Why architecture matters here
Payment rail architecture matters because rail choice affects cost, speed, and dispute rights. Wrong rail = user pain (delay, wrong currency, unrefundable).
Cost per rail varies: cards ~2-3%; ACH cents; wire $10-50; RTP low; crypto network fee.
Reliability differs by rail. Cards reversible; ACH slower but reversible; wire final; crypto final (immutable).
The architecture: every rail explained
Walk the diagram top to bottom.
Payment intent. Structured description of what to pay.
Rail selection. Policy engine picks rail based on merchant, amount, geo, user preference.
Executor. Rail-specific integration (Stripe for cards, Plaid for ACH, etc.).
Cards (Visa/MasterCard/etc.). Highest acceptance; 2-3% fee; chargebacks give consumer protection but merchant risk.
ACH. US bank-to-bank; slow (1-3 days); cheap ($0.10-1); reversible within R-code windows.
Wire (SWIFT/Fedwire). High value; ~$10-50 fee; same-day final; used for large B2B.
Real-time (RTP/FedNow/SEPA Instant). Instant settlement; low cost; growing but not universal.
Crypto. Permissionless; global; immutable; regulatory grey area.
Cost / speed / reversibility. Fundamental trade-offs per rail.
Regulatory (KYC/AML/PCI). Different requirements per rail.
End-to-end rail selection flow
Trace rail selection. Agent has intent for $150 hotel booking, merchant in Germany.
Policy engine: merchant category = travel; amount = $150; geo = EU; user has card + SEPA on file.
Selection: prefer card (high acceptance for travel; chargeback protection); currency conversion applied.
Execute via Stripe: card charge; auth immediately; capture on room check-in.
Alternative: $50k B2B invoice payment. Card fees too high; ACH slow but doable in US; wire common.
Policy: wire selected for high-value B2B; fees acceptable relative to amount.
Executor: initiate wire via bank API; confirm; audit.
Alternative: cross-border micropayment. Traditional rails too expensive/slow.
Selection: crypto (stablecoin) if user has enabled; otherwise fall back to cards with FX.
User preference always considered when policy allows.