x402 vs Mastercard AP4M: two bets on how agents pay

x402 and Mastercard's Agent Pay for Machines both let AI agents pay autonomously — but they split the problem in opposite places. x402 collapses authorization and settlement into one on-chain act; AP4M puts authorization on public chains (Solana, Polygon, Base) and keeps settlement on Mastercard's rails. A dimension-by-dimension comparison and when to reach for each.