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Is Opus 5.5 nerfed? 3 checks to run before you blame the model

·5 min read

"Opus 5.5 got dumber" is all over X and Reddit today. One of the top posts in r/ClaudeCode is called "Opus 5.5 in the past two weeks" and X shows a trend about users split on a sudden reasoning shift. Anthropic has not said anything public about it that I could find.

I can't tell you if the model changed. Nobody outside Anthropic can. What I can do is lay out the numbers people are pointing at, and give you three checks that cost nothing and tell you whether the problem is in your setup.

The number behind the complaints

A user opened GitHub issue #105112 on Oct 26 and has been updating it since. They scored their own Opus 5.5 answers from local Claude Code transcripts. An answer counts as flagged if their next message corrects it, or if Claude's next reply opens by retracting ("you're right", "my mistake").

  • Sep 21 to Oct 18: 5.2% of 5,738 answers were corrected.
  • Since Oct 19: 25.4% of 900 answers.
  • The Nov 5 update says the weekly rate was 23.0%, then 20.5%, then 17.0% for Nov 2 to 4.

The author says the Claude Code build (2.1.303), the system prompt, the tool definitions, their CLAUDE.md and the effort level were the same on both sides of the break. That is their claim and nobody has verified it. The issue has 6 comments, none from Anthropic staff, and it is one person's data.

What the public trackers say

Two projects try to catch quiet changes with a fixed test:

  • LiveNerf is open source and runs a frozen panel of 78 questions every day through a pinned Claude Code CLI (2.1.280). On Nov 4 (day 42) it reported no change: window 3 was +0.028 ± 0.023 against launch week. Its rule needs a shift of at least 3 points in two windows in a row.
  • BridgeBench's NerfBench has kept every Opus 5.5 run between 94.2% and 105.8% of its launch score. Its most recent reading I found is from Oct 16, which is before the Oct 19 break, so it says nothing about the last three weeks. Its tasks are private and API only.

So the trackers do not show a drop, and one power user does. Both can be true. A fixed panel of 78 questions is not your repo, your CLAUDE.md or your habits.

Check 1: set effort yourself

This is the cheapest explanation, so start here. Anthropic's effort docs say Claude Opus 5.5 defaults to medium, and a request that omits effort runs one level lower than it did on Opus 5, which defaulted to high.

Claude Code's model config docs list the ways to set it:

/effort high            # saves a level for the current model
claude --effort high    # this session only

One catch from the same page: a top-level effortLevel in your user settings file doesn't apply to Opus 5.5. It stays on medium until you set a level for it with /effort or the /model picker. If you set effortLevel in your user settings months ago and assumed it covered everything, check.

Check 2: pin your Claude Code version

The model is one moving part. Claude Code is another, and it updates itself in the background. Anthropic's April 23 postmortem traced an earlier quality drop to Claude Code changes (a lowered default effort, a caching bug and a prompt change), not the API. I read that postmortem through secondary coverage, so check the original for the dates.

To stop background updates, per the setup docs:

{
  "env": {
    "DISABLE_AUTOUPDATER": "1"
  }
}

Then run claude doctor and look for Auto-updates: disabled. Run claude --version and write the number next to any test you do. If you want a specific version, the native installer takes one: curl -fsSL https://claude.ai/install.sh | bash -s 2.1.89 (swap in the version you want, and use the PowerShell form on Windows).

Check 3: measure your own corrections

This is the one that tells you if it's real. Use the same metric the 25.4% used, on your own work.

  1. Count. For a week, note every answer you had to correct, plus every time Claude opens with "you're right". Divide by how many prompts you sent. Do it again the next week.
  2. Freeze. Pick 5 real prompts from your own work, with the files they need. Save them in a folder. Rerun them at the same effort and the same version, each in a fresh session.
  3. Decide. If your rate rose and effort and version did not change, that is worth a GitHub issue with the transcript attached. If it didn't, you probably found a settings problem.

A simple log is enough:

date | version | effort | prompts | corrected | notes

What the data can't tell you

I can't say Opus 5.5 is fine, and I can't say it was changed. One person's log is a signal, not proof. Trackers with fixed questions are proof of something narrower. Anthropic's docs say a model ID is a pinned snapshot, but that doesn't cover everything around it, like routing and the Claude Code harness. The honest read is that the checks above are the only part you control.

What to do this week

  • Run /effort high and see if the feeling goes away.
  • Pin your version and write it down.
  • Start the correction count today. A week of numbers beats a week of arguing on X.

If you did the count, did your rate move? Tell me what you found.

Sources: GitHub #105112, LiveNerf, i10x, Nov 7, abZ Global, Oct 4, Anthropic effort docs, Claude Code model config, Claude Code setup.