While I don't like to say anything is 100%.... these harnesses are close to Escape Proof! 
Designed to fit snuggly around the belly and neck so that cats and small dogs cannot back out of them. We took one of our cats outside in it and he did have a complete freak out moment unfortunately, but the harness did not come off! 
 
Please make sure if it is your first time using a harness with your cat that you watch plenty of videos how to acclimate/ get them used to harness walking... they tend to be a bit different than just throwing a harness on a dog! 
 
When ordering, please indicate which fabric you would like under the "available fabrics" section of the website, but make sure it's a fabric that says "1/2 width+" 
 
Harnesses are made with 1/2" width. Checkout the premades section for current available ones!