I built a terminal app that paces slow breathing at 6 breaths per minute for vagal tone training. It's a single Python file, stdlib only, no dependencies — just run breathe and follow the bar.I'm a cardiology patient (HFrEF). Slow breathing at resonance frequency is one of the few non-pharmacological interventions shown to improve cardiac vagal tone and baroreflex sensitivity (Bernardi et al., Circulation 2002; Lancet 1998). I wanted a frictionless daily habit tool — no app store, no account, no subscription, just open terminal and go.Design constraints, all grounded in the clinical literature:- No breath retention — Valsalva risk in cardiac patients- No rapid breathing — minimum 8-second cycles- Exhale ≤ 2x inhale — no evidence for extreme ratios- Immediate exit, always — q or Ctrl+C restores the terminal even on crashThe README includes a resonance frequency measurement protocol for anyone with a chest-strap HRV monitor who wants to find their individual optimum instead of using the 6 bpm default.macOS only (uses afplay for audio cues). MIT licensed.pip install breathe-cliorbrew tap marekkowalczyk/breathe && brew install breathe.