I discovered by chance that by placing a wooden stick or wooden spoon inside my jar of coffee powder, this will prevent the coffee powder from hardening. It was a pure fluke discovery!
I have always hated scraping hardened coffee off the glass bottle each time the coffee powder reaches the bottom of the jar. Recently, I noticed that each time I put a recycled ice-cream stick inside the coffee powder bottle, the coffee powder remains fresh and loose until the last granule. After finishing a few bottles, I realized that it must be the wooden stick absorbing moisture from the coffee powder. And that's the reason why my coffee powder stays fresh and loose. I've tried placing a stainless steel spoon in the bottle and the coffee powder hardened even faster. It must be the compounds in coffee that reacted oddly with the metal spoon.
Hardly any hardened coffee powder at the bottom of the bottle. The coffee granules stayed fresh and loose till the last bit.
If you've always struggled with scraping and hitting your bottle to get the dang hardened coffee powder out, try the wooden spoon/stick method and see if it works!