Claude Opus 5.5 vs. Fable 5.1: One overthinks, the other cuts corners
Anthropic released Claude Opus 5.5 on September 22, saying it performs at the level of Claude Fable 5.1 on most The post Claude Opus 5.5 vs. Fable 5.1: One overthinks…
Since it was released Opus 5.5 has solved almost all the problems I've had with previous Anthropic models over the last year. On medium thinking level (default in Claude code) it's been remarkably more succinct and really good at understanding my questions and even anticipating my needs. Has been a markedly huge improvement over previous models and even had me commenting to a colleague the other day that if all advancement were to stop, it would be a useful tool for at least the next 5 years.But over the last 2-3 days, it's started exhibiting some of the problems previous models presented. It's been making absurd assumptions without asking about my preferences. More definitively, in 3 instances I've had to either correct it on previous decisions that were made but it ignored. 2 times I've had to double check it on some quite obvious high level performance choices I've come to anticipate the model already understanding from prior experience, especially in the same project. At least twice I've had to call it out only get the remedial "you're totally right I should have done X instead".Also to be clear, I'm not playing around with more advanced expensive models that fall back. This…
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