A new benchmark comparing Claude Fable 5 and GPT-5.6 Sol on three complex backend design tasks (webhooks, billing, permissions) revealed a significant flaw where one model's proposed solution would have double-billed customers on retry. GPT-5.6 Sol scored 9.10 on the rubric, while Claude Fable 5 scored 8.04, despite the critical error in one of the solutions.
Frontier models still struggle with critical edge cases in complex backend design, requiring rigorous human oversight for production-grade code generation.