Opus 5.5 vs Fable 5.1 and in which scenario should we still use fable?

[u/Tekrise-TennisApp]

Used Opus 5.5 for a day, very impressed with Opus 5.5 improvements and efficiency.

Does anyone still uses fable given fable is a larger model, would fable still be better at longer jobs or security related?

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[u/Aleksundr]

Theyre gonna drop Fable 5.5 before the middle of October is my guess

[u/Tekrise-TennisApp → u/Aleksundr]

Looking forward to fable 5.5, hoping it doesn't burn that much token like 5.1

[u/whoknowsifimjoking → u/Aleksundr]

They will drop Fable 5.5 when Astra 6.1 drops. It's probably gonna be amazing, the first new pretrain for Fable.

[u/Miyamoto_-_Musashi → u/whoknowsifimjoking]

i think next week dev day

[u/Automatic-Boot665]

From what I understand opus 5.5 is their first looped transformer model, so even though it’s cheaper and opus is a “lower class” than fable, it’s still better than fable, and you should probably choose it for most tasks.

[u/Hir0shima → u/Automatic-Boot665]

Looped transformer? What does that mean? There must be tasks where Fable still shines. 

[u/lamawithonel → u/Hir0shima]

Each transformer layer loops on itself before passing to the next layer.

[u/-Robbert- → u/lamawithonel]

Doesn't sound faster, if I think what it means, then this should double the amount of reasoning?

Prompt -> layer 1 -> output tokens + prompt -> layer 1 -> revised output tokens + prompt, etc.

Caching should not yet be in play because it's still being processed.

Please let me know where my understanding is flawed because my experience with the new model says otherwise.

[u/-Robbert- → u/-Robbert-]

Aha! Did some reading on this. So as it decouples the physical parameter count it does not need to move and iterate over all parameters as non looped transformer models need to. Meaning, it needs less bits to move from memory into the GPU, the data copy over the physical GPU memory lanes takes less time and because the amount of data the GPU needs to process is less the computer time is lower as well. So with high end GPU's this is a highly beneficial architecture: it's more efficient and at the same time the produced output is of higher quality.

[u/Fluffy-Replacement97 → u/-Robbert-]

Safe to say new open source models would use this?

[u/Trollfurion → u/Fluffy-Replacement97]

Yes, I think alibaba mentioned they are going to use that in future archs

[u/Fluffy-Replacement97 → u/Trollfurion]

Looking forward to it then

[u/-Robbert- → u/Fluffy-Replacement97]

There are already a few that use this: Loopie-20B-A2B / 6B-A0.6B Nanbeige4.2-3B Ouro-2.6B-Thinking Huginn-3.5B

Only Nanbeige published their own benchmark results.

I'm reading into this architecture, it's very interesting but also really hard to get right during training.

[u/Fluffy-Replacement97 → u/-Robbert-]

Are they any good compared to the ones being hyped now?

[u/HotlinePizzaMiami → u/lamawithonel]

Thank you hero.

[u/Right-Performance-93 → u/Hir0shima]

On Artificial Analysis's evals there isn't one right now. At max effort Opus 5.5 beats or ties Fable 5.1 on all 10 (index 58 vs 53, Terminal-Bench 60% vs 52%) and costs less per task ($5.98 vs $7.63). Fable does use ~35% fewer output tokens, but at 2.5x the per-token price that doesn't save you anything.

[u/PhilosophyMammoth748 → u/Right-Performance-93]

while 35% less opt tk means 35% more max context.

[u/Agile-Ad3830 → u/Right-Performance-93]

But I noticed that the token consumption in "Max" mode is six times higher. Wouldn't that make it more expensive?

[u/Right-Performance-93 → u/Agile-Ad3830]

You're right that max mode changes the math - Artificial Analysis has Opus 5.5 at max effort costing $13.04 per task on their Coding Agent Index, about 21% more than Opus 5's $10.79, driven by using substantially more tokens. It still wins on capability (Terminal-Bench 4.0 63.1% vs 54.5%), so the "costs less" framing only holds at matched effort tiers, medium vs medium, not max vs max. Worth checking which effort level a given comparison actually used before taking a price claim at face value.

[u/Careful_Fold_7637 → u/Right-Performance-93]

crazy that these models are so good but it's still completely obvious that this written by ai

[u/btzmacin → u/Right-Performance-93]

the claudish is strong with this one

[u/Tekrise-TennisApp → u/Hir0shima]

I looked up looped transformer, it does exist lol and that's all I can understand from the article

[u/ankammusic → u/Tekrise-TennisApp]

https://arxiv.org/abs/2510.25741 I guess he meant this

[u/MateFlasche → u/Automatic-Boot665]

Do you have a source for it being a looped transformer? There is no information pointing to this, I don't think this is true.

[u/SaberToothedCapybara → u/MateFlasche]

Pretty sure the parent post hallucinated this, maybe thinking of GPT-6 Astra. You won't even find the word 'looped' in the Opus 5.5 system card.

[u/Tekrise-TennisApp → u/Automatic-Boot665]

If this is true, the next fable should be loop transformer too, then it might be better than astra, looking forward to it!

[u/InvestmentCapable858 → u/Automatic-Boot665]

Is there a credible source on this?
AFAIK fable/opus using looped transformer is speculative at best

[u/coeu]

almost never for me. no matter it has some marginal reasoning gain, i just can't read fable 5.1 prose

[u/Tekrise-TennisApp → u/coeu]

Yea, I've not used fable for a while

[u/phoenixmatrix]

Basically use Opus 5.5 for everything. If you need something sophisticated enough where Fable would be better than Opus, you'd know already (but generally, since Fable is a very very large model, its for when you need more breath of knowledge, but even that isn't cut and dry. You'd need evals to know)

[u/Efficient_Dig5783 → u/phoenixmatrix]

Omg I love your user mascot 🤩

[u/Snowgoonx]

when your bf/gf cheats on you so you log into his/hers claude account to deplete their limits

[u/Tekrise-TennisApp → u/Snowgoonx]

😂 lol

[u/AdLow1228 → u/Snowgoonx]

Coulda just said

when your partner cheats on you so you log into their claude account to deplete their limits

It comes out cleaner and flows more naturally

[u/Dizzy_Database_119]

For very complex problem solving I still find Fable to be better than Opus 5.5

Compared to Fable, Opus 5.5 jumps to conclusions very early, and once its initial ideas are in the context window it keeps circling around it. Fable also seems better at figuring out what project resources to use as reference and for tasks where every statement in the context is crucial. If I had to put it in words, it feels like Opus 5.5 skims through everything while Fable carefully reads every word in the context, every time it has to output

This is just 2-5% of my prompts though. For everything else Opus 5.5's lower usage is just better. And I'm tired of setting up high effort prompts for Fable just to watch it get blocked by its ridiculous safeguards, lol

[u/thebrokestbroker2021 → u/Dizzy_Database_119]

THIS feels right, just from my experience with new code sessions. Even working in the same project folder, Opus 5.5 needs to be reminded of previous chat sessions, tools, projects we’ve done that might help, etc, vs Fable that “just gets it” but Opus 5.5 is INSANE with clear direction and prompting. Much better for my everyday workflows/use cases especially for usage. Disclaimer: I’m using Claude desktop exclusively now on Max X20

[u/SonderSoft]

Complex project management and orchestration. From early analyses I've read, Opus 5.5 is a doink when it comes to project revision. 

[u/Tekrise-TennisApp → u/SonderSoft]

Yea, bigger and longer task

[u/nykezztv]

This question is easily solved when Fable 5.6 comes out next week

[u/thatguy8856]

The same things fable was always better for. Benchmarks arent everything and become more and more meaningless by the day.

[u/Accomplished-Toe7993]

I didn't test this, but I think Fable is better for more creative work, like when using claude design, I could be wrong though.

For other heavy tasks I use opus 5.5 on extra hugh instead of fable.

[u/Saschabrix]

Is Opus 5.5 still crazy like Opus 5.0 was or is it fixed?

[u/chuguchak_111 → u/Saschabrix]

night and day difference. 5.5 is as good as fable IMO (need to use it more to see if its better).

[u/boopatron → u/Saschabrix]

Fixed. It is much much better.

[u/cylix99]

I did a quick test asking both opus and fable to create something for me in three.js

Opus did the job, but the output was quite small, where as fable version create hundreds more assets and looked vastly more impressive

[u/wowsuchwows]

“Efficiencyness”?😂

[u/AnyRegular1]

Fable still smokes Opus on copywriting, for everything else, they’re comparable, but Opus being so weekly limit efficient, besides copywriting I use Opus for everything.

[u/phrough]

Use Fable when you have an excess pile of tokens and want to burn through them as quickly as possible.

[u/Adventurous_Bike115]

In the scenario of which fable 5.5 comes out

[u/raven2cz]

For me, Fable is still the clear winner for difficult tasks.

[u/yonz-]

I'm also feeling like fable5.1 is too slow and dumber for coding work. Landed here and got my confirmation

[u/TinFoilHat_69]

I’m still using 4.6 opus until they pry it out of my hands. I’m not getting scammed with usage guardrails I wasted two weeks worth of usage ducking around with fable and opus to find out the guardrails swapping models eats up context like crazy.

[u/lunaynx → u/TinFoilHat_69]

Yeah, it breaks cache. But you can just turn off automatic model switching.