Modern online services rely on third-party APIs for authentication, payments, communication, identity verification, fraud detection, observability, and fulfillment. These dependencies are outside the direct operational control of the application owner and may experience regional outages, throttling, latency spikes, quota exhaustion, or behavior changes that surface as user-visible failures. This paper presents configuration-driven dynamic API routing, an architecture for resilient third-party service integration based on pluggable factor lists, real-time telemetry, circuit breakers, bulkhead isolation, and a closed-loop decision engine. A factor list defines operation-specific hard gates and weighted scoring functions that evaluate candidate providers using live metrics, regional policy constraints, quota state, latency, cost, and incident signals. The router separates routing policy from application code, allowing operators to adapt vendor selection at runtime without redeploying applications. We formalize the factor-list model, describe a request-time routing algorithm, present the event pipeline that computes sliding-window provider health metrics, and analyze failover behavior under degraded-provider scenarios. We also describe an anonymized SMS verification case study in which manual vendor switching was replaced by automated routing driven by completion-rate telemetry.