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.

I also found it to forget some obvious cases in a quite simple flow (validate the email address of a user who register), that surprised me a lot. Maybe it's because I got used to Fable? But I am quite sure Opus 4.8 wouldn't have make this mistake. If I had time I would try the same prompt with it to see. Anyway, back on 100% Fable for me.

I don't know what I could be doing differently to you but I found Opus 5 to be more reliable than even myself at times. Maybe your stack is unusual or you have conflicting commands in your prompts vs CLAUDE.md (that really confuses it)? It could be anything but this huge error bar in delivered quality is one of the biggest issues with LLMs.

Getting to grips with each new model does require some tweaking and experimentation. So far I've found Opus 5 to repeatedly pause its work and give me some seemingly randomly invented decisions to make.

Opus 5 isn't very reliable for coding and introduces a lot of regressions every single time I use it.

And GPT 5.6 Sol over engineers just about everything. No LLM is perfect, its about learning the issues with each LLM and figuring out if you can live with it. Knowledge means that you can anticipate if it tries to pull something funny, and harness it against that behavior.

Same here, also it lies often to me or implements something else that what was planned. It feels quite strange to see it say casually "I didn't tell you the full truth on X" when I notice the issues. At the same time, maybe it is more honest?

I do not experience any regressions, I don't really notice much difference either.

Maybe it's my harness but I haven't seen it introducing regressions.

Do you not have unit tests, or how does it introduce regressions? You can tell it how to run the test suite in CLAUDE.md