Fix optional tool argument defaults

MCPSharp.Test/ToolHandlerTests.cs

@@ -0,0 +1,36 @@
+using MCPSharp.Core.Tools;
+using MCPSharp.Model;
+
+namespace MCPSharp.Test;
+
+[TestClass]
+public class ToolHandlerTests
+{
+    [TestMethod]
+    public async Task HandleAsync_UsesDeclaredDefaultForMissingOptionalString()
+    {
+        var method = GetType().GetMethod(nameof(OptionalString))!;
+        var handler = new ToolHandler(new Tool { Name = method.Name }, method);
+
+        var result = await handler.HandleAsync(new Dictionary<string, object>());
+
+        Assert.IsFalse(result.IsError);
+        Assert.AreEqual("fallback", result.Content[0].Text);
+    }
+
+    [TestMethod]
+    public async Task HandleAsync_UsesDeclaredDefaultForMissingOptionalValueType()
+    {
+        var method = GetType().GetMethod(nameof(OptionalCount))!;
+        var handler = new ToolHandler(new Tool { Name = method.Name }, method);
+
+        var result = await handler.HandleAsync(new Dictionary<string, object>());
+
+        Assert.IsFalse(result.IsError);
+        Assert.AreEqual("7", result.Content[0].Text);
+    }
+
+    public string OptionalString(string value = "fallback") => value;
+
+    public int OptionalCount(int count = 7) => count;
+}

MCPSharp/Core/Tools/ToolHandler.cs

@@ -29,7 +29,9 @@ public async Task<CallToolResult> HandleAsync(Dictionary<string, object> paramet
                     }
                     else
                     {
-                        inputValues.Add(par.Name, par.ParameterType.IsValueType ? Activator.CreateInstance(par.ParameterType) : null);
+                        inputValues.Add(par.Name, par.HasDefaultValue
+                            ? par.DefaultValue
+                            : par.ParameterType.IsValueType ? Activator.CreateInstance(par.ParameterType) : null);
                     }
                 }