For that I can only think of writing custom JsonConverter. I will give you the steps + initial implementation. Further details you will have to figure out.

So, let’s start by implementing sample JsonConverter. Since you want to hide som fields, I assume you know the type and you know, based on user, what to hide and what not. For simplicity, I implemented “return nothing for unauthenticated user”:

public class JsonHideConverter<T> : JsonConverter<T>
{
    private readonly IHttpContextAccessor _httpContextAccessor;

    public JsonHideConverter(IHttpContextAccessor httpContextAccessor)
    {
        _httpContextAccessor = httpContextAccessor;
    }

    public override T? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        // For other types than specified,
        // do not do nothing, just return full info.
        if (typeof(T) != typeof(WeatherForecast))
        {
            JsonSerializer.Serialize(writer, value);
            return;
        }

        // If user is authenticated, return full info.
        if (_httpContextAccessor.HttpContext.User.Identity.IsAuthenticated)
        {
            JsonSerializer.Serialize(writer, value);
            return;
        }

        // Hide all together for unauthenticated user.
        return;
    }
}

All good, but it is very tricky to get IHttpContextAccessor into the converter. For that we have to implement another interface IConfigureOptions<JsonOptions> (JsonOptions from Microsoft.AspNetCore.Mvc namespace):

public class ConfigOptions : IConfigureOptions<JsonOptions>
{
    private readonly IHttpContextAccessor _httpContextAccessor;

    public ConfigOptions(IHttpContextAccessor httpContextAccessor)
    {
        _httpContextAccessor = httpContextAccessor;
    }

    public void Configure(JsonOptions options)
    {
        options.JsonSerializerOptions.Converters.Add(new JsonHideConverter<WeatherForecast>(_httpContextAccessor));
    }
}

And last, we need to register ConfigOptions as singleton, but it required IHttpContextAccessor, which by default is scoped service, but it is perfectly fine to register it as singleton:

builder.Services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
builder.Services.AddSingleton<IConfigureOptions<JsonOptions>, ConfigOptions>();

With all that, you can start using our converter and enhance it with you custom checks based on type of response being written and the user information at hand.

Hope that helps.

References:

• ASP.Net Core 2.1 – Is it possible to use DI inside of a custom JsonConverter class
• System.Text.Json how to get converter to utilise default Write() behaviour

According to your requirement, I implement this feature in my side. Here is the detailed steps.

ClaimBasedJsonConverter.cs

using System.Reflection;
using System.Text.Json.Serialization;
using System.Text.Json;

namespace WebApplicationApi
{
    public class ClaimBasedJsonConverter : JsonConverter<object>
    {
        private readonly IHttpContextAccessor _httpContextAccessor;

        public ClaimBasedJsonConverter(IHttpContextAccessor httpContextAccessor)
        {
            _httpContextAccessor = httpContextAccessor;
        }

        public override bool CanConvert(Type typeToConvert)
        {
            return typeToConvert.GetProperties().Any(prop => prop.IsDefined(typeof(ClaimSensitiveAttribute), true));
        }

        public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            throw new NotImplementedException();
        }

        public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
        {
            var httpContext = _httpContextAccessor.HttpContext;
            var userClaims = httpContext.User.Claims;

            writer.WriteStartObject();

            foreach (var property in value.GetType().GetProperties())
            {
                var propertyValue = property.GetValue(value);
                var claimSensitiveAttribute = property.GetCustomAttribute<ClaimSensitiveAttribute>();

                if (claimSensitiveAttribute == null)
                {
                    writer.WritePropertyName(property.Name);
                    JsonSerializer.Serialize(writer, propertyValue, property.PropertyType, options);
                    continue;
                }

                var hasRequiredClaim = userClaims.Any(c =>
                    c.Type == claimSensitiveAttribute.ClaimType &&
                    claimSensitiveAttribute.ClaimValues.Contains(c.Value));

                if (hasRequiredClaim)
                {
                    writer.WritePropertyName(property.Name);
                    JsonSerializer.Serialize(writer, propertyValue, property.PropertyType, options);
                }
            }

            writer.WriteEndObject();
        }
    }

}

ClaimSensitiveAttribute.cs

namespace WebApplicationApi
{
    [AttributeUsage(AttributeTargets.Property)]
    public class ClaimSensitiveAttribute : Attribute
    {
        public string ClaimType { get; }
        public string[] ClaimValues { get; }

        public ClaimSensitiveAttribute(string claimType, params string[] claimValues)
        {
            ClaimType = claimType;
            ClaimValues = claimValues;
        }
    }

}

Register it

using WebApplicationApi;

var builder = WebApplication.CreateBuilder(args);
// Register it 
builder.Services.AddHttpContextAccessor();

builder.Services.AddControllers().AddJsonOptions(options =>
{
    options.JsonSerializerOptions.Converters.Add(new ClaimBasedJsonConverter(builder.Services.BuildServiceProvider()?.GetRequiredService<IHttpContextAccessor>()));
});
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();
// Make sure you have this line
app.UseAuthentication();
app.UseAuthorization();

app.MapControllers();

app.Run();

Apply it in model

namespace WebApplicationApi.Models
{
    public class User
    {
        public string? Name { get; set; }
        public string? Email { get; set; }

        [ClaimSensitive("Plan", "Free", "Premium")]
        public string? SensitiveData { get; set; }

        [ClaimSensitive("Role", "Admin")]
        public string? AdminNotes { get; set; }
    }
}

Test it in controller

using Microsoft.AspNetCore.Mvc;
using System.Security.Claims;
using WebApplicationApi.Models;

namespace WebApplicationApi.Controllers
{
    [ApiController]
    [Route("api/[controller]")]
    public class UsersController : ControllerBase
    {

        [HttpGet]
        public IActionResult GetUser()
        {
            // If change Plan value to `Unknown`, 
            //{
            // "Name": "John Doe",
            // "Email": "[email protected]",
            // "AdminNotes": "Admin Only Notes"
            //}
            var claims = new List<Claim>
            {
                
                new Claim("Plan", "Premium"),  
                new Claim("Role", "Admin")     
            };

            var identity = new ClaimsIdentity(claims, "TestAuth");
            var user = new ClaimsPrincipal(identity);
            HttpContext.User = user;

            var userModel = new User
            {
                Name = "John Doe",
                Email = "[email protected]",
                SensitiveData = "Sensitive Premium Info",
                AdminNotes = "Admin Only Notes"
            };

            return Ok(userModel);
        }

    }
}

Test Result