Personal opinion: don't put floating point numbers into an API. If you really want real numbers passing them as decimal string is actually sensible. This can be done without using JSON, of course. Can also do fixed precision as well, which will store reasonably well into a VLQ.

OTOH this is pretty cherry picked. It is questionable if you really need floating point numbers if your numbers are all this small and low precision, but in actuality unless this comprises your entire message, you'll probably still lose to JSON overhead eventually anyways, whereas non-self-describing serialization like capnp has effectively no overhead by default. The advantage of encoding as doubles is that it is predictable and won't explode into taking massively more, its always the same size per number. If you want it to be smaller for some reason, compression is always an option, though I suspect for IPC it's the wrong thing to be concerned about.