FedNow is the US Federal Reserve's instant-payments rail, launched in July 2023 as a domestic 24/7/365 service. The rail settles in real time on the Fed's books, with no batching window and no business-day cutoff. The per-transaction cap is currently $500,000 (configurable by the participating FI up to that ceiling).
FedNow was built ISO 20022 from day one — the customer-facing message is the FI's own choice (most use pain.001 internally), but the inter-bank rail uses pacs.008.001.08 for the credit transfer, pacs.002.001.10 for status, and camt.054.001.08 for confirmation. The Fed publishes the FedNow ISO 20022 implementation guide as a freely-downloadable specification.
The "Request for Payment" (RfP) extension is the differentiator vs the older legacy ACH credit pattern: FedNow supports a pain.013 RfP that the creditor sends to the debtor; the debtor approves (in their banking app) and the FI triggers the pacs.008 transfer. This is the US analogue to PIX's request-to-pay flow and is the closest the US gets to a UPI-like flow.
Coexistence with The Clearing House RTP rail is the rail-level decision US FIs need to make — both rails are real-time, both ISO 20022, but they are not interoperable. Most large FIs participate in both. The corporate experience is identical at the API surface (a pain.001 with an instant-payment indicator); the FI chooses which rail to route on.
Not yet implemented in iso-compliant; see FUTURES.md (Phase 3 — US instant) for the roadmap context.