ASP.NET Custom Response on Model-Validation Error

InvalidModelStateResponseFactory.cs

using System;
using Microsoft.AspNetCore.Mvc;

public class InvalidModelStateResponseFactory
{
    public static ProblemDetails Create(ActionContext _) => new();

    public class ProblemDetails : IActionResult
    {
        public Task ExecuteResultAsync(ActionContext context)
        {
            context.HttpContext.Response.StatusCode = 418;
            context.HttpContext.Response.Headers.Add("custom-header", "custom-value");
            return Task.CompletedTask;
        }
    }
}

Program.cs

builder.Services.AddControllers().ConfigureApiBehaviorOptions(opt =>
{
    opt.SuppressMapClientErrors = true; // Automatic problem-details generation must be disabled to activate the custom factory.
    opt.InvalidModelStateResponseFactory = InvalidModelStateResponseFactory.Create;
});