Yes I had a similar experience with Opus 5. It is very token efficient, fast, and gets reasonable part of the work right but makes a LOT of mistakes. In a month+ use of Fable completed each task without ANY errors. Opus could not complete a single of ~5 tasks without some issue or the other - either not getting it fully right or actually introducing regressions. To their credit it was able to catch regressions and fix competently. It seems like a pre Opus 4.6 model in terms of reliability with a lot more power and spiky intelligence. When it gets things right it's powerful and efficient but without reliability I had to 'downgrade' to Opus 4.8 forcibly (since it was not a default option on claude code). I really miss Fable on the pro plan and will likely churn to K3.