Custom handler isn't being called

While handlers that handle 'known' events such as IHoverHandler work, I cannot find a way to get a custom handler to register.

In the code below, placing a breakpoint on server.WaitForExit shows that the handler isn't in the server.HandersManager's list of handlers. The other known handlers are registered and work well.

I cant find any errors in any output as to why this wouldn't register in the output, it appears to just silently fail.

What would cause the handler to be ignored? Shouldn't there be an error or warning somewhere?

using MediatR;
using OmniSharp.Extensions.JsonRpc;

namespace BitMagic.X16Debugger.LSP;

[Method("bitmagic/preview", Direction.ClientToServer)]
public class PreviewHandler : IJsonRpcRequestHandler<PreviewHandler.PreviewParameters, PreviewHandler.PreviewResult>
{
    public async Task<PreviewHandler.PreviewResult> Handle(PreviewHandler.PreviewParameters request, CancellationToken cancellationToken)
    {
        return new PreviewResult() { Content = "This is something" };
    }
    public class PreviewParameters : IRequest<PreviewResult>
    {
        public string Filename { get; set; } = "";
    }

    public class PreviewResult
    {
        public string Content { get; set; } = "";
    }
}

    public async Task Run()
    {
        var server = await LanguageServer.From(options =>
        {
            options.Services
                .AddSingleton<DocumentCache>()
                .AddSingleton<TokenDescriptionService>()
                .AddSingleton<ITokenDescriptionProvider, X16KernelDocumentation>()
                .AddSingleton<ProjectService>()
                .AddSingleton<ProjectBuilder>()
                .AddSingleton<ServiceManager>(e => new ServiceManager(getEmulator, e.GetService<IEmulatorLogger>()))
                .AddSingleton<FileChangeHandler>()
                .AddSingleton<PreviewHandler>()
                .AddSingleton<IEmulatorLogger, Logger>()
                ;

            options
                //.WithInput(new LoggingStreamWrapper(inputStream, "input"))
                //.WithOutput(new LoggingStreamWrapper(outputStream, "output"))
                .WithInput(inputStream)
                .WithOutput(outputStream)
                .WithHandler<HoverHandler>()
                .WithHandler<FileChangeHandler>()
                .WithHandler<PreviewHandler>()
                .WithLoggerFactory(new LogFactory())
                .ConfigureLogging(logging =>
                {
                    logging.SetMinimumLevel(LogLevel.Trace); // Trace logs everything
                })
                .OnInitialize(OnInitialise)
                ;
        });

        await server.WaitForExit;
    }

The code generator appears to be working:

using OmniSharp.Extensions.JsonRpc;

[assembly: AssemblyJsonRpcHandlers(typeof(BitMagic.X16Debugger.LSP.PreviewHandler))]

Begin with the Run method's options.Services registration and the subsequent WithHandler() call; compare that path with the working HoverHandler registration and inspect the generated AssemblyJsonRpcHandlers attribute. Done means identifying why PreviewHandler is omitted and establishing whether the behavior should produce a warning or register the handler.