Core Banking Modernisation: How AI-Native Development Accelerates the Journey Off Legacy Systems

The Core Banking Modernisation Challenge The core banking systems running most established banks were designed for batch-processing world that no longer exists. They cannot easily support real-time payments, open banking API requirements, or the personalisation that digital-first competitors offer natively. But replacing them is extraordinarily risky — the systems that have run reliably for decades contain decades of business logic that is poorly documented and incompletely understood. The conventional approach — full core replacement — has an alarming failure rate. TSB's 2018 migration, Australia's Commonwealth Bank's decade-long SAP replacement, and dozens of others illustrate how often these programmes go catastrophically wrong. A different approach — the strangler fig pattern combined with AI-native build velocity — offers a better path. The Strangler Fig Approach Rather than replacing the core system in a single big-bang migration, the strangler fig approach progressively builds modern systems that take over specific functions from the legacy core — one capability at a time. New products are built on the modern stack. Specific customer journeys are migrated to the new platform. Gradually, the legacy core handles less and less until it can be decommissioned. This approach requires building new capabilities fast enough that the migration momentum is maintained. AI-native development — delivering new banking capabilities in 6–14 weeks rather than 12–18 months — makes this pace achievable. Digital Banking Layers Most core modernisation programmes include building a digital banking layer that sits in front of the core: API gateway, product catalogue, customer onboarding, account servicing, and notification management. These components are relatively well-understood and are excellent candidates for AI-native development — the specifications are clear, the patterns are established, and the ability to parallelize development across components creates a significant speed advantage. Payment Modernisation Real-time payment rails (RTP, FedNow, PIX, SEPA Instant) require processing infrastructure that legacy batch systems cannot support. SIGMA builds payment processing layers — message transformation, routing logic, confirmation flows, and settlement reconciliation — that can connect to modern payment rails while maintaining the interfaces that existing core systems expect. Frequently Asked Questions What banking platforms does SIGMA build? Digital banking layers, payment processing infrastructure, customer onboarding platforms, open banking API gateways, regulatory reporting systems, and wealth management platforms. Contact us at sigmasoft.app to discuss your specific modernisation requirements. How does SIGMA handle the complexity of banking regulatory compliance? Regulatory compliance is an architectural input. Engineers define the compliance architecture — audit trail requirements, data retention rules, reporting specifications — before AI agents generate any application code. SIGMA has delivered platforms compliant with regulations from central banks across multiple jurisdictions.