Admittedly, this is a huge weakness of Cap'n Proto. The C++ implementation (which is the one I use and work on personally) is mature. There are pretty solid implementations in Rust and Go, too. But it falls off after that, with most implementations being serialization-only and at various levels of (im)maturity.
There's not a lot I can do about this. Cap'n Proto adoption doesn't directly drive revenue for anyone in particular, so I can't hire an army of engineers to throw at it... People who want better Cap'n Proto support in each language need to step up to help make it happen.
One thing I am looking at doing is making it easier for per-language serialization implementations to bind to the C++ RPC implementation. This might make a lot of sense, since the serialization implementations have wide APIs but shallow implementation details, while the RPC implementation is a pretty narrow API with very complex implementation. And it turns out Cap'n Proto messages are super-easy to pass between languages since the in-memory format is by design the same across languages -- passing around byte buffers tends to be pretty easy.
There's not a lot I can do about this. Cap'n Proto adoption doesn't directly drive revenue for anyone in particular, so I can't hire an army of engineers to throw at it... People who want better Cap'n Proto support in each language need to step up to help make it happen.
One thing I am looking at doing is making it easier for per-language serialization implementations to bind to the C++ RPC implementation. This might make a lot of sense, since the serialization implementations have wide APIs but shallow implementation details, while the RPC implementation is a pretty narrow API with very complex implementation. And it turns out Cap'n Proto messages are super-easy to pass between languages since the in-memory format is by design the same across languages -- passing around byte buffers tends to be pretty easy.