Cap'n Proto is only sortof-mutable. You can update e.g. an integer in-place, but if you want to change anything that is variable-width, e.g. modifying a string,

Cap'n Proto is only sortof-mutable. You can update e.g. an integer in-place, but if you want to change anything that is variable-width, e.g. modifying a string, adding/removing inner objects, etc., then it can get awkward. New objects will be added to the end of the buffer, and removed objects will just be zero'd out, wasting the memory. You can "garbage-collect" by making a whole-tree copy, of course. But yeah all the stuff about untrusted data is BS. NoProto's readme lists "Safely handle untrusted data" as an advantage vs. every other format it compares against, including Protobuf and JSON. What? Every serious serialization format supports untrusted data.