Our code is fairly interface agnostic - before iOS 18.0, we worked correctly on iPhone over Wi-Fi, an ethernet dongle, and Personal Hotspot (USB and Wi-Fi), and on macOS across every interface type we could find (Wi-Fi, Ethernet, Thunderbolt, etc). This seems to be caused by a larger change in iOS 18, of which you can see a symptom in my original post in this thread - every client connected to an iPhone Personal Hotspot is given an IP address of 192.0.0.2 I understand that this is an implementation detail, and iPhone Personal Hotspot isn't necessarily designed for this functionality, but it'd be really useful to understand the intention behind this change, and what we can do to convince Apple to re-enable the STA->STA use case.

For anyone reading this in future, here's what we've been told by the team on Feedback Assistant - I simplified the example to using ping to ping another client on the network, which doesn't work as of iOS 18.0. Please know you can’t use IPv4 to communicate between client devices, you have to use IPv6. IPv6 link-local is perfect for this. You can use ping6 with any of the configured IPv6 addresses to verify. I can now confirm that using one of the configured IPv6 addresses, we can communicate between clients on the iPhone Hotspot. It's an unfortunate change for us, but one I expect we'll need to handle - our products don't currently support IPv6.