Using OneOf for polymorphic schema

Program.cs

using System.Text.Json.Serialization;

var bld = WebApplication.CreateBuilder(args);
bld.Services
   .SwaggerDocument(o => o.UseOneOfForPolymorphism = true) //enable the setting
   .AddFastEndpoints();

var app = bld.Build();
app.UseFastEndpoints()
   .UseSwaggerGen();
app.Run();

public class Apple : Fruit
{
    public string Foo { get; set; }
}

public class Orange : Fruit
{
    public string Bar { get; set; }
}

[
    JsonPolymorphic(TypeDiscriminatorPropertyName = "_t"),
    JsonDerivedType(typeof(Apple), "a"),
    JsonDerivedType(typeof(Orange), "o")
]
public class Fruit
{
    public string Baz { get; set; }
}

sealed class MyEndpoint : Endpoint<Fruit>
{
    public override void Configure()
    {
        Post("/api/fruit");
        AllowAnonymous();
    }

    public override async Task HandleAsync(Fruit r, CancellationToken c)
    {
        await SendAsync(r as Apple);
    }
}