Can I answer with a question @LetMeLayAmongTheStars: What's the goal of your fasting? Lots of people fast for different reasons: Religious, weight loss, curiosity, fat burning, etc.
Depending on WHY you're doing the fast will help decide whether or not a drink (other than water) breaks a fast, although the guidance from @OverByYer is pretty safe in almost all situations.
From what I've seen lately, most people are fasting for some kind of health reason, either weight loss or burning fat or they're just curious to know if you can run 100 miles without eating. (You can.)
The general rule for what's OK to drink while you're aiming for a performance or weight loss goal will be low to no sugars, so that knocks out even weak squash.
@Zipadeedoodah & @QueenOfOversharing those methods (a couple of drinks instead of one, and all of 'em low calorie) with the goal of losing weight seem pretty sound. The biggest thing I noticed doing a full-on 5 Day Water Fast was not an intense hunger, but just that I had so much time on my hands.
As far as fat & fasting, I wrote this up as a more in-depth dive specifically for the butter coffee idea people are using now. May be useful to you, hope it helps.
~Nik
www.paleotreats.com/blogs/paleo-desserts-and-paleo-recipes/fat-fasting-should-you-drink-butter-coffee