As a coffee addict and someone who fasts I keep looking into this. Strictly speaking a fast is supposed to allow a period of detox for the body so coffee isn't really a good idea.

That said it doesn't break your fast since it has a trace calorie content (provided no milk and sugar). After all some proponents of fasting recommend water with lemons or apple cider vinegar (for killing bio films/candida effects). I can't imagine coffee would be that different to lemon water calorie wise.

Pros and cons - if no coffee makes your fast unbearable have some (I always do), but ideally you're looking to consume only water.