.IsSuccess.Should().BeTrue();
+    result.Value.Should().BeNull();
+    result.Error.Should().BeNull();
+  }
 }
 
 class Person