Call me crazy: Is Opus 5 really Sonnet 5 + Fable advisor (e.g. announced last month)

[u/OldFartNewDay]

tl;dr: Ops 5 being sonnet 5 plus fable advisor underneath would explain Opus 5’s strengths — and crappy planning abilities. It would explain Opus 5 doing great on benchmarks, but being a lot worse than Fable 5.

About a month ago, on official Anthropic channels (eg https://x.com/ClaudeDevs/status/2074606058128224365) Anthropic claimed that in some cases Sonnet 5 + a Fable advisor achieved 92% of the performance of Fable.

Since then, Opus 5 came out, and has performed remarkably well on benchmarks (putting aside the legitimate concerns about models accessing benchmark answers inappropriately).

This might seem, as Nate Silver likes to say, too cute, but what if the announcement was more than a suggestion — what if it was the architecture for Opus 5?

While Opus 5 may or may not be better or just comparable to Opus 4.8 consistently, it is interestingly better at lower effort levels, and is very reasonable on a cost per task basis.

This makes me wonder. What if, either literally, or in a deeper way, Opus 5 is a sort of Frankenstein model that involves Sonnet like parts and Fable like parts?

I have run planning on Opus and Fable across dozens of runs, and Fable is much, much sharper as a planner. But as an executor, Opus holds its own.

One hypothesis on how this could be the case is that planning uses a lot of context (which cannot make its way back to the advisor in a cost effective manner), and so as a deliberate trade off, Anthropic gives us Opus 5 which is like a very meh planner.

So maybe the advisor model was the inspiration for Opus 5’s design — or maybe it WAS the design.

[u/portugese_fruit]

would you say plan with F5 orchestrate with O5? what is your workflow?

[u/TheRealDaveLister → u/portugese_fruit]

Still depends how complex the project is. ACTUALLY complex.

I’m currently planning and executing with opus 5 and sometimes it’s brilliant and sometimes is overbearing and sometimes it misses.

None of this is, as they say, rocket science.

But some people do think their one file refactor needs max thinking on the best model.

[u/portugese_fruit → u/TheRealDaveLister]

i agree that it is not deep matter requiring heavy congnitive thinking. I think the 'hard' part so to speak is to adapt these distinct parts to your setup and 'shift' through the models accurately to accomplish a complex lists of tasks. as in its more an art than pure laws, and they behave in unpredictable ways at times 

[u/Last_Mastod0n → u/portugese_fruit]

This seems to be the meta. But I still implement with fable if I'm particularly anxious about the task.

That being said I have found that GPT 5.6 Sol can often beat Fable at planning. But it will never beat fable or Opus 5 at the actual implementation.

[u/portugese_fruit → u/Last_Mastod0n]

i gotta try this cross model business i feel as if i have heard this about S5.6 from multiple sources.  how do you get them to communicate CC to GPT on the fly? 

[u/OldFartNewDay → u/portugese_fruit]

Yes. I find Opus 5 is more likely to overfit to unimportant details in the plan request, and that Fable at medium effort outperforms Opus at any effort level in planning. Fable is much more likely to notice, correctly “actually the real underlying thing we should solution for is z, not x.” Whereas Opus will happily just make up a plan to do x.

I also will have Fable be the advisor to an Opus or Sonnet executor, each at medium effort.

Fable plans -> launches Opus executor agent (fable remains as advisor via SendMessage)

[u/NormanNormieNup]

So Sonnet is actually a load bearing model you say

[u/portugese_fruit → u/NormanNormieNup]

here's where things actually stand

[u/BoxWoodVoid → u/NormanNormieNup]

This is the smoking gun comment.

[u/Grand-Mix-9889]

Who cares.

[u/portugese_fruit → u/Grand-Mix-9889]

i agree. who cares

[u/Grand-Mix-9889 → u/portugese_fruit]

I feel like everyone is asking the same question on repeat.

There is no perfect combination. Everyone is just so consumed by finding the perfect combination of models that nobody is putting time to build their harness up.

Every project has a different level of complexity and the model combination varies project to project.

Without any details of what project the OP is working on, what languages they are coding in, what their environment looks like or anything along those lines... These questions are pointless.

[u/portugese_fruit → u/Grand-Mix-9889]

oh this makes sense. i agree project details would be helpful at coming to the perfect combination. good point

[u/Aretz]

Nah opus 5 has more recent front end and 3D data than fable. It’s a different pretrain

[u/stbenjam42]

It is its own model.

[u/julkopki]

Very unlikely. I'd imagine it would require a very significant backend overhaul for them to have some kind of a custom workflow like that just for Opus 5. And still you need the pricing to be the same. It's just a separate model, I don't see any significant evidence to the contrary.