XmlToDescriptionGenerator copies async modifier to partial declarations (CS1994)

[jongalloway]

The XmlToDescriptionGenerator copies all modifiers from the original method, including async, to the generated partial declaration. This is invalid C# - partial declarations cannot have the async modifier.

Reproduction

[McpServerToolType]
public partial class Tools
{
    /// <summary>Async tool</summary>
    [McpServerTool]
    public async partial Task<string> DoWorkAsync()
    {
        await Task.Delay(100);
        return "done";
    }
}

Compiler error

CS1994: The 'async' modifier can only be used in methods that have a body.

Generated code

// Invalid - partial declaration has async modifier
public async partial Task<string> DoWorkAsync();

Current workaround

Remove async from the implementation and delegate to a private async helper:

public partial Task<string> DoWorkAsync() => DoWorkAsyncCore();

private async Task<string> DoWorkAsyncCore()
{
    await Task.Delay(100);
    return "done";
}

This adds boilerplate and obscures the implementation.

Expected behavior

The generator should exclude the async modifier when copying modifiers to the partial declaration.

Source location

All modifiers are copied without filtering:
https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs#L323-L325

writer.Write(string.Join(" ", methodDeclaration.Modifiers.Select(m => m.Text)));

Suggested fix

Filter out the async modifier:

var modifiers = methodDeclaration.Modifiers
    .Where(m => m.Kind() != SyntaxKind.AsyncKeyword)
    .Select(m => m.Text);
writer.Write(string.Join(" ", modifiers));

[stephentoub]

Addressed by #1059