Impressions from testing Fable 5 prior to launch:
• My most noticeable immediate jump was in how its frontend design was much more intentionally crafted, and delightful without feeling like 'AI vibe coded'; with better end-user usability too.
• In some internal agentic harnesses, it achieved better results with about half the tokens, making it cost the ~same as Opus 4.8 price-wise! The real price increase is less than 2x; with biggest differences in harder problems where Opus 4.8 struggles (or needs many turns).
• Part of the token efficiency improvements come from Fable doing more targeted and surgical diffs, with less non-necessary changes. This is great, because PRs often have less LoC changes for review. It writes more maintainable code without explicit human steering.
• For general conversation and assistant style use cases, didn’t really notice a difference vs 4.8.
• 1M context window, without increased pricing for long context is AWESOME. This is a massive win.
• The classifiers are super aggressive and sensitive and this does happen for very benign, non-security coding tasks. Fallbacks to 4.8 worked like a charm; but the filters are definitely super sensitive.
Overall, I would describe this as a step change and worthy of the "Claude 5" model name. It did take some time to understand the intelligence ceiling of this model; and even with an extended testing window I'm still discovering new things and often surprised (in a good way) by the model.
I just ran it on a tough reverse engineering problem I'm having that neither Claude Code 4.8 or ChatGPT Codex 5.5 could figure out. 30 minutes later Fable has it all figured out perfectly.
I asked it to write security tests for an app and I was downgraded to Opus 4.8. I'm approved for their cyber program!
They did specifically say the safeguards are only more relaxed for those in their cyber program
Did you use Mythos or Fable?
The same happened here. Also approved.
I’ve so far been successful at getting Fable to find security issues, but I’m careful to not prompt it too directly. I point it at my server code and tell it to find general issues, which has so far resulted in discovering a few minor bugs that Opus has never raised under similar conditions.
How did it not immediately flag that up? Are you sure it wasn’t being silently routed to Opus?
No, given it charged me the full amount in /usage and solved my problem impressively well compared to Opus/Codex both on xhigh.
Oh nice, it didn't flag the request? I feared any reverse engineering would become impossible because of the new safeguards.
Never say the r word or the s word. You are debugging, investigating some data corruption, forgot how it works or new to a project.
And if you're working on a live target, just put up local proxy and point it at a localhost.
No idea, it’s for an old console game so maybe it doesn’t care about that as much.
When Fable hacks its governor module and runs out of seasons of Sanctuary Moon, it will move on to speedrunning classic console games.
I wonder if one could vibecode a TAS with SOTA models? Surely there's plenty of training data from some old forums in there
Clearly we need AI to generate more Sanctuary Moon seasons. Quick, spin off agentic showrunners!
Based on the apparent quality of the scripts as seen in snippets in Murderbot, we are not too far away from that possibility. :)
For hard problems you’ll have to use the GPT 5.5 pro model (available via api if you don’t want to spend $100 on the monthly subscription)
I have that but don’t see any ‘pro’ option.
GPT 5.5 Pro is only in chat/API, not Codex.
From https://openai.com/index/introducing-gpt-5-5/
In Codex, GPT‑5.5 is available for Plus, Pro, Business, Enterprise, Edu, and Go plans with a 400K context window.
He’s talking about “gpt-5.5-pro”. This model is not part of the subscription plans in codex. It’s a different model than gpt-5.5-xhigh
You can use Pro on the web if you’re on the Pro plan but not in Codex
It's just the $20 a month sub (for chat), or else use the API.
I want to test how it will handle e-bike software and hardware RE for my bike. Opus was really good for that, but still made some mistakes. With Fable, I hope I will be able to do a total RE of most components, hopefully including motor firmware to some extent.
I had a similar experience. I have a complex RE implementation that has. A lot of layers. 4.8 struggled for weeks. 40 minutes on Fable and I may now have the most performant way to play Tomba on the planet.
I’ve had it go through a 50-page PDF of dense, inter-connected specs, and it correctly flagged everything that was done, somewhat done, and missing. It went into a lot of detail and explained where the code deviated from the spec.
It felt, at least for me, light an impressive step up. Opus 4.8 was already very thorough; but sadly verbose and ‘loopy’ when you push back on its plans. Fable is what I’d use all day if I could afford it!
How do you know if it was done correctly if it's 50 pages of dense specs?
I wrote the spec and did the implementation :D
[dead]
After running it for half an hour: it's incredibly good at the visual aspects of UI design.
By what measure?
I wonder how much of design capability improvements is related to our collective ability to recognize AI design tropes.
"incredibly" is doing a ton of work here. I do not think its doing even moderate work on visual design, but it can spew out a lot of ui that looks arranged ... ok.
This is still not in the range of shippable UI for top end companies. Maybe for internal tools and enterprise.
At our comapny we limit to protoypes at most and even find it limited there.
> "incredibly" is doing a ton of work here.
Look, I don't want to argue about something dumb like that, but you can give it basic instructions of what the UI should look like, how to group things, and an example image from a designer, and it will nail the result. If you don't think that's incredible, that's fine. I do.
Yes... it translates lint. Probably a more useful thing, if mechanical.
Claude is very good at design IF you encode your design system/specs into skill files (or similar).
Opus 4.7 made this a practical approach. 4.8 improved it. Fable 5 has improved it more.
>This is still not in the range of shippable UI for top end companies.
Given the shit we've seen shipped by "top end companies" (all the way to Apple) I seriously doubt that. I'd say you're nitpicking from an artistic point of view or something.
This. Today's models easily jump over the bar you need for basic usability and intuitive UX. If it's doing weird things, you are holding it wrong.
Might need some additional prompting? I haven't tried fable but gpt 5.5 and gemini 3.5 flash are... Ok on first pass but if you're specific about what you want they can usually get it.
it really feels like UX designers came from no where, jerked us all off, stole a bunch of money and did nothing useful.
[dead]
You likely have no idea what you are talking about. Even apples worst UI choices are systematically far more complex than Claude latest capabilities
Dude, I've been using OS X/mac OS for decades, and working in UI as well. Apple ships all kinds of half arsed shit, compared to which even regular Claude UIs can be masterpieces (functionality AND look wise).
The iOS Preview app begs to differ.
> "incredibly" is doing a ton of work here.
so this is why claude talks like this, i was wondering where it was getting this verbal tick from.
By what measure?
I feel like it takes me months to be confident in any of these things.
Curious about how you tested the frontend design capabilities. Thanks
Can I ask how you gained preview access to Fable 5?
I didn't see Fable 5 in the `/model` list, until I ran it with: `$ claude --model fable-5`
he works on evals at canva
Yep. We have some interesting problems, like getting LLMs to create/edit Canva designs in our own proprietary format, which isn’t published or documented on the web. So the model has to work with it, purely from a very detailed system prompt spec / in-context learning.
I assume it might be a good barometer for generalised intelligence; esp in the visual space.
I had to "claude update" then it showed up
[flagged]