In Hevy: Profile → ⚙ Settings → Export & Import Data → Export Workouts. Then upload workout_data.csv here. Re-importing is safe — duplicate sets are skipped.

Unofficial sync — MyFitnessPal has no public API, so this can break if they change their site, and it's against MFP's terms. Your login is stored encrypted on the server and only used to fetch your diary totals.