I would to advise members here to refrain from carrying certain bike tools on their body. With this, I mean, in your jersey pockets or in any bags (including hydration packs) you have on your back.
Bicycle pumps, shock pumps and multi tools are all tools made from metal/hard materials. If you fall on your back, these hard items can cause injuries to back or even severe injuries to your vertebrae.
The best things to carry in your hydration bags are water and other soft items. One way of testing them out..put whatever you want into these bags. Put them on. Lean against a wall..if there's no pain or nothing hard poking into your back, then it should be fine.