Developer turns Claude Code into an open-source personal trainer: CoAIch and 4 hard-won lessons

cubetes · reddit · 2026-10-13

Since March, the author has been building CoAIch, an open-source (AGPL-3.0) Claude Code plugin acting as a personal trainer, with 14 skills mostly written by Claude Code itself (v0.3.0 preview, used weekly by the author). It imports workouts from Strong/Hevy/openGym and steps/weight/sleep/body metrics from Apple Health, builds and audits programs, sets weekly targets with per-target reasoning, and runs reviews.

Lessons learned:

• Don't let the model do the maths — early versions drifted; now every number comes from a small tested Python script, and Claude just runs it and explains results.
• Safety rules in code, not prompts — prompts get skipped in long conversations; medical clearance, pain red flags, per-lift explicit OK for spine-loaded increases, and frozen weekly targets are enforced by scripts that refuse to continue.
• Plain files beat a database — CSV for sets, Markdown for plans, JSON for targets: Claude-readable, human-readable, git-friendly.
• Privacy took more work than expected — no server, no telemetry; optional photo review strips faces on-device with Apple Vision before Claude sees anything.

Try it with made-up data via the examples/demo-athlete folder in the repo.

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