I asked Opus why it used raw http client instead of api client that is already a dependency and it said: "you're right it's overkill" and proceeded to implement api client on top of raw tcp socket.

Or an eager contractor who bills by the hour with a big unallocated budget.

I think the improvement on how it codes is pretty much represented correctly by the benchmarks (a nice bump, but not some crazy leap)

But where it really shines is in how NOT lazy it is. Fable requires less hand-holding. And I can understand how someone who uses Claude-Code sparingly and with very focused prompts would not see a lot of improvement there.

But simple example: if you ask Opus to do a review of the codebase (with a short prompt and not too much guidance), I've had it basically read the `git log` output, do a simple `ls` and have it declare "Everything looks great! No problems found!", when Fable really does what you would expect it to do.

And you might think: "oh, so it's just capable of handling crap prompts?", well sure. But even if you make THE PERFECT Opus plan (a plan that would take many turns/hours to finish), Opus will fake out, say everything is done, and then you see that half of the plan was deferred, half of the functions are ridiculous stubs, ...

If you give the same plan to Fable, it'll just DO IT. And it WILL get it done. And in the end it'll tell you "Oh, I also found 30 other bugs and I fixed all of them properly" (where Opus would have started crying, or WORSE, worked around the bugs)

20 yoe, application/systems stuff, and I always run models on xhigh or max effort level.

Fable has been more intelligent, with better taste and defaults (e.g. make impossible states impossible without being told, build for testability), and considers/solves things that Opus did not.

My workflow is to run Claude in planning mode first to spit out a plan file and then review->revise cycle it with Codex or other agents.

One big tell is that Opus will say that it can't find any more revision advice for a plan file, yet Fable will find more issues but also smart pivots into better solutions. This is probably the best test since it's not based on vibes.

I'm doing work with fairly complicated cryptographic algorithms and math. I'm finding Fable 5 to be a significant stop better than Opus 4.8, but that Opus occasionally comes up with something small but nontrivial that Fable missed. (The reverse is true much more often.)

15+YOE. Fable 5 is well above the level of Opus. I have used it alongside Opus for a range of hard problems, including porting a large static analysis tool to Rust, building various tooling around .pptx and .xlsx documents.

In all cases, Fable clearly outperformed Opus.

What is your view on how experience and problem space relate to subjective experience.

For example will inexperienced or experienced users see a bigger jump in subjective quality?

It still does stupid stuff like leave unnecessary abstractions around after refactoring instead of proactively suggesting to remove them.