WithListToolsHandler not overriding default functionality fully

[dowdoliver]

Describe the bug
When defining a custom tools .WithListToolsHandler(SOME_FUNCTION), it doesn't fully override the tool handler

To Reproduce

builder.Services
    .AddMcpServer()
    .WithListToolsHandler((context, cancellationToken) =>
    {
        var result = new ListToolsResult();
        return ValueTask.FromResult(result);
    })
    .WithStdioServerTransport() 
    .WithTools<MyTool>()

When calling the tools/list method MyTool will still be listed

Expected behavior
Just the result from the WithListToolsHandler (empty list) should be returned

Additional context
From McpServer.cs

var originalListToolsHandler = listToolsHandler;
listToolsHandler = async (request, cancellationToken) =>
{
    ListToolsResult result = originalListToolsHandler is not null ?
        await originalListToolsHandler(request, cancellationToken).ConfigureAwait(false) :
        new();

    if (request.Params?.Cursor is null)
    {
        foreach (var t in tools)
        {
            result.Tools.Add(t.ProtocolTool);
        }
    }

    return result;
};

It feels like there should be a default list tool handler that just returns the list of tools, and it gets replaced by the listToolsHandler specified in options if it is not null. Let me know if I'm not understanding how it is supposed to be working.

[stephentoub]

This is by design. You're choosing to use both explicit tools and a manual handler, so we respect both. Listing will list out the explicit tools you added and also invoke your handler to augment the results. Invocation will check if it can match to one you explicitly registered, and if it can't then fall back to your invocation handler.