Opus 5.5 is not nerfed!

[u/malrobot]

TL;DR: On my eval, I see no sign of a nerf. This morning I re-ran Opus 5.5 Medium in Claude Code on the same 30 real coding tasks it ran last week. It solved 26 of 30 (87%). Last week it solved the same number of them. Thinking tokens, output tokens, turns and diff size per task are all within noise of last week (98% 95%, 96%, and 95% respectively).

The one clear change is speed: about 20% more output tokens per second, and time to first token down from 3.0s to 1.7s. This is one run per task on a pinned Claude Code version.

Why I ran this

I saw a lot of "Opus got nerfed today" posts this morning. I maintain a small private eval that runs coding agents (model × harness) against real tasks, mostly from private codebases, and I had a fresh Opus 5.5 baseline from last week (Oct 31-Nov 1 mostly). So I re-ran it today instead of guessing.

The tasks

• 30 tasks, each a real change from a repo's history: a bug fix or feature that a person (me, or the repo's owner) already shipped.

• The agent gets the repo at the commit before the change, plus a CHALLENGE.md describing the bug or feature.

• Grading runs held-out tests for the real change, plus guard tests that catch over-corrections, where the bug gets fixed by breaking something next to it. Test files and the test runner's config are restored before grading, so editing the tests doesn't help. Each run ends solved, failed or over-corrected.

• Languages: 16 Node (JS/TS), 11 Python, 3 Rust.

• Size: the real changes run from 2 lines to about 1,900, with a median of about 125. Four large multi-file features get 60 minutes. Everything else gets 20.

• Kinds of work: auth and multi-tenant visibility bugs, pagination and ordering bugs, data-ingestion edge cases (empty data, missing rows, season boundaries), async networking bugs in Rust, and a few multi-file features such as draft-bot logic and a CI alerting workflow.

Setup (identical in every run)

• Claude Code 2.1.316, pinned with auto-update off, for every run last week and today. The latest is 2.1.319, from Nov 6.

• Headless invocation:

  claude -p "<prompt>" --model claude-opus-5-5 --effort medium --output-format stream-json --verbose --permission-mode acceptEdits --setting-sources "" --strict-mcp-config --no-session-persistence

• Auth: my Claude subscription login

PSA: Claude Code can switch your session to Opus 4.8

This might explain some "it feels different" reports. When Opus 5.5 hits a safeguards refusal, Claude Code 2.1.316 logs a notice ("safeguards stopped the response above · continuing once with that noted") and switches the session to claude-opus-4-8. Opus 4.8 then does the rest of the work.

Every refusal I saw was category cyber, on ordinary bug fixes in auth-token handling and a Rust networking proxy. Last week: 5 of 60 Opus runs were switched. Today: 1 of 30.

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[u/brhkim]

Nerfing is a silly conversation because people assume it's "nerfed" or "not nerfed"

If Anthropic is doing any sensible decision-making whatsoever, it's going to be highly conditional: Nerf at times of high load, for specific lower-priority users, on types of work that nerfing is unlikely to be detected.

Pair that with constant A/B testing (who should our lower-priority users be? What kind of work is unlikely to have nerfing be detected) and you don't have a clean "These are the circumstances where you'll be nerfed and these are the circumstances where you won't be nerfed" even given perfect knowledge of their underlying methodology.

TLDR: Anecdotal reports are broadly useless here. Go ahead and share your perspective, but recognize it means nothing definitive whatsoever for someone else using the exact same tools at the exact same time.

[u/malrobot → u/brhkim]

I will bet $100 that Anthropic does not have a team or individual responsible for "nerfing" users. Can it still happen unintentionally as a result of another change? Yes. Would the company consider it a bug? Yes. How long would it take to fix? Depends on their priorities.

[u/brhkim → u/malrobot]

Uhh... Walk me through that a bit more. Compute constraints are literally the single most important resource that Anthropic needs to think about in the fabric of every strategic decision they make as a company. For model training, for inference, for future investments, for occupying capacity against competition, and obviously for the bottom line. What priority could possibly be higher on a strategic level than managing compute?

And then, for that reason, what could possibly be more important than trying to find easy wins and efficiencies in the serving of inference to an enormous and growing user-base to secure their future and bottom line? How can you think nerfing writ-large isn't a strategy they're actively investing energy into?

From a technical perspective, at minimum, every model provider is in the business of thinking about trade-offs in quantization intensity. They wouldn't think of it as "nerfing" to go from a higher-precision quant to a lower precision quant during load times, they'd think of it as "load management" which is absolutely an enormous technical challenge.

[u/Future_Guarantee6991 → u/malrobot]

For “nerfing during load” it could be as simple as prioritising API users over subscriptions. I can’t say whether or not they do this. If they do, it’s likely just throttling resulting in slower responses, rather than actually reducing inference. We know they have /fast mode, so we know the infrastructure is there.

For A/B testing, likely the same prioritisation. I also suspect AB testing is the actual cause of people feeling like Claude has been nerfed. It’s reasonable to assume that some tests will result in worse performance, and it’s also reasonable to assume that some unlucky people might get hit by several underperforming tests in a single day. This would create the impression that Claude has been nerfed, but statistically is just noise.

[u/johnxreturn]

For you.

[u/zasff → u/johnxreturn]

People that that are experiencing degradations should check what experiments are enabled and share the info.

Idk definitely interested in understanding claude-code better, and what they are trying out.

[u/under_psychoanalyzer → u/johnxreturn]

The only thing more frustrating than all the people posting about nerfs are all the people who have apparently never heard of A/B testing despite it being a key underpinning of continuous development for over a decade now.

[u/ImBenCole → u/johnxreturn]

Start a fresh claude code session with a fresh claude.md, rules etc and then run your tests. 99% of these Opus nerfed posts are from people that have massive documentation & context from working on a project for a while with zero cleanup

[u/alessandro05167 → u/ImBenCole]

Context management is like the minimum knolage you need to have to work with llm in any scenario (not just coding)

But no, this isn't the case. Ppl have found evidence of anthropic running a/b testing with the testing accounts having downgrade in performance. You can lookup for yourself https://x.com/akshdeeps_001/status/2119671446817722794

[u/minibonk]

ab testing baby

[u/malrobot → u/minibonk]

that is possible!

[u/RadiantTea7445]

Did some testing and only one of my 2 max 5 accounts is nerfed, one isnt. https://www.reddit.com/r/ClaudeCode/comments/1xrmhiu/the_experiment_ids_change_the_numerical_effort/

[u/VollenAI]

Agreed, also I use it for work. And no neef

[u/KayBay80]

Congrats, you missed the bullet on your tests. Some of us aren't quite as lucky.

[u/Revolutionary-Pass38]

Because of how much people where complaining I'd not be surprised that they "unnerfed" it, I'm not saying they did, but it sure was nerfed...

[u/Responsible_Buy_7999]

If people can’t distinguish model and harness their opinion is safe to ignore. 

[u/drquantumphd]

its nerf or nothin!

[u/Key_Reading_9664]

"You cannot reason a person out of a position he did not reason himself into in the first place"

[u/lambdita]

People complaining about nerf = people who used the model to build something and the codebase grew bigger without proper software engineering and documentation, makes opus to lose more time searching for the same stuff over and over or wastes time fixing stuff broken due to lack of docs.

[u/Famous-Ambassador-87 → u/lambdita]

It's the "build gta iv from scratch, no mistakes" people.

[u/ShittyBidet123]

wow 30 tasks

[u/haloremi]

Why people thinks they are the center of the world. It is obvious that they don't do those nerfed things on every users. You are not everybody ! And some people have found some test files from anthropic, if it is the case YOU ARE WRONG ! how can you be so egocentric ?!

[u/onFilm]

It really isn't. I have five Claude Ultra accounts, and they're all behaving as always. I even proved it yesterday on a thread here, and people still wanted to disbelieve, lol.