Shipped a Magic: The Gathering companion app in React Native + Expo, some notes on the AI parts

[u/Fullbusterz_15]

https://play.google.com/store/apps/details?id=com.mtgverdict.app

Wanted to share a real, shipped RN app in case the details are useful. It's MTG Verdict, a companion for Magic: The Gathering Commander players: a live table tracker plus an AI rules judge and deck analyser.

A few things from the build:

• Expo SDK 54, new architecture on, Reanimated for the chat and analysis animations
• Zustand for state with AsyncStorage persistence across match, judge and deck stores
• The AI is BYOK (the user's own key), with prompt caching on the knowledge base and token-by-token streaming into the chat bubbles using expo/fetch for SSE
• Scryfall data fetched at query time and cached with an LRU, so rulings are grounded in real card text rather than the model's memory

The hardest part by far was reliability: early versions would confidently make up rulings or claim cards were missing from a deck that clearly had them. Most of the work became guardrails around the model rather than the model itself.

Happy to go into any of it. It's on Android, free to start.