Comment by astrobe_
4 hours ago
This reminds me of a comment I read here a long time ago; it was about XML and how DTDs were supposed to permit one to be strict. However, in reality, the person said, if the the other end who is sending you broken XML is a big corp who refuses to fix it, then you have no choice but accept it.
Bottom line: it's all a matter of balance of powers. If you're the smaller guy in the equation, you'll be "Postel'ed" anyway.
Yet Postel's law is still in the "the road to hell is paved with good intentions" category, for the reason you explain very well (AKA XKCD #1172 "Workflow"). Wikipedia even lists a couple of major critics about it [1].
Would be tempted to stick a proxy in there that checks if the data is malformed in the expected way, and if so converts it to the valid form before forwarding to the real service.