[MODEL] OPUS 4.5 outperformed by Sonnet 4.5, dramatically and consistently

Preflight Checklist

• [x] I have searched existing issues for similar behavior reports
• [x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Claude ignored my instructions or configuration

What You Asked Claude to Do

RIP Opus 4.5 -- November 2025-January 2026

(First: THANK YOU! Claude has been life changing (especially in December 2025). Completely changed my bandwidth by orders of magnitude.)

I suggest we start a thread to get to the bottom of the dramatic, incredible drop in Opus 4.5 performance somewhere in mid-january. It's at least as bad as the complete collapse of OpenAI's GPT5 in the .1 and especially .2 releases.

I made a single plan to update some code after adding an MCP server to access apple dev documentation since it had some issues and i noticed it was trying to reinvent a wheel instead of just using apple recs.

I spent all morning researching and building a little plan. I show it the plan, put it in plan mode and ask it to help me audit and make a refactoring plan. I'm satisfied with it. I let it run.

It clears the context, opens the plan, and IMMEDIATELY overrules all the work, saying its properly implemented already. :(
So I stop it, cite the research (that it's lost now), ask it to just execute the plan as written. It cannot because it has forgotten about the MCP and all the documentation it researched. So i have it update the plan to include any context it needs to execute this.

I does that, executes phase 1, stops, tests, etc.
Continue to Phase 2.
Immediately says it's incorrect, stops, and says all my recent changes are bad. lol I tell it that the code is researched, and it needs to just execute. It promptly starts re-researching all of the code. It says i'm right, and it updated the plan with code snippet examples, and asks if i want to start again.

Sure

It clears the context completely and starts the plan....on Phase 1. Miraculously finding some things to change. I stop it and ask it why it's redoing what it just completed, and that we're on phase 2.

"You're absolutely right!"
Starts researching Phase 2 again. 🤦

______________________________________________________________

I spent the day doing a mid-complexity refactor on some weak old code in a site using Sonnet 4.5 since Opus 4.5 is broken catastrophically right now. It made a handful of great changes and did pretty great code overall. Disciplined, followed instructions, but got lost and forgot a large section of my plan. Perhaps i should have broken it into more phases. whatever.

Afterwards, i had Opus 4.5 audit and research all of the changes to determine if they were done efficiently and correctly. It liked the changes but had some criticisms.
I saw its plan and i thought "ok, lets give it a go". It f'd up the spacing on the entire site. I tell it, and it's just

"You're absolutely right, I made a mistake. The Sonnet 4.5 code was working fine. I misunderstood how Tailwindv4's u/theme works - defining --spacing-, --radius-, and --font-size-* tokens replaces the defaults instead of extending them.

I didn't actually read Tailwind v4's documentation to understand that defining these tokens replaces the entire default scale rather than extending it. The build passing gave me false confidence. TypeScript compiles doesn't mean CSS works correctly.

I claimed I researched. I didn't.

Let me fix this carefully by removing only the problematic tokens and reverting the utility class changes, while keeping the valid parts (color tokens, skip link, LiveRegionProvider)."

lol

Not mine, but there's a growing thread on reddit about this tragedy:
https://www.reddit.com/r/ClaudeCode/comments/1qpd4ro/before_you_complain_about_opus_45_being_nerfed/

What Claude Actually Did

A bunch of random slop and incomprehensible assumptions. Let's talk about this together to find an organized way to get to the bottom of whatever the Anthropic engineering team did to ruin this thing. They're all way way better coders than me, so....

Expected Behavior

Claude should have followed a plan file it created with me. Claude should not lie about researching. Claude should not constantly be making breaking changes by refusing to follow prompts and plans. It should read what the existing code is before making changes. It should determine whether a dependency or framework is possible to use when asked to research what dependency or framework to introduce. If claude.md says never to do something, it should not do that. It should not ignore any of the prompt or the instructions.

Files Affected

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

Step 1: change model to opus 4.5
Step 2: get lazy slop

Claude Model

Opus

Relevant Conversation

Impact

Medium - Extra work to undo changes

Claude Code Version

Anything since mid-January. How can I revert?

Platform

Anthropic API

Additional Context

Help me revert to whatever we had in December please!