Do you not have unit tests, or how does it introduce regressions? You can tell it how to run the test suite in CLAUDE.md
It is a well known fact that projects with unit tests never have regressions.
I detect the regression already in planning with Opus 5, so I do not let Opus 5 implement anything. But it is a waste of time and tokens! Does planning with Opus 5 works out for you?
Opus 5 tries to modify the unit tests as a cover to its own regressions - thinking its own logic is correct and the test must be wrongly specified