How NestJS Can Power Real-Time Fraud Detection on Instant Payment Rails

Instant payment systems like FedNow and RTP require fraud decisions to be made within seconds, before a transaction settles and becomes irreversible. Unlike older systems that relied on overnight batch reviews, these rails leave no room for after-the-fact checks. To handle this, fraud detection logic must sit directly in the transaction path, structured as modular, independently updatable rules rather than embedded in core payment code. A NestJS-based approach using rule interfaces and interceptors allows each fraud check — such as unusual amounts or high transaction velocity — to be isolated and evaluated before settlement is confirmed. This architecture lets teams add or adjust fraud rules without disrupting the underlying payment flow, making it both fast and maintainable.