Update to .NET 10.x packages (#957)

Directory.Packages.props

@@ -1,72 +1,69 @@
 <Project>
   <PropertyGroup>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
-    <System9Version>9.0.10</System9Version>
+    <System8Version>8.0.22</System8Version>
-    <System10Version>10.0.0-rc.2.25502.107</System10Version>
+    <System9Version>9.0.11</System9Version>
-    <MicrosoftExtensionsAIVersion>9.10.2</MicrosoftExtensionsAIVersion>
+    <System10Version>10.0.0</System10Version>
   </PropertyGroup>
 
-  <!-- Product dependencies netstandard -->
+  <!-- Product dependencies shared -->
-  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+  <ItemGroup>
-    <PackageVersion Include="Microsoft.Bcl.Memory" Version="$(System9Version)" />
+    <PackageVersion Include="Microsoft.Extensions.AI" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Hosting.Abstractions" Version="8.0.1" />
+    <PackageVersion Include="Microsoft.Extensions.AI.Abstractions" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Logging.Abstractions" Version="8.0.3" />
+    <PackageVersion Include="Microsoft.Extensions.Hosting.Abstractions" Version="$(System10Version)" />
-    <PackageVersion Include="System.Diagnostics.DiagnosticSource" Version="8.0.1" />
+    <PackageVersion Include="Microsoft.Extensions.Logging.Abstractions" Version="$(System10Version)" />
-    <PackageVersion Include="System.IO.Pipelines" Version="8.0.0" />
-    <PackageVersion Include="System.Text.Json" Version="8.0.6" />
-    <PackageVersion Include="System.Threading.Channels" Version="8.0.0" />
   </ItemGroup>
 
-  <!-- Product dependencies LTS -->
+  <!-- Product dependencies < .NET 10 -->
+  <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible($(TargetFramework), 'net10.0'))">
+    <PackageVersion Include="System.Net.ServerSentEvents" Version="$(System10Version)" />
+  </ItemGroup>
+
+  <!-- Product dependencies .NET Standard -->
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <PackageVersion Include="Microsoft.Bcl.Memory" Version="$(System10Version)" />
+    <PackageVersion Include="System.Diagnostics.DiagnosticSource" Version="$(System10Version)" />
+    <PackageVersion Include="System.IO.Pipelines" Version="$(System10Version)" />
+    <PackageVersion Include="System.Text.Json" Version="$(System10Version)" />
+    <PackageVersion Include="System.Threading.Channels" Version="$(System10Version)" />
+  </ItemGroup>
+
+  <!-- Product dependencies .NET 8 -->
   <ItemGroup Condition="'$(TargetFramework)' == 'net8.0'">
-    <PackageVersion Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="8.0.15" />
+    <PackageVersion Include="System.IO.Pipelines" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Hosting.Abstractions" Version="8.0.1" />
+    <PackageVersion Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="$(System8Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Logging.Abstractions" Version="8.0.3" />
-    <PackageVersion Include="System.IO.Pipelines" Version="8.0.0" />
   </ItemGroup>
 
   <!-- Product dependencies .NET 9 -->
   <ItemGroup Condition="'$(TargetFramework)' == 'net9.0'">
-    <PackageVersion Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="$(System9Version)" />
     <PackageVersion Include="Microsoft.IdentityModel.Tokens" Version="8.14.0" />
-    <PackageVersion Include="Microsoft.Extensions.Hosting.Abstractions" Version="$(System9Version)" />
+    <PackageVersion Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="$(System9Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Logging.Abstractions" Version="$(System9Version)" />
   </ItemGroup>
 
   <!-- Product dependencies .NET 10 -->
   <ItemGroup Condition="'$(TargetFramework)' == 'net10.0'">
     <PackageVersion Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="$(System10Version)" />
     <PackageVersion Include="Microsoft.IdentityModel.Tokens" Version="8.14.0" />
-    <PackageVersion Include="Microsoft.Extensions.Hosting.Abstractions" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Logging.Abstractions" Version="$(System10Version)" />
-  </ItemGroup>
-
-  <!-- Product dependencies shared -->
-  <ItemGroup>
-    <PackageVersion Include="Microsoft.Extensions.AI.Abstractions" Version="$(MicrosoftExtensionsAIVersion)" />
-    <PackageVersion Include="Microsoft.Extensions.AI" Version="$(MicrosoftExtensionsAIVersion)" />
-    <PackageVersion Include="System.Net.ServerSentEvents" Version="$(System10Version)" />
   </ItemGroup>
 
   <ItemGroup>
-
     <!-- Build Infra & Packaging -->
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
 
     <!-- Testing dependencies -->
-    <PackageVersion Include="Anthropic.SDK" Version="5.6.0" />
+    <PackageVersion Include="Anthropic.SDK" Version="5.8.0" />
     <PackageVersion Include="coverlet.collector" Version="6.0.4">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageVersion>
     <PackageVersion Include="GitHubActionsTestLogger" Version="2.4.1" />
-    <PackageVersion Include="Microsoft.Extensions.AI.OpenAI" Version="9.10.2-preview.1.25552.1" />
+    <PackageVersion Include="Microsoft.Extensions.AI.OpenAI" Version="10.0.0-preview.1.25559.3" />
-    <PackageVersion Include="Microsoft.Extensions.DependencyInjection" Version="$(System9Version)" />
+    <PackageVersion Include="Microsoft.Extensions.DependencyInjection" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Hosting" Version="$(System9Version)" />
+    <PackageVersion Include="Microsoft.Extensions.Hosting" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Logging" Version="$(System9Version)" />
+    <PackageVersion Include="Microsoft.Extensions.Logging" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Logging.Console" Version="$(System9Version)" />
+    <PackageVersion Include="Microsoft.Extensions.Logging.Console" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.Options" Version="$(System9Version)" />
+    <PackageVersion Include="Microsoft.Extensions.Options" Version="$(System10Version)" />
-    <PackageVersion Include="Microsoft.Extensions.TimeProvider.Testing" Version="9.10.0" />
+    <PackageVersion Include="Microsoft.Extensions.TimeProvider.Testing" Version="$(System10Version)" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
     <PackageVersion Include="Moq" Version="4.20.72" />
     <PackageVersion Include="OpenTelemetry" Version="1.12.0" />

src/ModelContextProtocol.Core/CustomizableJsonStringEnumConverter.cs

@@ -1,125 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-using System.ComponentModel;
-using System.Diagnostics.CodeAnalysis;
-#if !NET9_0_OR_GREATER
-using System.Reflection;
-#endif
-using System.Text.Json;
-using System.Text.Json.Serialization;
-#if !NET9_0_OR_GREATER
-using ModelContextProtocol;
-#endif
-
-// NOTE:
-// This is a workaround for lack of System.Text.Json's JsonStringEnumConverter<T>
-// 9.x support for JsonStringEnumMemberNameAttribute. Once all builds use the System.Text.Json 9.x
-// version, this whole file can be removed. Note that the type is public so that external source
-// generators can use it, so removing it is a potential breaking change.
-
-namespace ModelContextProtocol
-{
-    /// <summary>
-    /// A JSON converter for enums that allows customizing the serialized string value of enum members
-    /// using the <see cref="JsonStringEnumMemberNameAttribute"/>.
-    /// </summary>
-    /// <typeparam name="TEnum">The enum type to convert.</typeparam>
-    /// <remarks>
-    /// This is a temporary workaround for lack of System.Text.Json's JsonStringEnumConverter&lt;T&gt;
-    /// 9.x support for custom enum member naming. It will be replaced by the built-in functionality
-    /// once .NET 9 is fully adopted.
-    /// </remarks>
-    [EditorBrowsable(EditorBrowsableState.Never)]
-    public sealed class CustomizableJsonStringEnumConverter<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)] TEnum> :
-        JsonStringEnumConverter<TEnum> where TEnum : struct, Enum
-    {
-#if !NET9_0_OR_GREATER
-        /// <summary>
-        /// Initializes a new instance of the <see cref="CustomizableJsonStringEnumConverter{TEnum}"/> class.
-        /// </summary>
-        /// <remarks>
-        /// The converter automatically detects any enum members decorated with <see cref="JsonStringEnumMemberNameAttribute"/>
-        /// and uses those values during serialization and deserialization.
-        /// </remarks>
-        public CustomizableJsonStringEnumConverter() :
-            base(namingPolicy: ResolveNamingPolicy())
-        {
-        }
-
-        private static JsonNamingPolicy? ResolveNamingPolicy()
-        {
-            var map = typeof(TEnum).GetFields(BindingFlags.Public | BindingFlags.Static)
-                .Select(f => (f.Name, AttributeName: f.GetCustomAttribute<JsonStringEnumMemberNameAttribute>()?.Name))
-                .Where(pair => pair.AttributeName != null)
-                .ToDictionary(pair => pair.Name, pair => pair.AttributeName);
-
-            return map.Count > 0 ? new EnumMemberNamingPolicy(map!) : null;
-        }
-
-        private sealed class EnumMemberNamingPolicy(Dictionary<string, string> map) : JsonNamingPolicy
-        {
-            public override string ConvertName(string name) =>
-                map.TryGetValue(name, out string? newName) ?
-                    newName :
-                    name;
-        }
-#endif
-    }
-
-    /// <summary>
-    /// A JSON converter for enums that allows customizing the serialized string value of enum members
-    /// using the <see cref="JsonStringEnumMemberNameAttribute"/>.
-    /// </summary>
-    /// <remarks>
-    /// This is a temporary workaround for lack of System.Text.Json's JsonStringEnumConverter&lt;T&gt;
-    /// 9.x support for custom enum member naming. It will be replaced by the built-in functionality
-    /// once .NET 9 is fully adopted.
-    /// </remarks>
-    [EditorBrowsable(EditorBrowsableState.Never)]
-    [RequiresUnreferencedCode("Requires unreferenced code to instantiate the generic enum converter.")]
-    [RequiresDynamicCode("Requires dynamic code to instantiate the generic enum converter.")]
-    public sealed class CustomizableJsonStringEnumConverter : JsonConverterFactory
-    {
-        /// <inheritdoc/>
-        public override bool CanConvert(Type typeToConvert) => typeToConvert.IsEnum;
-        /// <inheritdoc/>
-        public override JsonConverter? CreateConverter(Type typeToConvert, JsonSerializerOptions options)
-        {
-            Type converterType = typeof(CustomizableJsonStringEnumConverter<>).MakeGenericType(typeToConvert)!;
-            var factory = (JsonConverterFactory)Activator.CreateInstance(converterType)!;
-            return factory.CreateConverter(typeToConvert, options);
-        }
-    }
-}
-
-#if !NET9_0_OR_GREATER
-namespace System.Text.Json.Serialization
-{
-    /// <summary>
-    /// Determines the custom string value that should be used when serializing an enum member using JSON.
-    /// </summary>
-    /// <remarks>
-    /// This attribute is a temporary workaround for lack of System.Text.Json's support for custom enum member naming
-    /// in versions prior to .NET 9. It works together with <see cref="CustomizableJsonStringEnumConverter{TEnum}"/>
-    /// to provide customized string representations of enum values during JSON serialization and deserialization.
-    /// </remarks>
-    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
-    internal sealed class JsonStringEnumMemberNameAttribute : Attribute
-    {
-        /// <summary>
-        /// Creates new attribute instance with a specified enum member name.
-        /// </summary>
-        /// <param name="name">The name to apply to the current enum member when serialized to JSON.</param>
-        public JsonStringEnumMemberNameAttribute(string name)
-        {
-            Name = name;
-        }
-
-        /// <summary>
-        /// Gets the custom JSON name of the enum member.
-        /// </summary>
-        public string Name { get; }
-    }
-}
-#endif

src/ModelContextProtocol.Core/Diagnostics.cs

@@ -13,13 +13,8 @@ internal static class Diagnostics
     internal static Meter Meter { get; } = new("Experimental.ModelContextProtocol");
 
     internal static Histogram<double> CreateDurationHistogram(string name, string description, bool longBuckets) =>
-        Meter.CreateHistogram<double>(name, "s", description
+        Meter.CreateHistogram<double>(name, "s", description, advice: longBuckets ? LongSecondsBucketBoundaries : ShortSecondsBucketBoundaries);
-#if NET9_0_OR_GREATER
-        , advice: longBuckets ? LongSecondsBucketBoundaries : ShortSecondsBucketBoundaries
-#endif
-        );
 
-#if NET9_0_OR_GREATER
     /// <summary>
     /// Follows boundaries from http.server.request.duration/http.client.request.duration
     /// </summary>
@@ -36,7 +31,6 @@ internal static class Diagnostics
     {
         HistogramBucketBoundaries = [0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 30, 60, 120, 300],
     };
-#endif
 
     internal static ActivityContext ExtractActivityContext(this DistributedContextPropagator propagator, JsonRpcMessage message)
     {

src/ModelContextProtocol.Core/McpJsonUtilities.cs

@@ -47,7 +47,7 @@ public static partial class McpJsonUtilities
         // Add a converter for user-defined enums, if reflection is enabled by default.
         if (JsonSerializer.IsReflectionEnabledByDefault)
         {
-            options.Converters.Add(new CustomizableJsonStringEnumConverter());
+            options.Converters.Add(new JsonStringEnumConverter());
         }
 
         options.MakeReadOnly();

src/ModelContextProtocol.Core/Protocol/ContextInclusion.cs

@@ -8,7 +8,7 @@ namespace ModelContextProtocol.Protocol;
 /// <remarks>
 /// See the <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">schema</see> for details.
 /// </remarks>
-[JsonConverter(typeof(CustomizableJsonStringEnumConverter<ContextInclusion>))]
+[JsonConverter(typeof(JsonStringEnumConverter<ContextInclusion>))]
 public enum ContextInclusion
 {
     /// <summary>

src/ModelContextProtocol.Core/Protocol/LoggingLevel.cs

@@ -8,7 +8,7 @@ namespace ModelContextProtocol.Protocol;
 /// <remarks>
 /// These map to syslog message severities, as specified in <see href="https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.1">RFC-5424</see>.
 /// </remarks>
-[JsonConverter(typeof(CustomizableJsonStringEnumConverter<LoggingLevel>))]
+[JsonConverter(typeof(JsonStringEnumConverter<LoggingLevel>))]
 public enum LoggingLevel
 {
     /// <summary>Detailed debug information, typically only valuable to developers.</summary>

src/ModelContextProtocol.Core/Protocol/Role.cs

@@ -5,7 +5,7 @@ namespace ModelContextProtocol.Protocol;
 /// <summary>
 /// Represents the type of role in the Model Context Protocol conversation.
 /// </summary>
-[JsonConverter(typeof(CustomizableJsonStringEnumConverter<Role>))]
+[JsonConverter(typeof(JsonStringEnumConverter<Role>))]
 public enum Role
 {
     /// <summary>

tests/ModelContextProtocol.Tests/Protocol/ElicitationTypedTests.cs

@@ -309,7 +309,7 @@ public partial class ElicitationTypedTests : ClientServerTestBase
         Assert.Contains(typeof(string).FullName!, ex.Message);
     }
 
-    [JsonConverter(typeof(CustomizableJsonStringEnumConverter<SampleRole>))]
+    [JsonConverter(typeof(JsonStringEnumConverter<SampleRole>))]
 
     public enum SampleRole
     {

tests/ModelContextProtocol.Tests/Protocol/UnknownPropertiesTests.cs

@@ -0,0 +1,258 @@
+using System.Text.Json;
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+/// <summary>
+/// Tests that custom JSON converters properly handle unknown additional properties
+/// for forward compatibility with future protocol versions.
+/// </summary>
+public class UnknownPropertiesTests
+{
+    [Fact]
+    public void ContentBlock_DeserializationWithUnknownProperty_SkipsProperty()
+    {
+        // Arrange - JSON with unknown "unknownField" property
+        const string Json = """
+            {
+                "type": "text",
+                "text": "Hello, world!",
+                "unknownField": "should be skipped"
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<ContentBlock>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        var textBlock = Assert.IsType<TextContentBlock>(deserialized);
+        Assert.Equal("Hello, world!", textBlock.Text);
+    }
+
+    [Fact]
+    public void ContentBlock_DeserializationWithStructuredContentInContent_SkipsProperty()
+    {
+        // Arrange - This was the actual bug case: structuredContent incorrectly placed
+        // inside a ContentBlock instead of at CallToolResult level
+        const string Json = """
+            {
+                "type": "text",
+                "text": "Result text",
+                "structuredContent": {
+                    "data": "this should be ignored"
+                }
+            }
+            """;
+
+        // Act - Should not throw
+        var deserialized = JsonSerializer.Deserialize<ContentBlock>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        var textBlock = Assert.IsType<TextContentBlock>(deserialized);
+        Assert.Equal("Result text", textBlock.Text);
+    }
+
+    [Fact]
+    public void ContentBlock_DeserializationWithMultipleUnknownProperties_SkipsAll()
+    {
+        // Arrange - JSON with multiple unknown properties
+        const string Json = """
+            {
+                "type": "image",
+                "data": "base64data",
+                "mimeType": "image/png",
+                "futureProperty1": "value1",
+                "futureProperty2": {"nested": "object"},
+                "futureProperty3": [1, 2, 3]
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<ContentBlock>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        var imageBlock = Assert.IsType<ImageContentBlock>(deserialized);
+        Assert.Equal("base64data", imageBlock.Data);
+        Assert.Equal("image/png", imageBlock.MimeType);
+    }
+
+    [Fact]
+    public void Reference_DeserializationWithUnknownProperty_SkipsProperty()
+    {
+        // Arrange - JSON with unknown "metadata" property
+        const string Json = """
+            {
+                "type": "ref/prompt",
+                "name": "test-prompt",
+                "metadata": {
+                    "version": "2.0",
+                    "author": "future"
+                }
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<Reference>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        var promptRef = Assert.IsType<PromptReference>(deserialized);
+        Assert.Equal("test-prompt", promptRef.Name);
+    }
+
+    [Fact]
+    public void Reference_DeserializationWithMultipleUnknownProperties_SkipsAll()
+    {
+        // Arrange
+        const string Json = """
+            {
+                "type": "ref/resource",
+                "uri": "file:///test.txt",
+                "extraField1": "ignored",
+                "extraField2": 123,
+                "extraField3": true
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<Reference>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        var resourceRef = Assert.IsType<ResourceTemplateReference>(deserialized);
+        Assert.Equal("file:///test.txt", resourceRef.Uri);
+    }
+
+    [Fact]
+    public void ResourceContents_DeserializationWithUnknownProperty_SkipsProperty()
+    {
+        // Arrange
+        const string Json = """
+            {
+                "uri": "file:///document.txt",
+                "text": "File contents here",
+                "mimeType": "text/plain",
+                "futureEncoding": "utf-16",
+                "futureChecksum": "abc123"
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<ResourceContents>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        var textResource = Assert.IsType<TextResourceContents>(deserialized);
+        Assert.Equal("file:///document.txt", textResource.Uri);
+        Assert.Equal("File contents here", textResource.Text);
+        Assert.Equal("text/plain", textResource.MimeType);
+    }
+
+    [Fact]
+    public void ProgressNotificationParams_DeserializationWithUnknownProperty_SkipsProperty()
+    {
+        // Arrange
+        const string Json = """
+            {
+                "progressToken": "token123",
+                "progress": 50,
+                "newProgressFormat": "percentage",
+                "estimatedTimeRemaining": 30
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<ProgressNotificationParams>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.Equal("token123", deserialized.ProgressToken.ToString());
+        Assert.Equal(50.0f, deserialized.Progress.Progress);
+    }
+
+    [Fact]
+    public void PrimitiveSchemaDefinition_DeserializationWithUnknownProperty_SkipsProperty()
+    {
+        // Arrange
+        const string Json = """
+            {
+                "type": "string",
+                "description": "A test string",
+                "futureValidation": "regex:.*",
+                "futureMaxLength": 100
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<ElicitRequestParams.PrimitiveSchemaDefinition>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        var stringSchema = Assert.IsType<ElicitRequestParams.StringSchema>(deserialized);
+        Assert.Equal("A test string", stringSchema.Description);
+    }
+
+    [Fact]
+    public void CallToolResult_WithContentBlockContainingUnknownProperties_Succeeds()
+    {
+        // Arrange - Simulates the real-world bug scenario: a malformed response where
+        // structuredContent was incorrectly nested inside content blocks
+        const string Json = """
+            {
+                "content": [
+                    {
+                        "type": "text",
+                        "text": "Tool executed successfully",
+                        "structuredContent": {
+                            "result": "this was incorrectly placed here"
+                        }
+                    }
+                ],
+                "isError": false
+            }
+            """;
+
+        // Act - Should not throw an exception
+        var deserialized = JsonSerializer.Deserialize<CallToolResult>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.Single(deserialized.Content);
+        var textBlock = Assert.IsType<TextContentBlock>(deserialized.Content[0]);
+        Assert.Equal("Tool executed successfully", textBlock.Text);
+        Assert.False(deserialized.IsError);
+    }
+
+    [Fact]
+    public void CallToolResult_WithStructuredContentAtCorrectLevel_PreservesProperty()
+    {
+        // Arrange - Correct placement of structuredContent at CallToolResult level
+        const string Json = """
+            {
+                "content": [
+                    {
+                        "type": "text",
+                        "text": "Tool executed"
+                    }
+                ],
+                "structuredContent": {
+                    "result": "correctly placed here",
+                    "value": 42
+                },
+                "isError": false
+            }
+            """;
+
+        // Act
+        var deserialized = JsonSerializer.Deserialize<CallToolResult>(Json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.StructuredContent);
+        Assert.Equal("correctly placed here", deserialized.StructuredContent["result"]?.ToString());
+        Assert.Equal(42, (int?)deserialized.StructuredContent["value"]);
+    }
+}