tools list does not contain descriptions for nested properties in the input schema

[Jarryd-Lansdell]

Is your feature request related to a problem? Please describe.
When calling list_tools the json structure generated does not have descriptions for the models properties and nested properties. In the image below the left-hand side is what I have used with a custom tools registration to include various metadata about the properties. I made the code traverse all nested classes and include their xml summary as descriptions for the properties.

Describe the solution you'd like
I would like to remove my tool registration and use the standard 'WithToolsFromAssembly' and let it handle everything for me.

Describe alternatives you've considered
I've created my own work around for now.

Additional context
Here is a class and a unit test(Xunit) that I am using to verify when the fix is in.

 /// <summary>
 /// A sample MCP tool container used to verify extraction of XML summaries from instance methods.
 /// </summary>
 [McpServerToolType]
 public partial class McpTestToolsForXMLSummary
 {
     public string Prefix { get; set; } = "Hello from C#: ";

     /// <summary>
     /// Returns the provided message with the configured `Prefix` to validate summary extraction
     /// for instance methods when generating MCP tool descriptions.
     /// </summary>
     /// <param name="message">A non-empty message to be returned with the current prefix.</param>
     /// <param name="testObject">A sample object used to confirm parameter documentation is included in tool metadata.</param>
     /// <returns>A string that concatenates `Prefix` and `message`.</returns>
     [McpServerTool]
     public partial string SeeSummary([Required] string message, [Required] TestObject? testObject) => $"{Prefix}{message}";
 }

public void BuilderRegistration_ToolHasRequiredParameters_RequiredIsReflectedInSchema()
{
    // Arrange: use the test tools assembly that defines the 'echo' MCP tool
    var exampleTypeFromApplication = typeof(McpTestToolsForXMLSummary);
    var path = (exampleTypeFromApplication.Assembly.GetName().Name ?? "") + ".xml";
    XmlDocHelper.LoadXmlDocumentation(path);
    var toolAssembly = Assembly.GetAssembly(exampleTypeFromApplication)!;

    // Build via McpServerBuilder extensions instead of McpToolHandler
    var sc = new ServiceCollection();
    sc.AddMcpServer().WithToolsFromAssembly(toolAssembly);

    // Act
    var serviceProvider = sc.BuildServiceProvider();
    var toolsAsServices = serviceProvider.GetServices<McpServerTool>();
    var echoTool = toolsAsServices.FirstOrDefault(x => x?.ProtocolTool?.Name == "see_summary");

    // Assert
    Assert.NotNull(echoTool);
    // Tool-level description from XML summary
    Assert.False(string.IsNullOrWhiteSpace(echoTool!.ProtocolTool.Description));
    Assert.True(echoTool!.ProtocolTool.InputSchema.TryGetProperty("properties", out var properties));

    // Prefer canonical JSON Schema: parent-level required array
    Assert.True(echoTool!.ProtocolTool.InputSchema.TryGetProperty("required", out var requiredArray));
    var requiredList = requiredArray.EnumerateArray().Select(e => e.GetString()).ToList();
    Assert.Contains("message", requiredList);
    Assert.Contains("testObject", requiredList);

    // Also check nullable hints are present on nodes (implementation detail)
    Assert.True(properties.TryGetProperty("message", out var messageSchema));
    if (messageSchema.TryGetProperty("nullable", out var messageNullable))
    {
        Assert.False(messageNullable.GetBoolean());
    }
    // Property description for 'message'
    Assert.True(messageSchema.TryGetProperty("description", out var messageDescription));
    Assert.False(string.IsNullOrWhiteSpace(messageDescription.GetString()));
    Assert.True(properties.TryGetProperty("testObject", out var objSchema));
    if (objSchema.TryGetProperty("nullable", out var objNullable))
    {
        Assert.False(objNullable.GetBoolean());
    }
    // Property description for 'testObject'
    Assert.True(objSchema.TryGetProperty("description", out var objDescription));
    Assert.False(string.IsNullOrWhiteSpace(objDescription.GetString()));

    // Check nested property description matches XML summary on TestObject.TestProperty
    Assert.True(objSchema.TryGetProperty("properties", out var objProperties));
    Assert.True(objProperties.TryGetProperty("TestProperty", out var nestedPropSchema));
    Assert.True(nestedPropSchema.TryGetProperty("description", out var nestedDescription));
    Assert.Equal("An example property used to validate XML documentation propagation to schema nodes.", nestedDescription.GetString());
}

[stephentoub]

The source generator that rewrites XML comments to be [Description(...)] attributes is only able to do so for the method itself; it does not have the ability to add [Description(...)] attributes to the types used in the signature nor anything they reference. And if those types aren't in the same compilation unit, it may not even have the ability to see the XML comments, as they're not persisted as part of the compiled .dll.

For those types and properties on those types, you will need to use [Description(...)] instead of or in addition to XML comments.