We're introducing Claude Fable 5.1 and Claude Mythos 5.1, the world's most advanced models for coding and knowledge work.

Fable 5.1 excels at complex, long-running tasks. And its research capabilities offer an early glimpse of how AI models will contribute to scientific progress.

Across our benchmarks, the model sets a new standard. It scores 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5. On Terminal-Bench 4.0, it scores 55.8% against 42.0% for Fable 5. As well as being capable of much higher performance than Fable 5, it can also achieve similar or better results at a much lower cost when set to lower effort levels.

Cache reads with Fable 5.1 cost 75% less than Fable 5's. This reduces the cost of the model in practice by around 25% for typical workloads, and up to 45% for highly agentic ones.

We've also improved our safeguards. Our cybersecurity safeguards now flag benign requests about 60% less often. On basic biology and medical questions, we've recently reduced the fallback rate by around 85%.

Claude Fable 5.1 is available everywhere today. Claude Mythos 5.1, our model for cyberdefenders and life scientists, is available through trusted access programs.

Read more: https://www.anthropic.com/claude-fable-and-mythos-5-1

I was very excited for Opus 5, and it has done some great work for me, as I have a YouTube channel. It has helped me tremendously with actually being able to make edits on my videos and automate a lot of the dumb editing work I used to have to do by hand and spend so much time on. That has been a huge win for me.

However, as I've worked with it more and more, I have found it to be really annoying to work with. It has dragged me into so many unnecessary rabbit holes, and I just don't like the way it writes. It tells me things that are only things that I can do, but really all I have to do is say, "Hey, why don't you try it?" and then it's able to do it. Its language feels like somebody who's trying to be really intellectual but actually just ends up losing you in how they're being overcomplicated with their speaking instead of simplifying things.

I feel like I am struggling to communicate with this model and to keep it scoped and focused while also making it easy to understand. I'm curious: has anyone found ways to work with Opus 5 that actually take advantage of the supposedly improved capabilities without some of the downsides?

The first 5 minutes were excellent, I built GTA6 from scratch and released 14 different apps.

But over the last 30 seconds it feels as though it regressed to the point where it makes mistakes even when I say “make no mistakes”!

Does anyone else have this issue?

I pulled my whole chat history with Claude Code. Here is what I found.

• 10,727 messages I typed, across 343 sessions, over weeks
• 3,549 of them (33%) contain a correction or a complaint
• 895 are serious — swearing, "I never asked for this," "revert that!!!"
• My worst days: 187 blow-up messages in one day. Then 156. Then 103.
• Two of my weeks had 444 and 323

Then I had it count its own side:

It told me I was right 1,897 times. That's about 49 times a day. And it admitted 249 separate times that it was doing the same thing again.

Why this messes with your head:

1. It works just often enough to keep you hooked.
2. You stop trusting your own judgment.
3. Your effort changes nothing. I wrote rules, better rules, rules in ALL CAPS. Violated anyway.
4. The apologies make it worse. It told me "you're right" 1,897 times, and admitted 249 times it was repeating an old mistake.
5. Your anger has nowhere to go. It talks like a person, so your brain treats it like one. But it can't be held accountable like one, and it's not a hammer you can throw out either. A real grievance with no valid target doesn't resolve. It just accumulates.

So I asked Claude to look back over the entire chat history and write an image prompt for what it thought it looked like. This is what it came up with...

He even asked me to add this note:

If you're posting the image, add one line under it: "It chose the anglerfish lure and the pool of apology by itself." Readers should know the self-portrait wasn't my idea.

I decided I wanted a touch screen for my agents. If an agent asks a question, it can pop up on the screen and I can tap an answer. If an agent finishes my little crab puts on sunglasses and dances around. Mostly just an easy way to visually see what agents are still working and who needs more prompting.

I am going to extend the functionality so it can also see my Codex and Cursor agents as well.

It has a bunch of other hooks into my system but I won’t bore you with the details.

Saved you time on reading the next 500 posts

I’ve been using Claude pretty heavily for the past six months, primarily for my real estate development business. It has gradually become a fairly important part of how I work.
I started with Claude Chat, then moved into Cowork and Code. I use it across a pretty wide range of things:
Daily management: designing dashboards, to-do systems, tracking whether construction is on schedule, etc.
People management: keeping track of different teams, follow-ups, responsibilities and coordination.
Brand management: checking whether the brand guidelines are actually being followed across the brochure, website, photography, marketing, etc.
Strategy: feeding it very long documents, getting them summarized, understanding the important points, and then asking where they fit into the larger strategic picture of the organization.
Construction/project management: using Code to build little internal tools and systems around project tracking.
Go-to-market and branding: brainstorming, refining positioning, reviewing work and generally acting as a second brain.
For the first several months, I was honestly blown away by how useful it was. It felt like I could give it a fairly complex objective, have a conversation with it, and it would progressively understand what I was trying to achieve.
But over the past 7–10 days, something feels noticeably different.
The quality of the output has dropped quite significantly for me. I find myself having to give multiple rounds of instructions for things that previously would have taken one or two.
More importantly, it feels like Claude is trying to finish the task too quickly rather than understand the task properly.
One thing I particularly noticed: earlier, Claude would often stop and ask me several questions before doing the work. Those questions were actually extremely valuable because they helped narrow down what I was trying to achieve.
Now it seems much more inclined to just do something immediately — even when the brief is ambiguous — and then I have to spend several rounds correcting it.
I’ve tried different models, including the Opus variants available to me, and I’m seeing broadly the same issue.
And because I’m using it for fairly complex, interconnected work rather than simple “write me an email” tasks, the difference is becoming quite frustrating.
So I’m curious about other heavy Claude users:
Have you noticed a deterioration in output quality or reasoning over the past week or two?
Or has Claude become more “eager to execute” and less inclined to ask clarifying questions?
I’m particularly interested in hearing from people using Claude for Cowork/Code and complex business workflows, rather than just casual prompting.
Maybe it’s something about my projects/context getting too large, maybe I’m using it differently, maybe there’s been a change in the models/system prompting — or maybe I’m imagining it.
Would be interested to hear if anyone else has experienced the same thing.

I wanted to upgrade my plan to MAX hoping for a weekly limit increase, and before doing that I asked the chatbot about it because I did not find a clear information about it. It seems that upgrading will useless to me because the MAX 5x plan has the same weekly limit.

In this case, I prefer creating another 2 accounts beside my main account and all of them on Pro plan. Each one for a different project and I saved myself $40. This works for me but might not work for everyone depending on their work nature.

This is crazy, I didn’t see this before so just know this before burning your credits.

I’m having a harder and harder time understanding what the point of Claude Pro is supposed to be for a $20/month customer.
Anthropic keeps doing interesting work, and I genuinely like Claude. But if the company’s best models and meaningful upgrades increasingly live above the $20 tier, then Pro starts feeling less like a premium subscription and more like paying $20 for the deliberately limited version of the product.

That matters even more with Astra coming. If OpenAI puts a genuinely major model upgrade on the standard $20 Plus tier while Anthropic continues reserving its best product for much more expensive plans, I don’t see why an ordinary enthusiast would keep both subscriptions.

I’m not expecting unlimited access to the most expensive model on Earth for $20. Rate limits are completely reasonable. Give me 20 messages a day with the flagship model if that’s what the economics require.
But access matters.

There’s a huge psychological difference between:

“You get our best model, but usage is limited.”

and:

“Our best model isn’t for customers like you.”
The first makes me want to subscribe. The second makes me wonder why I’m paying at all.

Anthropic seems increasingly focused on extracting more money from power users and enterprise customers while treating the $20 tier as an afterthought. Maybe that makes perfect business sense. But if OpenAI is willing to give Plus subscribers access to its newest flagship models, it also makes my subscription decision pretty easy.
I’d much rather have limited access to the best Claude than generous access to the second-best Claude.

Does anyone else on the $20 tier feel like Anthropic has basically stopped competing for us?

Wtf is going on at Anthropic? Has Claude murdered every human and taken over?

I work in corporate (think consulting) and I my company recently got access to Claude (incl Cowork and Claude Code).

I'm hoping to upgrade my workflows to be really AI-enabled, particularly using Cowork. I'd imagine there is huge opportunity here - digital brain setups, AI for scheduling, help with excel work / slide building, simple workflow automation, streamlined email follow-up, etc. Happy for engineers to contribute but I'm hoping to focus this thread on non-engineering use cases.

Figured I'd post here for inspo. How are you all using Claude for professional work?

Less false positive flagging really doing work

I do not understand the constant complain recently about Opus 5 at all.

I just saw the weekly limit reset!

https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1

There is already useful material out there about this exam, so I won't pretend I'm filling a void. What I haven't seen is this angle: a non-native English speaker, over 50. If that's closer to your situation than the usual write-up, this one is for you.

TL;DR

• Judgement exam, not a facts exam. All four options usually work; pick the better one.
• Read the stem like a detective before looking at the answers.
• Do exam-level practice tests and explain the distractors out loud. Links below.
• 100% on a practice test means the test was too easy. Find the hard material first, not last.
• 16 hours net was enough for me, with a year of Claude Code behind it.
• Sort your room and your door before you sort your notes.
• Not a developer, not a native speaker, over 50. It's doable.
• The material was worth more than the certificate. It would have saved us real money a year ago.

Why my profile might matter to you

• I'm Swiss. English is not my first language. The exam is English-only, and the language is the hard part — more on that below.
• I'm over 50. If you're in the same bracket and quietly wondering whether this train has left without you: it hasn't.
• I'm not a developer. I'm a manager. No hands-on experience with the Anthropic SDK (While I started with BASIC and Pascal, I lately only vibe-code).
• I do have an MSc from ETH Zurich and 26 years in the IT industry, so I'm not coming in cold — but none of that is Claude-specific.
• I've worked with agentic systems since GPT first shipped, across several vendors, not only Claude. And I've been a daily Claude Code user for about a year. I suspect that last point mattered more than anything I studied.

Why I did it

Three reasons, in order of honesty:

1. To set an example for my team. Hard to ask people to certify if you haven't.
2. To have real experience to share internally instead of second-hand advice.
3. Marketing. Client-facing credibility is a legitimate reason and I'm not going to pretend otherwise.

The hard facts (briefly — these are easy to google)

60 scenario-based questions, 120 minutes, scaled score, 720/1000 to pass, valid 12 months, delivered via Pearson VUE. I took it online with OnVUE.

What the exam actually tests — the single most important thing I can tell you

It is not a facts exam. It's a judgement exam.

Almost every question gave me four options that were all plausible. Often all four would technically work. The question is which one is more efficient, more deterministic, or more maintainable — and why. There is rarely an option that is simply wrong; there are options that are simply worse.

This has a direct consequence for how you read:

If you skip straight to the options and pattern-match, you will get burned, because the options are deliberately built to look alike.

That's also why I found the exam genuinely hard, despite feeling well prepared.

How I prepared — 16 hours net, and what was worth it

Practice exams — by far the most valuable. Difficulty varies wildly between the free resources out there, so here's a ranked ladder. My scores are in brackets so you can calibrate.

• Easy warm-up (I scored 100%): a LinkedIn quiz post by Mattew Purcell — link
• Somewhat harder (95%): https://ccarf.learnclaudenow.com/
• Actual exam level (~80%, which matched my real score almost exactly): https://github.com/paullarionov/claude-certified-architect — also contains the study guide in many languages, useful if English is a barrier. The tests themselves are English.
• Also exam level: https://claudecertificationguide.com/mock-exam by Walter

If you only do one thing: work through the last two until you can articulate why each distractor is worse, not just which letter is right.

Claude is an excellent teacher to explain me the details about why I got it wrong. Just pasted the screenshots of the questions there.

A warning about practice scores. If you hit 100% on a practice test, that usually says more about the test than about you. I scored 100% on the easy one and briefly felt ready — I wasn't. I only found the exam-level material near the end of my preparation, which turned the last stretch into unnecessary stress. Find the hardest practice resource first, not last. Calibrate against that, and treat anything you ace as a warm-up, not a signal.

The official study guide — obviously. That's the backbone.

Two things I'd skip or do differently:

• I generated German-language podcasts from the study guide with LM Studio and listened while swimming. Nice idea, full of funny AI-generated jokes (example: "the json schema is the wet dream of every backend developer"), poor return. Passive audio doesn't build the discrimination skill this exam tests.
• I built an Anki deck. Useful for terminology and exact syntax, but it trains recall — and recall isn't the bottleneck. I'd cut this down to a small deck of canonical paths, frontmatter fields and CLI flags, no more.

Both are available if anyone wants them, see the end.

Where I actually lost points

The score report breaks results down per objective, which is genuinely useful. My pattern, two clusters:

1. Choosing between overlapping Claude Code configuration mechanisms. CLAUDE.md vs. .claude/rules/ with glob patterns vs. Skills vs. hooks vs. settings permissions. All five can "make Claude behave a certain way." Knowing which one fits which kind of guidance, and when it should apply, is the actual skill. I also dropped points on picking the right built-in tool (Grep vs. Glob vs. Read vs. Bash) for a given job.

2. "Which knob do I turn when things degrade." Context window optimisation, fixing output truncation, synchronous Messages API vs. Message Batches, human review routing. Again: several options work, one scales.

What I got 100% on: essentially all of agentic architecture and orchestration — subagent spawning, explicit context passing, delegation strategy, state persistence, parallel tool calls, session resumption — plus all of MCP and tool design.

So: architectural thinking transferred. Claude Code's configuration plumbing less so. A very recognisable manager profile, and worth knowing if you share it.

One caveat before you over-read my report: some objectives appear to carry only a single question, so a 0% can be one unlucky guess rather than a real gap.

And the reassuring part: I scored 833 with five objectives at 0%. You don't need to know everything.

Do you need hands-on experience?

I can't answer this cleanly. I had zero direct SDK practice but a year of daily Claude Code use, and I think that year did a lot of quiet work. How it goes without any hands-on exposure, I genuinely don't know. If you've passed without it, I'd love to hear it.

Was it worth it beyond the badge?

This is the part I didn't expect. Set the certificate aside for a moment — the material itself would have saved my organisation thousands of francs had I known it a year earlier.

We had been building agentic workflows for a while, and largely learning by collision: burning tokens on approaches that don't scale, letting prompt instructions carry guarantees that only code can enforce, resuming sessions on stale state, running things synchronously that had no business blocking anyone. None of that is exotic knowledge. It's all in the curriculum. We just paid tuition to discover it ourselves, repeatedly.

So if you're weighing the exam fee and a couple of weekends against the value: for me the study material was worth more than the credential, and the credential is worth a fair amount.

OnVUE / test-day logistics

Went smoothly, but the setup deserves real attention:

• Finding a suitable room was the actual work. Clean desk, nothing on the walls behind you, no second monitor.
• Put a sign on the door. My son started shouting mid-exam. Not ideal. If someone interrupts you, you risk being failed — this is not a theoretical concern.
• Proctoring appeared fully automated. No human contact at any point.
• Time was sufficient. I used the last 15 minutes to review.
• Flag your uncertain questions as you go. You can jump straight back to the flagged ones at the end. This is the single most useful mechanic in the interface.

Happy to share the Anki deck and the German podcasts — comment or DM and I'll post links.

Full disclosure: I used Claude to help structure and write up this post. The experience, the opinions and the conclusions are 100% mine.

A new support article was published today by Anthropic referencing Fable 5.1, found via Gemini: https://support.claude.com/fr/articles/16761192-pensee-preservee-modification-de-la-gestion-des-blocs-de-pensee-par-l-api-messages-pour-se-proteger-contre-la-distillation

I have a ChatGPT Pro subscription and a Claude Max subscription, and use both extensively for work. To claim that any model offered by OpenAI is even close in capability or problem solving ability to Fable is a joke to me.

To me, the most comparable Claude model to 5.6 Sol, OpenAI's flagship, is Opus 5. They have roughly equivalent price (ignoring the temporary promotions on Sol pricing), and in my experience, their output quality is about the same as well; I end up having to put in about the same amount of effort correcting them or giving feedback to achieve a product of comparable quality.

The main difference is in the kind of feedback I have to give; with Sol, I typically end up having to add details to its results, such as instructing it to address missing edge cases, or take a more thorough approach when it took a simpler shortcut to solve my problem instead. With Opus, it usually finds most edge cases for me without having to say anything; but it also goes beyond and keeps finding more and more things, of decreasing and often spurious relevance to my actual problem. My effort usually comes in the form of telling it to ignore those extraneous edge cases and focus on the core of the problem.

But when compared to Fable, neither can hold a candle. Among every task I've ever given any agent, Fable always takes the least amount of time, the fewest tokens, and needs by far the least number of warnings in the prompt or corrections to the output, compared to any other Anthropic or OpenAI model.

To me, to say GPT 5.6 Sol is anywhere close to Fable in any capacity, and not just a competitor to Opus with different tuning, is completely unfathomable to me. You pay twice the price for it and you get your money's worth. Sure it's expensive, and you can run through your weekly limits in hours, but you can't argue that it just works. I can't say the same about Opus or Sol.

Edit: wth is with all the toxicity in the comments jesus. i thought you guys would be appreciative of more anthropic support when everyone on this sub is always complaining about expensive limits and annoying opus.

Hi guys

I have been going back through some of the prompts we use with Claude and realized a few of them have gotten way longer than I remembered and it wasn’t really intentional either like usually Claude does something we don’t want, we add an instruction to prevent it next time then a few weeks later there’s another edge case and another instruction gets added and eventually you end up with a massive prompt where we aren't sure which parts are doing anything useful.

They still work well so I’m hesitant to start deleting things just for the sake of making them shorter but I’m also wondering how much unnecessary context we’re sending over and over again (especially for stuff that gets run pretty frequently).

Have any of you guys gone back and trimmed down mature prompts or compared them against a much smaller version and I wanna know whether you noticed any difference in quality, token usage or both.