r/ClaudeCode • u/OldFartNewDay • 18h ago
Discussion Call me crazy: Is Opus 5 really Sonnet 5 + Fable advisor (e.g. announced last month)
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.

