I have encountered a related problem (see https://developer.apple.com/forums/thread/758255). I am wondering if you saw QUIC always enabled if assumesHTTP3Capable is true? (regardless enable_quic parameter in /Library/Preferences/com.apple.networkd.plist) In our case, we always use assumesHTTP3Capable to ensure a request is processed by QUIC. But we hit a blocker when a phone running iOS 16.0.2 always uses TCP (h2 I suppose) for some reason. (The backend only supports H3 / QUIC).

I compared the console log from a phone with this problem and another one that works: good case: (iOS 17.4) Connection 0: creating secure tcp or quic connection Connection 4: enabling TLS bad case: (iOS 16.0.2) Connection 1: enabling TLS I.e. the bad case is missing the log line "creating secure tcp or quic connection". Any ideas how this could happen?

Sorry about my late reply as I was out for the last week. So, this phone is unable to talk to your server at all? Yes, that's correct. We're looking into adding a HTTP/1.1 endpoint in the server as a backup. But that comes with own issues (TLS etc). Any ideas when / in what cases assumesHTTP3Capable does not take effect? P.S. the same phone also used to work a few months ago. But we cannot remember what updates have happened on this phone (currently it is running iOS 16.0.2). Thanks!