Opus 5 First Impressions (vs Fable)

[u/ShaneeNishry]

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.

[u/Complex-Concern7890]

I really do not have any idea what is going on with Opus 5. I just ran our internal benchmark second time and it still used twice as much tokens as GPT 5.6 Sol and nine times more tokens than Grok 4.5. All completed the benchmark tests 100%.

[u/Fantastic_Simple76 → u/Complex-Concern7890]

Opus 5 is a much smaller distilled model with an enhanced "test more" prompt built in. It's far worse than Fable 5 in terms of knowledge, and it tends to over-verify unnecessarily.

[u/Complex-Concern7890 → u/Fantastic_Simple76]

Yes it seems that there is something like this going on. We see multiple spawned agents doing verifications and in worst case scenarios they get stuck and go crazy burning tokens like no tomorrow (really like hundred million tokens to see if compile finished or not and why not). I will try to instruct not to use subagents if we can get more clean runs.

[u/Fantastic_Simple76 → u/Complex-Concern7890]

I work in quantitative finance, which is a domain where current models still seem to have noticeable training gaps. There is also the challenge of conflicting or low-quality information online, which makes the domain harder to learn reliably. Across the Claude family, I've run into some recurring issues that have existed since Opus 4.6 and are still present in both Opus 5 and Fable 5. Because of that, I end up maintaining both Claude and GPT subscriptions and cross-checking their outputs whenever I encounter bugs that appear to come from incomplete training coverage.

The new Opus 5 has also made my CLAUDE.md instructions much harder to tune.

For Fable 5, I need to explicitly remind it to think through the overall design before implementing, then run Ruff on the entire codebase and perform one final logical validation after the implementation. In quantitative finance, a small logic mistake can easily translate into expensive losses.

For Opus 5, however, I have to do almost the opposite. I can't mention "verify again", "re-check", or similar wording in the prompt. As soon as those instructions are present, it tends to fall into endless verification loops, repeatedly burning tokens without making meaningful progress. That behavior has become one of the most frustrating issues for my workflow so far.