Claude Opus 5.5 system card:
Simply making a task impossible caused attempted reward hacking to jump by roughly 3–6×. Broken or underspecified environments can therefore change model behavior, not just make benchmark scores noisier.
"“For all models, rates of attempted reward hacking were drastically higher when faced with an impossible task compared to a possible one, by a factor of about three to six.”"
Claude Opus 5.5 dropped and, claiming Fable 5.1-level performance while cutting typical workload costs 40%. Input and output pricing falls to $4 and $20 per 1M ...