> Is it input validation to say a string must be at least some length characters? What if that string is a password? What if it's an MD5 sum?

Think of it this way: pre-conditions are what the function assumes to be true when it’s called. If it would not be justified to make that assumption at the point where the function is called, it is the responsibility of the caller to perform a runtime check to ensure that it applies. So you might well have a precondition at the level of the function and an explicit check of the exact same condition on the caller side to ensure that it’s respected. If the caller fails to ensure that the condition is fulfilled, and the precondition thereby trips, that’s a programming error.

Yes. Yes it is. Input validation should happen at the public API and then contracts after that.

Contracts are very explicitly for things you don't want to ever see happen. They are "something went wrong".

So your API would accept a function that handles input validation and maybe has its own contract for things it explicitly doesn't handle but then internally you use contracts to enforce that the string is already input validated and you don't have to enforce edge cases.