The name Cap'n Proto is bit strange. The name makes me thing of CAP'N CRUNCH CEREAL and prototyping, but then I read about the connection to protocol buffers. Might want to think of a different name, e.g. Zero Data X "zero cost data interchange" or something.

I am using JSON to save structured data for an application, and this is interesting, although my app doesn't spend alot of time encoding/deconding the JSON since the data structures are fairly simple.

If I wanted to use this for Java, wouldn't there still be decode step to convert the fields into java objects?