Not to mention, JSON APIs start out simple because you just call `to_json` on whatever object you need to share and then move on. Except nobody’s ever documented the format, and handling ends up taking place in a million different places because you can just pass the relevant bits around. The API is “whatever it did before we split it up”.

Now when someone goes to replace one side, it’s often impossible to even figure out a full definition of the structure of the data, much less the semantics. You watch a handful of data come across the pipe, build a replacement that handles everything you’ve seen, and then spend the next few months playing whack-a-mole fixing bugs when data doesn’t conform to your structural or semantic expectations.

JSON lets you get away with never really specifying your APIs, and your APIs often devolve to garbage by default. Then it becomes wildly difficult to ever replace any of the parts. JSON for internal service APIs is unmitigated evil.