Global response modifier

Program.cs

var bld = WebApplication.CreateBuilder(args);
bld.Services
   .SwaggerDocument()
   .AddFastEndpoints();

var app = bld.Build();
app.UseFastEndpoints(
       c => c.Endpoints.GlobalResponseModifier
                = (ctx, content) =>
                  {
                      ctx.Response.Headers.Append("x-some-header", content as string);
                  })
   .UseSwaggerGen();
app.Run();

sealed class SomeEndpoint : Ep.NoReq.Res<string>
{
    public override void Configure()
    {
        Get("some-endpoint");
        AllowAnonymous();
    }

    public override async Task HandleAsync(CancellationToken ct)
    {
        await SendAsync("hello world!");
    }
}

@tmorejon
try this: https://fast-endpoints.com/docs/configuration-settings#custom-response-dto-serialization

That worked perfectly. Thank you for your help.