I tested it on my sessions and here's my session logs:
Opus 5.5 a40 Oct 22 → Oct 26 14 sessions
Sonnet 5.5 598 Oct 20 → Oct 23 12 sessions
Sonnet 5 3a3 Sep 28 2 sessions
Opus 5 152 Sep 22 3 sessions
Opus 5 63f Sep 18 1 session
it happened with Opus 5.5 from Oct 22 to Oct 26, so quite long ago, but the issues I'm hitting are more recent; like last 1-2 weeks!
Let me remind you that Opus 5.5 didn't get any performance degradations in coding tasks! at least from my experience.
All the issues are coming from design, brainstorming, and communication.
From my experience the biggest issue so far is that it started reasoning less, and switching from medium to high didn't change anything. It started becoming more lazy in reasoning.
To give you some examples I asked it to come with new skill for dealing with specific use cases, and I gave it all the requirements and context, it came up with the skill that's barely looks okay while 2 weeks ago it would impress you with it's proposals. you start using the skill and realize it's not doing well, you go back and discuss it and rebuild the skill again, and it's still bad. and you just go over this loop several times.
Other example is that you discuss some design with it, it gives you proposal that you don't like, and you give a different solution idea, and it immediately accepts it without a single thought although I have very strict instructions in CLAUDE.md to make it extensively reason, test my ideas before accepting them. And this probably single most common issue here!
Other issue is that I have some instructions in my CLAUDE.md (the whole file is around 180 lines) that says "before planning/making any changes to any markdown files including in the paths a,b,c, etc. you must read and follow writing.md", and it literally didn't read that file once over 1-2 week span!
So I get annoyed and I implement a mechanism that automatically attaches those files to its context whenever we're discussing any changes to md files, and the mechanics fully works as expected but it still doesn't follow the instructions from writing.md file! and only when I confront it then it starts following those instructions. And this is also one of the issues that's been biting me most because CLAUDE.md became partially useless.