I'm not sure I said exactly what is paraphrased above, but what I actually tried to refer to was https://secure.phabricator.com/T5051#91031

Ah then we misunderstood each other at FOSDEM. Editing description accordingly.

I don't think we are going to get any API promise. Then again, do we know about problems of incompatibility between Phabricator and Arc versions? https://secure.phabricator.com/project/board/2/query/all/ seems to be quiet in this respect.

Besides, we are updating Phabricator about every month, and Arc is "installed" as a local git repository, so in the worst case scenario your Arc version is behind, and all you need to do is to pull from master, right?

Can we get it documented that there is neither forward nor backwards compatibility if that is the case? Then we can act accordingly.

No arc might be installed as a package, that means it might be up to 5 years old. Or it might be installed from git as you said and then it might be 1 month younger.

From previous experience I can say that backwards and forwards compatibility are often broken. Indeed, for guaranteed compatibility, arcanist must be kept strictly in sync with the phabricator version we are running in production. Many changes are not compatibility-breaking, but some updates will definitely require coordination. We will need to push arcanist changes to our own fork of arcanist whenever we update phabricator and we will need to provide an easy way to pull these updates. The simplistic approach is simply to have everyone install arcanist from our git repo and periodically pull from there to stay updated.