Start with solving that.

IMHO commit hook should not modify code.

FYI this will be a real pain the first time somebody on your team needs to push up code that won’t compile so that a team member can help them.

I agree with @hauleth, this is the problem to solve. You can put specific rules about specific branches, rules about how PRs get merged etc, and it lets you handle mix credo and mix test centrally too.

I am strongly against imposing workflows on developers collaborating on a project. Everybody has a different order/process for producing code that is ready for review, and until its ready for review (and CI) all barriers and challenges should be minimized. If part of your process is to autoformat your code on every commit using a git hook, by all means do that, but please, please don’t impose that on your fellow devs who do not want to think about formatting at all until everything is ready.

Hard on hook. Fail on CI to enforce what you want

I like writing a custom mix ci task/alias. The CI server should be configured to execute that, but also each developer can choose to run that on their own machine in case they need help diagnosing a problem that only occurs on CI.