the problem with drinking coffee during a fast is that it can interfere with your stomach and make you sick.
coffee and tea are marketed as being healthy, but they aren't. the good things they have are swamped by the fact they are difficult to digest. that is why people drink them when they need to shit. during a fast the stomach is scrubbing so you want to stick with just water and electrolytes.
so in other words plain coffee technicality won't break your fast BUT you will get superior results especially when it comes to digestion/stomach healing if you avoid it
i discuss coffee in my Ebook in my signature. and my advice is if you are addicted to caffeine, sugar, alcohol, nicotine or any other substance you need to fix those addictions first because during your fast you will get major withdrawal problems. so quit your coffee before fasting!
now here is the thing if you must have a cup of coffee and still want to fast my advice is drink it JUST IN THE MORNING and 1 cup only PLAIN. that is all you get. then atleast the rest of the day your stomach can scrub so you would get good benefits