Nancy + problem+json extension using https://www.nuget.org/packages/Tavis.Problem/

module.cs

 public class HomeModule : NancyModule
    {
        public HomeModule()
        {
            Get["/"] = _ => "Hi";

            Post["/"] = _ =>
            {
                this.ModelValidationResult.Errors.Add("FirstName", "Firstname must start with a Z");
                this.ModelValidationResult.Errors.Add("LastName", "Lastname must rhyme with Orange");
                return Response.AsHTTPValidationProblem();
            };
        }
    }

response.json

{
  "type": "http://localhost:1234/validation-error.html",
  "title": "Unable to bind incoming data",
  "status": 422,
  "detail": "Unable to bind incoming data to the expected model",
  "validation-errors": [
    {
      "Key": "FirstName",
      "Errors": [
        "Firstname must start with a Z"
      ]
    },
    {
      "Key": "LastName",
      "Errors": [
        "Lastname must rhyme with Orange"
      ]
    }
  ]
}

responseextensions.cs

public static class FormatterExtensions
{
    public static Response AsHTTPValidationProblem(this IResponseFormatter formatter)
    {
        var problemDoc = new ProblemDocument
        {
            ProblemType = new Uri(formatter.Context.Request.Url.ToString() + "/validation-error.html"),
            Title = "Unable to bind incoming data",
            StatusCode = (System.Net.HttpStatusCode)422,
            Detail = "Unable to bind incoming data to the expected model",
        };
        var errors = formatter.Context.ModelValidationResult.Errors.Select(x => new { Key = x.Key, Errors = x.Value.Select(y => y.ErrorMessage) });
        problemDoc.Extensions.Add("validation-errors", JToken.FromObject(errors));
        var resp = new Response
        {
            StatusCode = HttpStatusCode.UnprocessableEntity,
            ContentType = "application/http-problem+json",
            Contents = (Stream stream) =>
            {
                problemDoc.Save(stream);
            }
        };
        return resp;
    }
}