> I'd like to understand which objects are being passed which can't be marshalled
AFAIK, the type for the objects which can't be marshelled is `collections.defaultdict`. 

> I'm worried that two different behaviours between the transports where one silently drops data isn't a good idea.
I agree. need to see if there is a way to convert to marshallizable object. 

> I'd also be interested to learn more about how you're using bitbake if you can share anything?
I'm developing bbclient that provides command interface for bitbake server. I'm working this as an part of visual studio language server for bitbake.
I'm sorry. Not very common use, I know😅