The type or namespace name NewtonsoftJsonOutputFormatter could not be found

Environment

Product	Progress® Telerik® Reporting
Version	20.0.26.211+

Description

After upgrading the Telerik Reporting packages to version 20.0.26.211(2026 Q1) or newer, I can no longer build the project due to the errors mentioned further down below.

I use one of the approaches from the Use Both 'System.Text.Json' and 'Newtonsoft.Json' for Serialization in ASP.NET Core article, so that I can use NewtonsoftJson for the Reporting REST Service, and System.Text.Json for everything else.

Error Messages

• The type or namespace name 'NewtonsoftJsonOutputFormatter' could not be found (are you missing a using directive or an assembly reference?)
• The type or namespace name 'NewtonsoftJsonInputFormatter' could not be found (are you missing a using directive or an assembly reference?)

Cause

Starting with the Progress® Telerik® Reporting 2026 Q1(20.0.26.211) release, the .NET implementation of the Reporting REST Service for the web based report viewer and report designer no longer depends on the Microsoft.AspNetCore.Mvc.NewtonsoftJson package.

Because of this change, after the Telerik Reporting packages are updated, the NewtonsoftJson assembly will no longer be available, and the types that depend on it will not be resolvable.

Solution

Case 1

If the workarounds. demonstrated in the Use Both 'System.Text.Json' and 'Newtonsoft.Json' for Serialization in ASP.NET Core article, were used only due to the dependency on NewtonsoftJson that the Reporting REST Service had previously, then you no longer need them, and they can be removed.

Starting with the Progress® Telerik® Reporting 2026 Q1(20.0.26.211) release, the Telerik Reporting packages, when used in .NET Standard 2.0+/.NET 8+ projects, will instead depend on the System.Text.Json package.

No additional setup will be needed to use the Reporting controllers, simply remove the AddNewtonsoftJson() extension method calls from the application startup file(Program.cs/Startup.cs).

If the Minimal API setup is used for the Reporting REST Service, it won't be affected even if the AddNewtonsoftJson() is not removed.

Case 2

If there are controllers in the project where NewtonsoftJson needs to be used, the approaches from the Use Both 'System.Text.Json' and 'Newtonsoft.Json' for Serialization in ASP.NET Core article can still be used, but opposite of what has been recommended there.

The approach that we would recommend being used if you do need to keep utilizing NewtonsoftJson in some places of the project, is the Local workaround.

You can create ServiceFilterAttribute, which you can attach to any controller that need to use a different JSON serialization.

It will take the result object of any endpoint, use the NewtonsoftJson serializer to produce a JSON string, and replace the endpoint result with that.

C#

using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc.Filters;

public class ToNewtonsoftActionFilter : IAsyncResultFilter
{

	public async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
	{
		if (context.Result is JsonResult jsonResult)
		{
			string jsonStr = Newtonsoft.Json.JsonConvert.SerializeObject(jsonResult.Value,
				(Newtonsoft.Json.JsonSerializerSettings?)jsonResult.SerializerSettings
				?? new Newtonsoft.Json.JsonSerializerSettings
				{
					ContractResolver = new Newtonsoft.Json.Serialization.DefaultContractResolver
					{
						NamingStrategy = new Newtonsoft.Json.Serialization.CamelCaseNamingStrategy()
					}
				});

			context.Result = new ContentResult { Content = jsonStr, ContentType = "application/json" };
		}

		await next().ConfigureAwait(false);
	}
}

C#

[ServiceFilter(typeof(ToNewtonsoftActionFilter))]
[Route("api/[controller]")]
public class ValuesController : ControllerBase

You would also need to create ModelBinderAttribute attached which will cause the objects to be deserialized by NewtonsoftJson.

C#

public class NewtonsoftJsonModelBinder : IModelBinder
{
	public async Task BindModelAsync(ModelBindingContext bindingContext)
	{
		if (bindingContext == null) throw new ArgumentNullException(nameof(bindingContext));

		using var reader = new StreamReader(bindingContext.HttpContext.Request.Body);

		string body = await reader.ReadToEndAsync().ConfigureAwait(continueOnCapturedContext: false);
		object? value = Newtonsoft.Json.JsonConvert.DeserializeObject(body, bindingContext.ModelType);
		bindingContext.Result = ModelBindingResult.Success(value);
	}
}

C#

public virtual new IActionResult GetMembers([ModelBinder(typeof(NewtonsoftJsonModelBinder))] TypeInfoWithFilter input) => base.GetMembers(input);

Finally, you would need to register ToNewtonsoftActionFilter as a Singleton service in the Program.cs. but the usage is completely confined to the specific controllers where you wish to still use that serializer instead of System.Text.Json.

C#

builder.Services.AddSingleton<ToNewtonsoftActionFilter>();

See Also

• How to configure two JSON serializers and select the correct one based on the route
• Use Both 'System.Text.Json' and 'Newtonsoft.Json' for Serialization in ASP.NET Core