Catalina Safari "Proxy Automatic Configuration" (PAC) is not used

Question

Running Catalina (10.15.6) My SOCKS5 proxy is a DynamicForwarddeclared port on a ssh session.
My pac file is ultra simple and tested good using online test tools, as well as Chrome which uses the OS's PAC setting.
To eliminate a filesystem sandbox issue, I host the PAC file using a python SimpleHTTPServer. I can see the file being downloaded when I configure it in Network prefs, as well as when I launch chrome and safari.
For some reason, Safari doesn't use my proxy. If I configure the proxy address and port directly under "Proxy" in network prefs, safari uses it fine.
Any clues or troubleshooting suggestion is welcome!

Jeff Dag · Answer

Here's what I learned:

Safari requires SOCKS localhost... not SOCK5 localhost..., but Chrome requires SOCKS5 localhost... So my pac file returns 'SOCKS5 localhost:12345;SOCKS localhost:12345'
Apps now being very sandboxed, you cannot use file:///Some/Path/proxy.pac that will be readable by all apps requiring the proxy config. World readable areas are now read-only in macOS.
If your pac file is not sensitive, save yourself the hassle of running an http server and host your file on some server somewhere. I used a gist.github.com file. Make sure you make it public and use the "raw" link to copy/paste into macOS' "Proxy Automatic configuration" URL.

Catalina Safari "Proxy Automatic Configuration" (PAC) is not used

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