It runs three places: on save in your editor, as a pre-commit step, and in CI. In CI, run it in check mode so it reports differences instead of writing them. A pipeline that reformats and commits on your behalf will eventually do it to a branch someone is working on.
Adding it to an existing project produces one very large commit that changes no behaviour at all. Make that its own commit, separate from any real change, so the diff stays readable and anyone can confirm nothing else moved.
Updated August 2026