I run a consulting company. Working with startups, cost-plus model is the only way to go. It is maddening when customer does not know exactly what they want to

I run a consulting company. Working with startups, cost-plus model is the only way to go. It is maddening when customer does not know exactly what they want to do because they figuring things as they go, often "pivioting", but yet asks you to provide a fixed cost estimate for the work and sign contract which often includes penalties or payment on deliveiry. Usually my answer is: I will be glad to give you fixed cost quote if you can provide me with detailed spec of whay you want me to do. But this is exactly what you will get. If you want to have a freedom of changing things on the fly, it must be cost-plus.