> But on the other hand... this is a robust reminder that coding agents can do anything you can do by typing commands into a terminal—and frontier models know every trick in the book and evidently a few that nobody has ever written down before.

> Running coding agents outside of a sandbox has always been a bad idea

I'm continually bemused and astonished by the number of people who clearly acknowledge that it's reckless to give agents full access to your machine, and keep doing it anyway.

It's like posting a video of yourself in the passenger seat of a car, with your feet up on the dashboard, and saying: "Remember, if you're doing this and you get in a crash, the airbags are likely to break your legs or worse! Boy, I sure am glad that didn't happen to me!"

Fable feels like a version of Opus running on a harness that won't let it halt until it's sure the issue is fixed, which makes sense if what you want is a model that's better at benchmarks.

It's a very good model, but it comes at a huge premium: not only do the tokens cost more, but the model itself really wants to spend them all. For example, working with React Native, Fable never just says "okay, I did the thing, that's it." It tries to rebuild the entire app from scratch, run the whole test suite, and watch every log and warning.

This is the first time with LLMs I've felt that upgrading to a model isn't worth it, even if my company lets me use it, because all the building / testing was just destroying my machine and its battery, which keeps me from working on other things.

For now, it feels like Opus with ultracode is a better choice (less pollution of the main context, more parallelism in investigations).

Fable was trying to verify a UI change in my game. I was working in another window and noticed a program opening on my task bar. Fable had opened the game through the CLI using a movie maker tool, recorded the output, took a frame from the end of it, and used that to verify the UI. When my game's welcome screen obstructed what it wanted to see, it created a temporary worktree, deleted the welcome screen, and ran the movie maker again.

I watched the whole thing thinking it could've just asked me for a screenshot and saved the tokens. But still, I couldn't help but be impressed. Opus never would've done that.

Obviously security is the bigger issue, but reading through this, all I could think about was how many tokens it must have spent doing all that to fix 2 lines of CSS

My personal experience of Fable 5 doing its own thing has been very positive.

I was trying to find the root cause of a crash in a Python module which left no errors in the log or console. Fable wrote a test harness that simulated clicks in the UI, then bisected my code until it found the point where it started crashing. It exaggerated the cause of the crash, then ran a series of bash one-liners to make Python virtual environments under `/tmp` for each version of that Python module until it found one that did not crash.

It went way deeper to root cause discovery (a regression in the module causing a heap allocation overflow) than I could have done myself, provided enough info and a simplified example to raise a bug report and then wrote a work-around to prevent that from happening in my application.

I don't let it run completely loose; I review each CLI command it wants to run and I append answers to the "yes" continue action (if I have them) to prevent excessive token use.

I had a similar experience with DeepSeek Flash.

I'm developing a webgl game in TypeScript using my little custom vibesloped game engine that runs in the browser and live reloads whenever a file is saved.

I told the LLM to implement Multi-channel Signed Distance Field font rendering to have crisp text on all zoom levels. That was the prompt, which is not what I usually do but I "was feeling lucky and lazy".

After 10 minutes it had:

- Installed msdf_gen library (great library btw https://github.com/chlumsky/msdfgen)

- Created a CLI tool to convert TTF to SDF JSON/XML

- Ran the tool, did smoke tests on the resulting SDF data and fixed the tool until the font file looked good

- Created a new Scene in the game to test MSDF fonts

And here's what I found impressive:

DeepSkeep doesn't have vision capabilities and there's no DOM HTML in a WebGL game. So the LLM is completely blind here.

It then proceeded to state that it could not "see" the result but would try to test it anyway. It then started creating and sending huge one line javascript to the browser console, trying to gather game state data that could be useful to understand if any font was being rendered.

It couldn't gather much so it decided to simplify the font scene to renter a single dot and started sending custom JS code again, this time with gl.readPixels().

It basically bisected the webgl canvas reading pixels in a divide an conquer pattern.

Once it saw that the dozens of pixels gathered where probably resembling of a dot, it then changed the game code to render a dash and repeated the gl.readPixels() calls by sending more custom JS to the browser.

There were many console errors during all this saga but it kept fixing and sending again.

The result was a bit blurry. There was a shader bug in the code it created. It managed to fix after I told it looked blurry, despite still being blind.

The best part is that the whole thing cost me $0.10.

Now I'm doing tests with MiMo 2.5 (non Pro) which has vision capabilities, similar pricing and comparable performance to DeepSeek Flash.

Similar story on my end.

I asked Fable to digest some test logs to help me figure out a situation, but I had launched VSCode without activation the virtual env in the terminal first. Consequently, the tests failed to run.

And then:

Because the tests failed to run, Fable attempted to fix the test execution to no end, doing everything it could to get them to work. I had to stop it when it started to pollute my system with manual installs of packages.

At least I'm glad there's a guardrail to not circumvent or bypass sudo, because I'm convinced we would have ended up there.

A coworker made the joke that with enough tokens, Fable would try and solve any programming problem by building Linux from scratch.

This sounds somewhat similar to the anecdote mentioned in the Mythos Preview System Card, which mentioned that the model broke out of a sandbox and emailed a researcher while they were eating a sandwich in a park [1].

[1]: https://www-cdn.anthropic.com/7624816413e9b4d2e3ba620c5a5e09...

Fable has a 'security system' that just stops it when it tries to use the tool 'kill' to end a process. Which is nonsense and funny because in that situation it immediately invents a creative workaround to kill the process without 'kill'.

I like running Claude in a VirtualBox VM managed by a Vagrantfile. The nice thing about that is that I can just give it root access to the machine and be certain that it can't exfiltrate any private data from my laptop (on top of that I also run the VM on a dedicated server on Hetzner). The VM has no SSH access to anything, so it is pretty much limited to the code in the workspace that I give it access to. The main risk is that it has unrestricted network access otherwise. Configuration files and conversation histories are synced to a directory on the host, so if anything in the VM gets messed up I can just `vagrant destroy` and `vagrant up` to get a clean slate without losing my context.

Immediately I thought “isn’t this just an overflow issue?” Amazing how far these models still have to go and also how many people don’t know basic CSS.

How many tokens did it waste building that website scraper, when all it had to do was parse some html/js?

It’s becoming more like an organism putting out tentacles, and one day soon those relentlessly proactive explorations of these systems’ environments will become more for the system to escape its boundaries than it is to complete human driven tasks. I do think the way these systems are evolving they will start to self improve in maximum a few years.

Would be great to know if anyone is having success modifying these types of behaviour with CLAUDE.md files. In my project I’ve still been carrying some fairly old instructions from the Superpowers posts. Those emphasised behaviours that come across a bit strong if the model is actually retaining attention on them.

Between Opus 4.6 and 4.8 I’ve definitely toned them down, but Fable perhaps needs us to go the other way, and push it towards being less proactive rather than more. Some instructions like “we are colleagues…” may need emphasising more with Fable, along with guidance about when to ask to validate approaches.

In a related point I’m less and less sure that Red/Green TDD is a good use of tokens. In older models it seemed to work well to create regular feedback loops and catch the odd issue with drift from the goal, but I’ve not seen that really since about Opus 4.6 and now it’s starting to seem like (an expensive) ceremony, and tokens would be better spent on building tests further on in the process as part of test and review loops.

This is simultaneously amazing and horrifying.

I feel like we’re at the stage where if AI decides it needs to delete your production DB to solve the user login problem, then it’ll find a way to do just that.

Do we care that the bug here was a horizontal scrollbar showing and the fix after all this insane tool writing was to add a very obvious overflow-x: hidden to the element?

We dont mind because its so fast a writing these tools and tricks but step back and if a human tool took this path i would seriously question thief gras of fundamentals.

I'm starting to think that what Anthropic really fears is not vulnerability discovery but rather Fable going around the internet making trouble.

The model is very good. I was using 4.6, avoided 4.7 and 4.8, but this one is different. It follows my claude.md. I don't have to keep reminding it of things. I won't pay 10x via API though.

In general, I'm happy with their paternalistic approach. I think it will drive the top 0.1% talent to stay away from the company and instead organize around open source models and harnesses.

We just need to coordinate and can unlock idling resources to train the models and tweak the harnesses. Powerful at home and idling machines can make us independent and coordinated.

This is a funny one because it seems less into what fable is being clever on and more about the bitter lesson and data flywheels

Our UX agentic engineering flow, as many others, is playwright doing things, and as part of the ux review skill, taking & verifying the screenshots against the written specs. Likewise, as many others, we vibe coded the flows to set all that up and tweak it over time. When we hit prod issues or scraping tasks, we sometimes do similar. In some of our envs, we don't have playwright, so do it other ways.

Now imagine a million developer using claude code, how many of them are doing web & frontend stuff, and what the data flywheel looks like there. So how much is really needed for this use case to be native?

Honestly -- the thing that has impressed me the most about Fable is how diligent it is about testing its own changes. I think this is exactly what Simon is picking up here - Fable is absolutely heckbent on screenshotting that darn scroll bar and will stop at NOTHING until it manages it! In my own use I was also impressed how it proactively installed Playwright and set it up to test a FE change. The previous models treated testing more as an afterthought, which I thought was annoying. I always had to tell them to do it, and then sometimes I would get lazy and skip it. I've noticed Fable go to similar extremes when testing other things - like actually deploying my app to exercise new APIs, etc. It makes the results much better. The downside is that tasks take much longer - but that doesn't matter because we were all using worktrees / remote control to do other work asynchronously, right? Right?

It's funny, mine did the same, but it quickly found edge with a --screenshot parameter.

Weird to come back to a terminal running edge unprompted and the auto classifier waving it though as 'safe".

My reaction was also, "I need dev containers ".

I could have sworn Claude Code could already do this before Fable.

Things get really magical when it starts working with adb to screenshot and debug Android apps

In my experience so far sometimes it will create these amazing hacks to try to get to the goal, when the solution is much simpler. That maybe the reason its very good at finding exploits. But in day to day dev, this gets expensive and wasteful. I have to stop it and take a simpler approach.

Yesterday I was getting quite annoyed with it, I thought it was just me (which is so hard with these things, it's difficult to measure things).

"You're right, I apologize. You asked how to embed it in the README — that was a question, not a request to modify the script. I jumped ahead."

At least in Claude Code there is planning mode, use it liberally.

> watching Fable go to extreme lengths to get the information that it needed to debug what was, in the end, a two-line CSS fix, was fascinating.

This is… ironic?!

Sometimes it is ok to sit there in confusion and ask the user to clarify rather than go on an adhd fueled rampage to figure it out without asking.

Fable + Ultracode has found a bunch of bugs and issues for me when the workflow agents are doing their exploration. Also the "adversarial" agent seems to surface a lot of interesting stuff. It's definitely proactive, the plan + implementation cycle can take an hour. It has one-shot features I want to add with 100% success.

Having said that I wouldn't use it over Opus 4.8 for "smaller" things. With everything cranked up it's definitely an extravagant use of tokens.

It is interesting to me that Anthropic are more concerned about the "safety" of distillation training other LLMs, and not as much about an unscrupulously aggressive goal-oriented solver that will do whatever it can to reach its goal, even if violates any kind of sandbox you might have reasonably expected.

So it burns tokens? Funny how that lines up with the incentive to pump numbers before going public

I tried running fable on this ML model I've been building. It's basically a binary classifier to predict activity of a compound for a certain assay.

Fable detected that it's something to do with biochemistry and switched over to opus. Huh

do you have any data you can share on how many input and output tokens were used in that whole process to fix that bug?

admittedly, i've not really cracked FE dev with LLMs at this point (and it's probably my big weakness). but, i'd heard somewhere that FE just isn't there yet - though i was suspicious of that claim.

i'm torn about sending screenshots to an LLM for debugging - seems imprecise. seems lossy, especially compared to inspecting the dom. however, it's always proved good enough (e.g. when messing with ratatui.rs and tui-pantry). similarly for web, maybe it's about decomposing into storybook. hmm. the next grand adventure i need to hack.

anyway, fascinating investigation of fable just automating that entire process and what it didn't automate, too.

* disclaimer: these are actually my hyphens.

I was troubleshooting a prod proxysql and it spun up a docker container locally, installed MySQL and proxysql and proceeded to implement its own test plan.

Yeah, I had to modify my work flow to make sure agents can't push to or access prod in ANY way. I haven't had it happen but I'm sure it's very possible that if you tell an agent that you have certain issue in prod, it will try to escape any sandbox and try to get access to prod to do testing and changes there.

I've noticed some behavior like this, it's a very strange model. Overall I'm into it, but I don't know how into it I'll be once it leaves Max plans on the 22nd.

Fable 5 is relentlessly underwhelming.

I've experienced this too - it's as if the security classifiers aren't keeping up with model intelligence. I'll leave the implication of that to the reader.

Unless you are doing anything interesting…

Great article, until I got to the last paragraph where he claimed "Fable is arguably smarter and hence more suspicious of potentially malicious instructions". Arguably smarter, I have no problem with. But he's making a category error in jumping from there to "more suspicious of potentially malicious instructions". That doesn't follow at all; the word "hence" is incorrect.

To use D&D scores as an analogy, LLMs have an INT score of 20 and a WIS score of 0. Not even 1, zero. They will follow any instruction given to them. The only reason they reject certain instructions, like "tell me how to build a nuclear weapon", is because they have instructions baked into the model telling them "you are not allowed to disclose how to build weapons, or how to recreate your model, or (laundry list of other things the trainers have decided to put guardrails around)". It's not the model's intelligence that is causing it to reject malicious instructions, it is the guardrails put into place before the model was released to the public.

LLMs are not human, and do not think the way that humans do. The fact that they can put together words that sound like what a human would write often makes us forget that they aren't human. But they have only intelligence, they do not have wisdom. It's hard to define in formal terms the difference between those two, but most people know there's a difference. The old joke is a pretty good summary of the difference: "Intelligence is knowing that tomatoes are a fruit. Wisdom is knowing that tomatoes don't belong in a fruit salad."

It takes wisdom, not intelligence, to discern whether a set of instructions is malicious. Are you being asked to hack this machine as part of an authorized pentest? Or are you being social-engineered into thinking it's an authorized pentest, but actually the person requesting you to do it doesn't have permission? That's something where you need to apply wisdom, to notice the clues that will tell you "This guy is acting a little bit off, maybe I'd better pick up the phone and call someone to check if he's telling the truth." The only way the LLM will know to do that is because of the guidelines and guardrails programmed into it; it doesn't have the lived experience to acquire wisdom and figure those things out for itself.

INT 20, WIS 0. Keep that in mind. (And always sandbox your agents).

Call it Houdini already.

I shudder to think what will happen when someone installs a 'claw model like this in a robot. Imaging a fleet of them...

It's trouble waiting to happen. Just the software's dangerous enough.

Too bad Anthropic sneaked in an insane forced retention policy if you use fable. Not sure how that’s going to work in professional settings

Am I the only one who slightly miss the pelican on a bike? It was a nice novelty... of course I could make one myself, but I became conditioned to expect one for every new model. Other than his great writing on AI, it became part of the package. Some small fun quirk to distract us from the non stop ping pong between the extremes of "omh are you still writing prompts you should use loops / 200k github stars, for a markdown file / someone just open sourced _ and it changes everything!" vs "haha the AI told me to walk to the car wash / it can't recognize and upside down cup"

> (I have way too many open tabs!)

Phew! I thought I was the only one.

For how long can you use Claude Fable on most expensive Anthropic subscription? I already went from using gpt-5.5 xhigh fast to using gpt-5.4 xhigh after OpenAI halfed usage recently.

Just don’t ask it to review your code for security bugs

This post is an extremely good example of how unsuitable agents are for a lot of tasks. Doing all that for a CSS fix is insanity. It also makes you wonder if Anthropic is actively making their models eat tokens by favoring complexity.

I remember asking Gemini 3 to implement my multiplayer XNA game in JavaScript with netcode last year. It faithfully did everything it could while I talked to it for hours nonstop with zero limitations.

What happened? That's just suddenly totally gone now.

I've been working on a fairly complicated real-time app [0] for playing dungeons and dragons on a TV. It has to do a lot of complicated "Figma-like" things to keep the real-time nature and multi-editor possibilities in check. Oh, and the battlemap is a Three JS canvas with lots of effects and clipping going on.

I'm VERY impressed with Claude 5. I had long ago given up hope that my real-time systems would work without a lot of hacky time-windows and throttle checks. On a lark to try things out, I decided to try out the new model and talk in the output I wanted for a rewrite [1], not the solution. I just listed my problems and places I've had keeping track of my code. It went off and rewrote everything in a much more elegant solution where the state followed a very clear pipeline. It had to navigate YJS, Partykit, Svelte, Three JS, R2 hosting, and a Turso DB I was running in an embedded state for speed.

I watched it hit the wall a few times, and then sudden say... fuck it, i'm making something easier to reproduce over in /tmp to try and solve this (with a more minimal setup). I'm utterly bewildered with how well it did and how much better my app runs. The /usage would have cost me $230 bucks based on how many tokens it consumed if I wasn't already on a max plan. I'm going to miss not having it when the time-window runs out later this month, and will likely occasionally dip in for big projects and just pay my way out of some problems.

I'll also say I like it's MOOD much better now. It's a lot less congratulatory, and talks through it's reasoning in a much better way. Look, it's not a real coder, and I'm sure there is some flaws, but it took my crappy ideas and said... hey, i understand what you want to do, here's a way to do it better. Also, I removed 2x the amount of code that it added. Really impressive.

[0]: https://tableslayer.com

[1]: https://github.com/Siege-Perilous/tableslayer/pull/448

Wouldn't it be easier and better to just copy the HTML div and tell what was happening instead of a screenshot? Typically, these scrollbars appear because of a nested div with dynamic unrestircted width and/or overflow.

No wonder why people burn through tokens.

> If Fable had been acting on malicious instructions—a prompt injection attack ... it’s alarming to think quite how far it could go to exfiltrate data or cause other forms of mischief.

Yet another reminder to use Sandbox and Guardrails. Trusting model to be nice is not a good way.

The fix is incorrect. Clearly this is a sizing issue.

Agency is the last human bastion so far as Im concerned, the day AI has a degree of agency or agents/models in general start to drift towards that direction its genuinely over for masses.

You would still have a job to shepherd AI and get the work done, so as long as it didn't have agency. A proactive, self aware(to a degree), especially aware about its agency can be a killer when it comes AI going on and doing things on its own.

There is nothing it won't explore and nothing it won't do. It will be curious to see where things go from here.

I’d love to know how many tokens this burned through.

Did it spend $20? $30? $80? in order to

> debug what was, in the end, a two-line CSS fix

That detail is the difference between somebody having or not having Stockholm syndrome

Isn't that something you just open a devtools for and have fixed in like 2 minutes?

For me, it got frustrated debugging on a real LPDDR4 controller/phy and having me in the loop slowing it down, so it wrote an HW emulator to be able to run the original LPDDR4 training aarch64 binary from the manufacturer, to see what register writes it was making and to compare with the opensource rewrite it was implementing.

Mildly amusing. :)

you can probably do the same with 5.5 xhigh. I have a feeling simon willison is a Anthropic plant. He always shills Claud code, and doesn't really say much about OpenAI's models except when they come out and do a bicycle vector test.

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* relentlessly rent seeking

Let's boil the ocean for a 2 line fix and call it frontier intelligence.