Building a Vedic Astrology API: thread-local bugs, 1,500-year-old test fixtures, and a 429 disguised as CORS

Astrology apps are one of India's quietest huge markets — panchang widgets, kundli generators, matrimonial matching, muhurta pickers. Under every one of them sits the same unforgiving requirement: the astronomy has to be exactly right , because your user's grandmother has a printed panchang on her wall and she will check. I spent the last few months building GrahaAPI — 237 REST endpoints across 23 modules of Vedic astrology, Hindi + English in every response. This post isn't a feature tour. It's the four engineering problems I didn't expect, because I think they're interesting even if you never touch astrology. First, 60 seconds of domain: what the computer actually calculates Strip away the mysticism and Vedic astrology is a coordinate system plus 1,500 years of lookup tables: Tithi (the "lunar date"): the Moon-Sun angular separation, divided into 12° slices. 30 per lunar month. Nakshatra : which of 27 equal 13°20′ segments of the ecliptic the Moon occupies. Dasha : a 120-year planetary period cycle, seeded entirely by the Moon's exact position at birth — a birth-time error of minutes shifts period boundaries by months . The whole thing runs on the sidereal zodiac, offset from the tropical zodiac by ~24° (the ayanamsa — we use Lahiri, the Indian government standard). So: an ephemeris gives you planetary longitudes, and everything else is careful classical bookkeeping. Which brings me to the first bug. Bug #1: the thread-local zodiac Our ephemeris core is a C library with Python bindings, and it holds "which zodiac mode are you in" as global state — per thread . FastAPI runs sync endpoints on a threadpool. First request warms up thread A: sidereal mode set, positions correct. Then a request lands on freshly-spawned thread B: mode silently defaults to tropical , every longitude comes back ~24° off, and — because 24° is almost exactly one nakshatra-and-a-bit — the Moon lands in a plausible but wrong nakshatra. Which seeds the dasha. Which means the API happily returne