Here is a working Converters for AggregateResult and AggregateFunction.

https://blazorrepl.telerik.com/mpYQaRbr37AmEXvM26

Note: Make sure to add the converters both to your client and server app:

 services.AddControllers()
     .AddJsonOptions(o =>
     {
         o.JsonSerializerOptions.Converters.Add(new AggregateResultJsonConverter());
         o.JsonSerializerOptions.Converters.Add(new AggregateFunctionConverter());
         o.JsonSerializerOptions.PropertyNameCaseInsensitive = true;
         o.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
     })

CamelCase is importent so the AggregateFunctionConverter works.

On Client, you have to set the JsonOptions with each request:

 private readonly JsonSerializerOptions jsonOptions = new()
 {
     PropertyNameCaseInsensitive = true,
     PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
 };

My custom HttpClient to handle this:

 public HttpClientHandeld(HttpClient client, IToastService toastService)
 {
     this.client = client;
     this.toastService = toastService;
     this.jsonOptions.Converters.Add(new AggregateResultJsonConverter());
 }

Get:

var response = await this.client.GetFromJsonAsync<T>(requestUri, this.jsonOptions);

Post:

var response = await this.client.PostAsJsonAsync(requestUri, value, this.jsonOptions);

Any update on this? Or any way we can do server-side grouping? For large grids, aggregate performance is very bad client-side - needs to be done server-side.