# DSF

- **Event:** [Built with Opus 4.7: a Claude Code hackathon](https://cerebralvalley.ai/e/built-with-4-7-hackathon)
- **When:** Apr 21 at 12:00 PM – Apr 27 at 2:00 AM (EDT)
- **Where:** Online
- **Team:** [Nathan Delacretaz](https://cerebralvalley.ai/u/Nathandela)
- **GitHub:** https://github.com/Nathandela/cahier
- **Demo video:** https://youtu.be/z3ugubz_1Ms
- **Gallery:** https://cerebralvalley.ai/e/built-with-4-7-hackathon/hackathon/gallery
- **Page:** https://cerebralvalley.ai/e/built-with-4-7-hackathon/hackathon/gallery/73

I built Cahier for myself first. I'm one of the 83% of European adults drowning in contradictory health information. The wellness industry sold me solutions; they failed; I concluded I was broken. Then I read the literature — the actual one — and the principles worked. So I built the tool I'd needed.

Cahier is a calm-tech health app: one concept a day from a primary-literature corpus of 310 papers across nutrition, training, cardio, sleep, and recovery — curated to 48 for the runtime RAG. Habit psychology shapes the cadence (small daily exposure beats 12-week transformations); Self-Determination Theory shapes the voice (17 banned fitness-bro words); calm technology (Weiser) shapes the surface (hand-painted, biomorphic, no streaks); trust epistemics shapes the chat (every reply passes a citation validator that strips any [^N] not actually retrieved above threshold).

Powered by Claude Opus 4.7 with adaptive thinking and 1M context, hybrid RAG (server-side embedding → on-device cosine top-k), and a hand-rolled SSE transport — no SDK.

The build held itself to the same evidence standard. compound-agent (github.com/Nathandela/compound-agent), an open-source harness I wrote, drives each task through a test-first cycle with multi-model review gates: Opus implements, fresh Sonnet and Opus review; failures become captured lessons. 70k+ lines of TypeScript shipped across multiple 5-to-15 hour autonomous Claude Code sessions on my laptop while I worked or slept.

The thesis: adults aren't broken. Given honest evidence, plain language, and the daily cadence of habit, people work themselves out.

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