@Szabolcs I got the type system working for Mathematica -> python. On the Mathematica side you make an encoder by providing a transformation rule and on the python side you provide a tuple specifying what to expect on the link.

Looks at PJLink/Mathematica/Encoders and PJLink/PJLink/Resources/Decoders for a sense of how this works

I need to make sure on-the-fly additions work as well. In Mathematica the rules are just held in PJLink`TypeHints`$TypeHints and new types ought to be possible to register with PJLink`TypeHints`RegisterTypeHint once I have that ready.

On the python side a KernelLink holds a TypeDecoder object that stores the decoder setups. Again, need to make sure on-the-fly loading works (although if you're down to work via files TypeDecoder.load_decoders loads all the decoders in that folder and adds them to itself).

Serializing these is non-trivial

I also updated the library source to hopefully compile properly

I pushed a 1.0.1 to the paclet server, so give that a PacletUpdate, now.