SPECA: Specification-to-Checklist Agentic Auditing for Multi-Implementation Systems -- A Case Study on Ethereum Clients

Multi-implementation systems are increasingly audited against natural-language specifications. Differential testing scales well when implementations disagree, but it provides little signal when all implementations converge on the same incorrect interpretation of an ambiguous requirement. We present SPECA, a Specification-to-Checklist Auditing framework that turns normative requirements into checklists, maps them to implementation locations, and supports cross-implementation reuse. We instantiate SPECA in an in-the-wild security audit contest for the Ethereum Fusaka upgrade, covering 11 production clients. Across 54 submissions, 17 were judged valid by the contest organizers. Cross-implementation checks account for 76.5 percent (13 of 17) of valid findings, suggesting that checklist-derived one-to-many reuse is a practical scaling mechanism in multi-implementation audits. To understand false positives, we manually coded the 37 invalid submissions and find that threat model misalignment explains 56.8 percent (21 of 37): reports that rely on assumptions about trust boundaries or scope that contradict the audit's rules. We detected no High or Medium findings in the V1 deployment; misses concentrated in specification details and implicit assumptions (57.1 percent), timing and concurrency issues (28.6 percent), and external library dependencies (14.3 percent). Our improved agent, evaluated against the ground truth of a competitive audit, achieved a strict recall of 27.3 percent on high-impact vulnerabilities, placing it in the top 4 percent of human auditors and outperforming 49 of 51 contestants on critical issues. These results, though from a single deployment, suggest that early, explicit threat modeling is essential for reducing false positives and focusing agentic auditing effort. The agent-driven process enables expert validation and submission in about 40 minutes on average.