Modern / classic mode in arcanist

Hello,

I have just updated arcanist to c471983697b3ad8260d32886bfd52fdc1a2486be (after several months without doing it), and now when I execute arc unit or arc lint they work, but show this message:

arc unit
 (::)  Workflow is unrecognized by modern "arc", falling through to classic mode.

Where can I find information about that “modern” or “classic” arc, so I can update the config files?

Here is my .arcconfig, if you need to see it:

{
    "phabricator.uri" : "https://XXX.XXX.com/",
    "repository.callsign": "XXX",
    "unit.engine": "PhpunitTestEngine",
    "unit.phpunit.binary": "vendor/bin/phpunit",
    "phpunit_config": "config/build/phpunit.xml.dist"
}

This is expected. No “modern” version of the unit workflow is available yet. See https://secure.phabricator.com/T13488 for some discussion.

You can use stable instead of master if you’d prefer not to see this message.

I’ll take a look at T13488, and consider changing to stable if somethings goes wrong. Thank you!