Your Agents Are Aging Too: Agent Lifespan Engineering for Deployed Systems [R]

Are agents aging after deployment?: On a new longitudinal deployment benchmark, switching the Claude Code CLI agent from Sonnet 4.6 to Opus 4.7 dropped PyTest pass rate by ~15%. This (to me) is a counterintuitive-enough result to pay attention to. The authors built AgingBench, to measure how coding agents hold up over a long deployment, not just on a single task. On their S7 coding scenario, swapping the backbone model from Sonnet 4.6 to Opus 4.7, within the same Claude Code CLI harness, produced a 15% mean drop in PyTest pass rate across the deployment horizon.