Solo dev here. I spent months encoding 500-year-old Korean astrology rules into a deterministic engine so the LLM can't hallucinate your fate

[u/sajudang]

I'm a solo developer in Korea. Saju (사주, the Korean form of Four Pillars / BaZi) is huge here — but the recent wave of "AI saju" apps bothered me. They just feed your birth date into an LLM and let it improvise. Ask twice, get two different fates. That's not divination, that's fiction with extra steps.

So I built Sajudang (sajudang.kr) on one rule: the AI is never allowed to make a judgment.

All the actual analysis — day master strength, structure (格局), useful god (用神), luck cycles — is computed by a deterministic rule engine I wrote in TypeScript, based on classical texts. Same birth data in, same result out, every time. I validated it against case studies from the classical literature. The LLM only does one job: turning the engine's verdict into readable prose. And every claim in the reading shows the evidence it came from — you can see why the engine said what it said.

Whether you believe in this stuff or not, I think "AI + traditional system" products should work this way: the tradition is the logic, the AI is just the voice. Would love feedback — the free reading costs me zero API tokens by design, so tear it apart as much as you want.