The script is here, if you're interested: https://source.chromium.org/chromium/chromium/src/+/main:chrome/installer/mac/sign_chrome.py

I've spoken with hmare, and wile she was very kind and helpful, we weren't able to root cause the problem. She suggested I post to the chromium-dev Google group, but I didn't get any traction on this question there: https://groups.google.com/a/chromium.org/g/chromium-dev/c/xkx_AhcLZ8Q Unfortunately my project isn't open source right now :(

We're using HTTPS CONNECT proxying. It's a L4 tunnel created by the proxy instance to enable end-to-end TLS between the client and the destination server. I'ved tried both the nw_proxy_config_create_relay API as well as the nw_proxy_config_create_http_connect API. The latter gives this error message: -[ViewController webView:didFailProvisionalNavigation:withError:]. Error Error Domain=NSURLErrorDomain Code=-1000 "bad URL" UserInfo={NSErrorFailingURLKey=https://api.ipify.org/, ...