I built an AI that reads stock charts — and made it grade its own homework

How KlineVision does AI technical analysis with Next.js + Gemini, draws it on the real candles, and verifies every call it makes after the fact — misses included. Most "AI stock analysis" tools confidently tell you a chart looks bullish and then never look back. I wanted the opposite: a tool that records every call it makes and later checks whether the market actually agreed — and publishes the hit rate, misses included. That one constraint — keep yourself honest — ended up shaping most of the interesting engineering. Here's how KlineVision is put together. What it does Type a ticker ( AAPL , 600519.SH , 0700.HK ) — or drop a chart screenshot. You get a structured read : trend, support/resistance, candlestick + chart patterns, and 缠论 (Chan theory) structure — drawn directly onto the real candles , not hand-waved in prose. Works across US, A-shares, and Hong Kong markets. The stack Next.js (App Router) on Vercel Supabase (Postgres + RLS) for data & auth Gemini 3 Flash for the analysis itself EODHD + Yahoo Finance for market data GitHub Actions + Vercel Cron for the background jobs PostHog for product analytics Now the parts that were actually interesting to build. 1. "Never analyze fake data" The market-data layer had a silent fallback: when a provider rate-limited, it returned synthetic candles so the UI never broke. Great for a demo, terrible for a product whose entire value is trust — the model would write a beautiful, confident analysis of a chart that never existed . The fix was to make the data layer fail honestly : Yahoo (primary) → EODHD (fallback) → if only demo data is available → 503, no analysis If we can't get real candles, the user gets "live market data temporarily unavailable" — not a hallucinated read. For a trust tool, a silent fallback to fake data is the worst bug on the board. 2. Make the AI show its work A text blob saying "there's a double bottom" isn't credible. You have to see it on the chart. So the model returns structured overlays keyed to