Deterministic Identification of Failed SEPA Instant Test Payments

This document presents a deterministic method for identifying failed SEPA Instant Credit Transfer (SCT Inst) test cases using only daily balance observations of a limited number of Nostro accounts. By assigning carefully structured payment amounts and observing end-of-day balance deltas, it becomes possible to determine exactly which test cases were not executed, without inspecting individual transaction identifiers such as EndToEndId or UETR. The method is designed for controlled test environments where opening balances are known, credits are exact, and observation windows are clean.The approach relies on partitioning test cases across multiple beneficiary Nostro accounts and assigning amounts from uniquely decodable (superincreasing) sequences. This guarantees that any missing payment produces a balance discrepancy that can be uniquely decomposed into the identifiers of the failed tests. The method is mathematically provable, operationally simple, compatible with SEPA Instant constraints (EUR-only, full-amount credit, no partial postings), and provides an immediate daily integrity check independent of transaction-level reconciliation.