Since March I've been building a Claude Code plugin to act as my coach and personal trainer my lifting. First time sharing it, here's what I learned

Hi all. I've been working on this since March, mostly evenings and weekends, and this is the first time I'm showing it to anyone. A bit nervous, so please be gentle. It started as my own private setup: a few prompts to plan my training. It grew into CoAIch, an open-source Claude Code plugin with 14 skills. I built most of it with Claude Code itself. It's an early preview (0.3.0, "Achilles preview") and I use it for my own training every week. A few things I learned on the way, in case they help someone building something similar: Don't let the model do the maths. My first versions let Claude work out next week's weights. They drifted: same data, different answer on a different day. Now every number comes from a small Python script with tests, and the skill files tell Claude to run the script and explain the result. Claude does the talking; the scripts do the numbers. Put the safety rules in code, not in the prompt. Prompts get skipped when the conversation goes long. So the checks are scripts that refuse to continue: a medical check before any program, which can stop the plan until a doctor clears you; a pain red flag that keeps exercises off that area; weight increases on spine-loaded lifts need your explicit OK, one by one; a week's targets are frozen once accepted, so the plan can't quietly rewrite its own history; it refuses to grade a week when the imported data stops early. Plain files beat a database for this. Everything lives in one folder: CSV for sets, Markdown for the plan and check-ins, JSON for targets. Claude reads it easily, I can read it, and it fits in git. Privacy took more work than I expected. There's no CoAIch server and no telemetry. Photo review (off by default) removes your face on your Mac with Apple Vision before Claude ever sees the picture. /feedback only drafts a GitHub post; you read it and post it yourself. What it does today: imports workouts from Strong, Hevy and openGym, and steps, weight, sleep and body measurements from Apple Health (via Health Auto Export). Builds and audits a program. Sets next week's targets with a reason for each. Weekly review. A longer roadmap. For now it's built for iPhone and Apple Health users. Try it with made-up data: clone the repo, open examples/demo-athlete, run claude, then /weekly-review. /plugin marketplace add cubetes/coaich /plugin install coaich@coaich /onboard Repo: github.com/cubetes/coaich (AGPL-3.0) I'd really like to hear how you structure big plugins: skills vs scripts, how you test them, what I'm doing the hard way. And anything that's confusing or breaks when you try it. Thanks for reading!