diff --git a/src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs b/src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs
index 0110ac018..40109158d 100644
--- a/src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs
+++ b/src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs
@@ -321,10 +321,13 @@ private static void AppendMethodDeclaration(
             writer.WriteLine($"[return: Description(\"{EscapeString(xmlDocs.Returns)}\")]");
         }
 
-        // Copy modifiers from original method syntax.
+        // Copy modifiers from original method syntax, excluding 'async' which is invalid on partial declarations (CS1994).
         // Add return type (without nullable annotations).
         // Add method name.
-        writer.Write(string.Join(" ", methodDeclaration.Modifiers.Select(m => m.Text)));
+        var modifiers = methodDeclaration.Modifiers
+            .Where(m => !m.IsKind(SyntaxKind.AsyncKeyword))
+            .Select(m => m.Text);
+        writer.Write(string.Join(" ", modifiers));
         writer.Write(' ');
         writer.Write(methodSymbol.ReturnType.ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat));
         writer.Write(' ');
@@ -332,9 +335,11 @@ private static void AppendMethodDeclaration(
 
         // Add parameters with their Description attributes.
         writer.Write("(");
+        var parameterSyntaxList = methodDeclaration.ParameterList.Parameters;
         for (int i = 0; i < methodSymbol.Parameters.Length; i++)
         {
             IParameterSymbol param = methodSymbol.Parameters[i];
+            ParameterSyntax? paramSyntax = i < parameterSyntaxList.Count ? parameterSyntaxList[i] : null;
 
             if (i > 0)
             {
@@ -352,6 +357,13 @@ private static void AppendMethodDeclaration(
             writer.Write(param.Type.ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat));
             writer.Write(' ');
             writer.Write(param.Name);
+
+            // Preserve default parameter values from the original syntax.
+            if (paramSyntax?.Default is { } defaultValue)
+            {
+                writer.Write(' ');
+                writer.Write(defaultValue.ToFullString().Trim());
+            }
         }
         writer.WriteLine(");");
     }
diff --git a/tests/ModelContextProtocol.Analyzers.Tests/XmlToDescriptionGeneratorTests.cs b/tests/ModelContextProtocol.Analyzers.Tests/XmlToDescriptionGeneratorTests.cs
index f9b13bf65..bffb24647 100644
--- a/tests/ModelContextProtocol.Analyzers.Tests/XmlToDescriptionGeneratorTests.cs
+++ b/tests/ModelContextProtocol.Analyzers.Tests/XmlToDescriptionGeneratorTests.cs
@@ -1463,6 +1463,260 @@ partial class GlobalTools
         AssertGeneratedSourceEquals(expected, result.GeneratedSources[0].SourceText.ToString());
     }
 
+    [Fact]
+    public void Generator_WithDefaultParameterValues_PreservesDefaults()
+    {
+        var result = RunGenerator("""
+            using ModelContextProtocol.Server;
+            using System.ComponentModel;
+
+            namespace Test;
+
+            [McpServerToolType]
+            public partial class TestTools
+            {
+                /// <summary>
+                /// Test tool with defaults
+                /// </summary>
+                /// <param name="project">The project name</param>
+                /// <param name="flag">Enable flag</param>
+                /// <param name="count">Item count</param>
+                [McpServerTool]
+                public static partial string TestMethod(
+                    string? project = null,
+                    bool flag = false,
+                    int count = 42)
+                {
+                    return project ?? "default";
+                }
+            }
+            """);
+
+        Assert.True(result.Success);
+        Assert.Single(result.GeneratedSources);
+
+        var expected = $$"""
+            // <auto-generated/>
+            // ModelContextProtocol.Analyzers {{typeof(XmlToDescriptionGenerator).Assembly.GetName().Version}}
+
+            #pragma warning disable
+
+            using System.ComponentModel;
+            using ModelContextProtocol.Server;
+
+            namespace Test
+            {
+                partial class TestTools
+                {
+                    [Description("Test tool with defaults")]
+                    public static partial string TestMethod([Description("The project name")] string? project = null, [Description("Enable flag")] bool flag = false, [Description("Item count")] int count = 42);
+                }
+            }
+            """;
+
+        AssertGeneratedSourceEquals(expected, result.GeneratedSources[0].SourceText.ToString());
+    }
+
+    [Fact]
+    public void Generator_WithAsyncMethod_ExcludesAsyncModifier()
+    {
+        var result = RunGenerator("""
+            using ModelContextProtocol.Server;
+            using System.ComponentModel;
+            using System.Threading.Tasks;
+
+            namespace Test;
+
+            [McpServerToolType]
+            public partial class TestTools
+            {
+                /// <summary>
+                /// Async tool
+                /// </summary>
+                [McpServerTool]
+                public async partial Task<string> DoWorkAsync(string input)
+                {
+                    await Task.Delay(100);
+                    return input;
+                }
+            }
+            """);
+
+        Assert.True(result.Success);
+        Assert.Single(result.GeneratedSources);
+
+        var expected = $$"""
+            // <auto-generated/>
+            // ModelContextProtocol.Analyzers {{typeof(XmlToDescriptionGenerator).Assembly.GetName().Version}}
+
+            #pragma warning disable
+
+            using System.ComponentModel;
+            using ModelContextProtocol.Server;
+
+            namespace Test
+            {
+                partial class TestTools
+                {
+                    [Description("Async tool")]
+                    public partial Task<string> DoWorkAsync(string input);
+                }
+            }
+            """;
+
+        AssertGeneratedSourceEquals(expected, result.GeneratedSources[0].SourceText.ToString());
+    }
+
+    [Fact]
+    public void Generator_WithAsyncStaticMethod_ExcludesAsyncModifier()
+    {
+        var result = RunGenerator("""
+            using ModelContextProtocol.Server;
+            using System.ComponentModel;
+            using System.Threading.Tasks;
+
+            namespace Test;
+
+            [McpServerToolType]
+            public partial class TestTools
+            {
+                /// <summary>
+                /// Static async tool
+                /// </summary>
+                [McpServerTool]
+                public static async partial Task<string> StaticAsyncMethod(string input)
+                {
+                    await Task.Delay(100);
+                    return input;
+                }
+            }
+            """);
+
+        Assert.True(result.Success);
+        Assert.Single(result.GeneratedSources);
+
+        var expected = $$"""
+            // <auto-generated/>
+            // ModelContextProtocol.Analyzers {{typeof(XmlToDescriptionGenerator).Assembly.GetName().Version}}
+
+            #pragma warning disable
+
+            using System.ComponentModel;
+            using ModelContextProtocol.Server;
+
+            namespace Test
+            {
+                partial class TestTools
+                {
+                    [Description("Static async tool")]
+                    public static partial Task<string> StaticAsyncMethod(string input);
+                }
+            }
+            """;
+
+        AssertGeneratedSourceEquals(expected, result.GeneratedSources[0].SourceText.ToString());
+    }
+
+    [Fact]
+    public void Generator_WithDefaultParameterValuesAndAsync_HandlesBothCorrectly()
+    {
+        var result = RunGenerator("""
+            using ModelContextProtocol.Server;
+            using System.ComponentModel;
+            using System.Threading.Tasks;
+
+            namespace Test;
+
+            [McpServerToolType]
+            public partial class TestTools
+            {
+                /// <summary>
+                /// Async tool with defaults
+                /// </summary>
+                /// <param name="input">The input</param>
+                /// <param name="timeout">Timeout in ms</param>
+                [McpServerTool]
+                public static async partial Task<string> AsyncWithDefaults(string input, int timeout = 1000)
+                {
+                    await Task.Delay(timeout);
+                    return input;
+                }
+            }
+            """);
+
+        Assert.True(result.Success);
+        Assert.Single(result.GeneratedSources);
+
+        var expected = $$"""
+            // <auto-generated/>
+            // ModelContextProtocol.Analyzers {{typeof(XmlToDescriptionGenerator).Assembly.GetName().Version}}
+
+            #pragma warning disable
+
+            using System.ComponentModel;
+            using ModelContextProtocol.Server;
+
+            namespace Test
+            {
+                partial class TestTools
+                {
+                    [Description("Async tool with defaults")]
+                    public static partial Task<string> AsyncWithDefaults([Description("The input")] string input, [Description("Timeout in ms")] int timeout = 1000);
+                }
+            }
+            """;
+
+        AssertGeneratedSourceEquals(expected, result.GeneratedSources[0].SourceText.ToString());
+    }
+
+    [Fact]
+    public void Generator_WithStringDefaultValue_PreservesQuotedDefault()
+    {
+        var result = RunGenerator("""
+            using ModelContextProtocol.Server;
+            using System.ComponentModel;
+
+            namespace Test;
+
+            [McpServerToolType]
+            public partial class TestTools
+            {
+                /// <summary>
+                /// Test tool with string default
+                /// </summary>
+                [McpServerTool]
+                public static partial string TestMethod(string name = "default value")
+                {
+                    return name;
+                }
+            }
+            """);
+
+        Assert.True(result.Success);
+        Assert.Single(result.GeneratedSources);
+
+        var expected = $$"""
+            // <auto-generated/>
+            // ModelContextProtocol.Analyzers {{typeof(XmlToDescriptionGenerator).Assembly.GetName().Version}}
+
+            #pragma warning disable
+
+            using System.ComponentModel;
+            using ModelContextProtocol.Server;
+
+            namespace Test
+            {
+                partial class TestTools
+                {
+                    [Description("Test tool with string default")]
+                    public static partial string TestMethod(string name = "default value");
+                }
+            }
+            """;
+
+        AssertGeneratedSourceEquals(expected, result.GeneratedSources[0].SourceText.ToString());
+    }
+
     private GeneratorRunResult RunGenerator([StringSyntax("C#-test")] string source)
     {
         var syntaxTree = CSharpSyntaxTree.ParseText(source);