Just ran into a somewhat similar issue (erlang deps didn’t seem to be cached) and in my case the problem was in not caching deps folder after running mix compile. This is important since rebar3 actually stores ebins in deps, and in _build mix only creates symlinks.

So, my approach before (which had the same problem as in OP):

# deps stage
- cache pull deps
- mix deps.get
- cache push deps

# compile stage
- cache pull deps, _build
- mix compile
- cache push _build

and my approach now:

# deps stage
- cache pull deps
- mix deps.get
- cache push deps

# compile stage
- cache pull deps, _build
- mix compile
- cache push deps, _build # <-- !!!

Once for each stage, yes. Otherwise mix complains about missing dependencies.

We were made aware of this recently and got it fixed. Didn’t realize all compilation wasn’t done to deps by mix

The rebar3 version installed by mix should now be 3.13.1 which supports an additional argument of where to send output.

I’m not sure what version of mix uses the new output option… Will go look in a bit.