[Spivak]

Is there a way to do this that doesn't require keeping two sets of books for every API? Because the JSON API is right now the canonical one and still has to work. I don't imagine the lift is worth it for the difference between compressed JSON and BSON.

Also how much can you realistically win when the payload for small messages where the difference matters are text?

[xx_ns]

Modern serialization libraries make supporting multiple serialization formats pretty transparent - of course, I'm not sure what the current situation is in Elixir land, which Discord seem to be using, but Go and Rust (as trivial examples) have serialization libraries which make serialization based on content negotiation pretty much transparent. Of course, this doesn't help with testing, you'll still need to be testing both content types separately, but the savings in bandwidth might just be worth it.