They're currently on the previous generation for Opus (3), it's kind of forgetful and has worse accuracy curve, so it can handle fewer instructions than Sonnet 3.5. Although I feel they may have cheated with Sonnet 3.5 a bit by adding a hidden temperature multiplier set to < 1, which made the model punch above its weight in accuracy, improved the lost-in-the-middle issue, and made instruction adherence much better, but also made the generation variety and multi-turn repetition way worse. (or maybe I'm entirely wrong about the cause)