The JSON specification doesn't support integers larger than 64-bits as far as I can tell, which makes sense given it's JavaScript-based origins. This leaves the question of how to represent larger numbers somewhat open today, leading to different approaches in the wild for dealing with such numbers, e.g. encode as string, ignore spec and use customized JSON parser, etc.

What do we expect folks to do in the case of System.Text.Json, and by extension, ASP.NET Core (dotnet/aspnetcore#43119)? Create and use a custom JsonConverter<Int128>? Would we consider including a default converter in the runtime, and if so, what would it be?