Coinbase price premium vs composite spot median (OKX + Kraken + Binance). premium_pct = (coinbase / composite_median − 1) × 100. Positive = US/institutional demand skew; negative = global selling pressure. Covers BTC/ETH/SOL/XRP/DOGE/ADA/AVAX/LINK/DOT/LTC/BCH/ATOM/NEAR (assets listed on Coinbase). Point-in-time only — no historical baseline stored. $0.02 USDC per call via x402.