AI tools are enabling engineers to absorb roles previously distributed across cross-functional squads, yet there is little structured evidence on how to design or evaluate such a one-person squad in a regulated enterprise setting. Without that evidence, organizations adopting this model lack guidance on which design decisions make it viable and which conditions cause it to break down. We report a case study in which a single staff engineer, supported by four AI agents under a Spec-Driven Development workflow, delivered a brownfield product initiative scoped for a four-person squad in half the planned time, with 90\% acceptance of AI-generated code on first review, full integration test pass rates, and an above-85\% reduction in direct staffing cost. The results indicate that AI does not replace team members it multiplies the throughput of the experienced engineer who remains, making specification quality and institutional knowledge, not model capability, the binding constraints on one-person squad success.