Ben Greenfield explains what breaks a fast: any drink with meaningful calories, like coffee with cream or butter, will end it. Black coffee, black tea, and water with stevia are fine. Caffeine actually boosts fat burning during fasting, as long as you skip the calories.
First of all, I'm often asked if coffee breaks a fast. Think about it this way: anything that contains an appreciable number of calories will break a fast. So black coffee does not break a fast. An orange mocha Frappuccino or an original black coffee with a whole bunch of cream and sugar in it does break a fast. Now, coffee is actually an interesting one, along with some teas, because the caffeine in them can also help to support a fast. They can shift your body into a higher amount of beta oxidation or fat burning. It can also help with alertness and overall metabolic rate. Just don't add a bunch of butter or MCT oil or coconut oil or creamer or sugar or anything else to that morning beverage if you truly want to stay fasted. Now, other things that don't contain calories like, uh, black tea, sparkling water, non-calorie sweeteners like stevia or monk fruit would also be generally fine. But if it has calories, it breaks a fast.
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