Domink:
Thanks for the solution, it works like a charm.  

@Momchil can you guys take a look at Dominics solution and see if its feasible to integrate into the solution.  I completely agree that adding the two serialization engines to the service container is a bad idea.  No matter the order, .Net 8 seems to want to use Newtonsoft instead of System.Text.Json which is the opposite of what we want.

I have attached a working sample based on "Progress\Telerik Reporting R3 2023\Examples\CSharp\.NET 8\Html5IntegrationDemo"

What I changed:
- Added 
--ToNewtonSoftActionFilter, NewtonSoftJsonModelBinder, ValidateModelStateAttribute
--ReportDesignerControllerSTJBase, ReportsControllerSTJBase
- Program.cs
-- Removed AddNewtonsoftJson
-- Added "builder.Services.AddSingleton<ToNewtonSoftActionFilter>();"
- ReportDesignerController and ReportsController use the new base classes

@Telerik You might want to update your NET8 sample. It still points to NET6.

I'd be very interested in your solution. Could you post a complete example?

Thank you.

I didn't like either option. So I've made a modification to the second one.

I didn't like the first because that would mean too much overhead and I really wanted to avoid adding NewtonsoftJson to the service container. The two aren't really mean to coexist and the hack may break on any change on either package.

So instead of using a "MySuperJsonInputFormatter" which I also dislike because of its global nature, I did the following:
I created a wrapper Controller for ReportDesignerControllerBase and ReportsControllerBase which has a ServiceFilterAttribute attached to it which will take the result object of any endpoint, use the newtonsoft serializer to produce a json string and replace the endpoint result with that. The request parameters which are classes have a ModelBinderAttribute attached which will cause the objects to be deserialized by newtonsoft.

Example:

[ServiceFilter(typeof(ToNewtonSoftActionFilter))]
public class ReportsControllerSTJBase : ReportsControllerBase {

...

[HttpPost("clients/{clientID}/instances/{instanceID}/documents", Order = -1)]
public virtual new IActionResult CreateDocument(string? clientID, string? instanceID, [FromBody][ModelBinder(typeof(NewtonSoftJsonModelBinder))] CreateDocumentArgs args) => base.CreateDocument(clientID, instanceID, args);

...

}

With this I only need to add the NewtonSoft package but the usage is completely confined to the two controllers. No ".AddNewtonsoftJson" or any global Formatters. And I haven't had any problems since I started using it many month ago.

To make this work less tedious, I've automated the creation (by using reflection) to create those wrapper classes.

Now here is my question:
I'd much rather not have to create those wrapper classes myself, instead it would be awesome if this or something like it was already contained in the package, for various reasons. So would it be possible to include something like that?

If interested I could clean up the code a bit more and post it here.

>> NOTE: This would be a breaking change.

Understood. But without this fix, one cannot use Kendo and Telerik Reporting in the same .NET6 app.