I am working on a freelance project for a software dev startup. The api service endpoints given to me is so full errors that you can boldly say it's zero percent tested and you'll be correct. The project was meant to last for a week but now it's going to a month due to the errors I have encountered while working with the given API service, so more like a back and forth wait for an update kind of thing. I am close to done building the client but yes they cannot test my last update because someone updated the login endpoint which now returns 500 internal server error. I really want to vent out my frustration to this company without loosing them to the project but honestly i don't know how to do it.

Edit: Just for a side note, about the relationship this client is my former company.