They are both zero-copy, but the designs have a number of differences. I compared them back in 2014: https://capnproto.org/news/2014-06-17-capnproto-flatbuffers

They are both zero-copy, but the designs have a number of differences. I compared them back in 2014: https://capnproto.org/news/2014-06-17-capnproto-flatbuffers-sbe.html https://capnproto.org/news/2014-06-17-capnproto-flatbuffers-... Disclaimers: 1) I'm the author of Cap'n Proto; I'm probably biased. 2) A lot could have changed since 2014. (Though, obviously, serialization formats need to be backwards-compatible which limits the amount they can change...)