You can build your systems with as few assumptions as possible. The fewer assumptions you make, the less probable it is that any of your future assumptions will conflict with your previous assumptions. Your code will be built for change.

If your API call to some external system assumes the existence of a particular button in the UI, then your system isn’t built for change. Maybe you want to change this button? Then you need to go through all places in the code that relies on this particular button to see if it doesn’t conflict with any of their assumptions.