Building Yarrow: an AI I Ching divination SaaS in a few weeks (Next.js + Supabase + Stripe + DeepSeek)

Hey 👋

Just shipped Yarrow — an I Ching ("Book of Changes") divination web app that uses an LLM to write the interpretation.

Live: https://iching.lifefinetuning.com

The flow

• User types a question
• 3 coins are tossed 6 times (the casting itself is pure logic, no AI)
• The resulting King Wen hexagram (1 of 64) is identified, plus any changing-line transformation
• An LLM steeped in the Wilhelm/Baynes tradition writes the reading
• Responds in whatever language you asked in (English, Chinese, etc. — handled by the model)

Tech stack

• Next.js 16 + Tailwind 4 + TypeScript on Vercel
• Supabase (auth + Postgres + magic-link email via Resend)
• Stripe (monthly $9.99 / annual $79)
• DeepSeek for the AI (cheap, works well for both English and Chinese)
• Resend for transactional email
• PostHog for product analytics + Session Replay

Things I think are interesting

• The hexagram engine is pure logic, not AI. I encode all 64 hexagrams as binary strings and look up the King Wen number. The AI only writes the interpretation — it can never hallucinate a hexagram.
• Each reading gets a public shareable URL (/r/[id]) with its own dynamically generated OG image (your question + the hexagram visual).
• Anti-abuse: anonymous quota uses an IP+UA hash (not a cookie), so clearing cookies doesn't reset it. Reading slots are reserved BEFORE the AI call so people can't disconnect mid-stream to skip the count.

Free tier

• 1 reading per day anonymous
• 3 per day signed-in
• Unlimited on subscription

What I'm looking for

• General UX feedback — is the page clear? Does it feel respectful or like fortune-cookie BS?
• I Ching practitioners — does the system prompt feel grounded or shallow?
• Other devs — anything you'd structure differently in the auth/payment/share flow?

Solo builder in Malaysia. Happy to answer technical questions or hear honest criticism. 🙏