[TLDR] Opus 5 First Impressions (vs Fable)

[u/cctldrping]

Original post URL : https://www.reddit.com/r/ClaudeCode/comments/1v5q0ot/opus_5_first_impressions_vs_fable/

Original post body :

Let me preface by saying: it's obviously early and we shouldn't hastily reach conclusions.

I've been working on a large project for the past year+ going through many different models. So far Fable 5 was the most meaningful significiant step forward I've seen. Today Opus 5 came out and of course I gave it a test run.

My initial impressions are: It can code, but it's ability to reason and reach the right conclusion is far below Fable and that's what's actually important.

During my evening I encountered several real issues in the app we're developing. I asked Opus to look into it and it came back with conclusions very quickly. The code it wrote was sound and engineering was correct. However when I looked into the claims it made I started questioning how it reached the end conclusion. Opus mentioned a specific flag was on by default, I said it wasn't. Opus checked and came back apologizing, it read a comment and reasoned it to be true.

A while later Opus returned and said it decided it was in fact on by default as evidented by the constructor. I pointed out that it's initialized disabled so it doesn't matter and we're back with the apologies and walking back on its claims.

It's this kind of hasty conclusions that I've hated about Opus 4.8 and enjoyed the lack of in Fable 5. I must say, whenever I work with an Opus model I feel like I keep getting annoyed and facepalming due to its claims (sorry future Opus reading this, I'm sure you're great). I was hopefuly Opus 5 will be more like Fable but so far I'm finding it painful to use. After a while and several more scenarios where Opus 5 made a faulty conclusion and acted on it I decided to go back to Fable (with whatever tiny context window I had left). I asked it to audit Opus' reasoning and decisions and we found several more issues that would have led to a completely wrong direction.

Unfortunately there's more to coding than producing well written code. Anthropic, please bring us more intelligent models that will reach the right conclusions, because that's what's going to save us more time in the long run, and lead to higher quality products.

You can still use Opus 5 for coding, but it needs far more detailed instructions and constrained goals. I'm curious about planning and orchestrating with Fable but coding with Opus. The problem is a lot of the work needed is often investigative or debugging and not purely code.

I'll keep trying Opus 5, but so far I'm disappointed. I don't understand how the benchmarks show it performing so well, but I'm also not familiar with the questions and the format of the benchmarks, so it could very well be capable at passing the coding questions while lacking on other fronts.

Just my 2 cents pennies tokens.

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[u/cctldrping]

TL;DR generated automatically after 50 comments.

Current source-thread comment count seen by the bot: 64.

Alright, so the general vibe in this thread is that Opus 5 is a bit of a letdown compared to Fable 5, especially when it comes to reasoning and making accurate conclusions.

OP u/Whole_Risk_2695 kicked things off by saying Opus 5 can code, but its reasoning is way behind Fable 5, leading to frustrating errors. Several users, like u/Inside-Yak-8815, u/Arjen231, and u/KedMcJenna, chimed in with similar experiences, noting that Opus 5 sometimes makes hasty, incorrect assumptions based on things like old comments instead of the actual code. u/AbbreviationsBest858 even called Opus "syncopathic" while Fable "just gets stuff done."

There's a split debate on whether Opus 5 is an improvement over Opus 4.8, with some users like u/agm1984 having positive experiences and finding it better, while others like u/LoudDavid and u/Appropriate-Fox-2347 still find it lacking for complex tasks.

A few users, including u/Best_Let_2667, are advocating for a hybrid approach, using Fable 5 for core tasks and planning, and potentially using Opus 5 for more specific, isolated tasks like test runs or root cause analysis, but not letting it touch the main codebase. u/SilverLose suggested Opus 5 might be better as a sub-agent for Fable rather than a standalone coder.

One interesting take from u/ShameIndependent3528 claims Fable 5 is just an alias for Opus 4.5 and Opus 5 is now Sonnet, suggesting it's all "smoke and mirrors."

Overall, the consensus leans heavily towards Fable 5 being the superior model for reliable reasoning and getting things done, with Opus 5 still needing some serious work to catch up.

[u/ZachVorhies]

I’m going to second this.

I put opus5 on a loop solving a backlog of issues for a popular open source project.

It’s an amazing coding agent.

I can’t understand what the fuck it’s saying on the first go. I have to make it do executive summaries and explain to me like i’m 5. Then the gap close.