I certainly noticed a significant drop in reasoning power at some point after I subscribed to Claude. Since then I've applied all sorts of fixes that range from disabling adaptive thinking to maxing out thinking tokens to patching system prompts with an ad-hoc shell script from a gist. Even after all this, Opus will still sometimes go round and round in illogical circles, self-correcting constantly with the telltale "no wait" and undoing everything until it ends up right where it started with nothing to show for it after 100k tokens spent.

Whether it's due to bugs or actual malice, it's not a good look. I genuinely can't tell if it's buggy, if it's been intentionally degraded, if it's placebo or if it's all just an elaborate OpenAI psyop.

The real question I see nobody asking is how GPT-5.4 beats Opus at a fraction of the price. I doubt it’s only a question of subsidization. My impression from the past is that GPT-5 was around a Sonnet-sized model, and 5-mini was Haiku-sized. At least on my codebase anyways, Codex one-shots tricky things that Opus needs several tries to fully get right.

There's a github issue for this: https://github.com/anthropics/claude-code/issues/42796