I know this is old, but here is my understanding of “100 Continue”
Your server is supposed to validate the request based on header alone from the client i.e. if the request is invalid, don’t send “100 Continue” but actual http error instead e.g. 403. This should prevent the client from posting the data which I understand is the whole point of roundtrip to the server (i.e. client waiting for “100 Continue”) in the first place.
If you are validating actual posted data, then you need to apply higher-level protocol here i.e. send your error wrapped in valid HTTP response content. Yes, it seems like limitation and I’m not assuming it’s protocol limitation; more likely client confusion having to handle server response more than once.