An annoying trend I've been seeing recently, which the GitHub repo behind this does, is having better documentation for the robots than there is for the users.
Compare the README.md to the skills/pay-for-http-request/SKILL.md
> AFAIK this wouldn't work because MAC addresses don't get forwarded to the next network segment
Finally! I was scrolling down the comments, starting to think nobody on HN knew networking enough to notice this.
MAC addresses never "leave" the local network segment, so can't be used for tracking tethering unless the phone has code to forward this info on. But, at that point, the MAC address isn't really needed anymore ;)
Compare the README.md to the skills/pay-for-http-request/SKILL.md