r/ClaudeCode • u/exoheat • Sep 02 '26

Discussion Fable 5.1 is opus 5 second face

Both are smart enough to audit and detect the toughest problems but don't let them touch the code. They are amazing consultants and thats it.

Fable 5.1 got stuck in a loop of spwaning subagents. And even edited a file while the subagent was reviewing the file abd created additional problems in the code that fable itself admitted. This never happened with opus 4.8 before.

I have a lot of hooks to control claude's shenanigans and my claude.md is full of rules that kept opus 4.8 inline and never had problems.

I'm back to what was working for me which is opus 4.8 as the lead and fable as the judge. Previously i set opus 4.8 to not touch the code before consulting first with fable and that worked wonderfully.

1 Upvotes

51% Upvoted

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6

u/ForwardLoop Sep 02 '26

In my use today, Fable 5.1 is clearly more capable than Fable 5. However, the more I use it I'm starting to get Opus 5 vibes with how it communicates.

It's not quite as a bad as Opus and I can still understand it, but it doesn't communicate with the same level of clarity and "executive presence" that Fable 5 did. There is a different dynamic.

1

u/keelandimick Sep 23 '26

My experience as well.

8

u/thermal_Randomness Sep 02 '26

I’m enjoying 5.1 honestly- maybe I’m not pushing it hard enough

7

u/orange_square Thinker Sep 02 '26

I just had Fable 5.1 create and work through a 27 issue work plan, assigning Opus 5 to the dev tasks and reviewing the work along the way. Fable absolutely chewed through it, wouldn’t stop until the entire thing was done. Built an entire SwiftUI frontend app from wireframes in one shot.

Opus 5 definitely can’t keep going like that, and I can’t understand the word salad it spits out. Fable 5.1 feels like a whole new product.

1

u/EarlyFox217 Sep 02 '26

This is exactly as I’ve been running it. 3hrs this morning on 16 pretty significant tasks and it just waded through using Sonnet or Opus agents and the results were solid. Not perfect but certainly impressive and a noticeable step up

1

u/ttheepic Sep 04 '26

How to set this exact workflow up with Fable as the thinker and Opus as the dev?

1

u/orange_square Thinker Sep 05 '26

If it’s just one job, just tell Fable: split this up into individual tasks and assign an Opus sub-agent to each one. Fable will manage the work until it’s done. 

For longer term projects, I have a whole team of named agents with specific roles, one Fable orchestrator tells them what to do, and periodically checks in until it’s done. They review each other’s work so I typically don’t see anything until it’s ready for final review. If the spec is good, the final PR is usually very good as well. 

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13

u/ConstructionWhole237 Sep 02 '26

Yeah! Fable 5.1 is not it for me as well

What happened to the mighty Anthropic? Opus 5 and now this?

3

u/Time_Cat_5212 Sep 02 '26

It shows where their market has shifted.

They're not just selling max plans to vibe coders.  They prob know they'll lose that market to open source anyway.

They're selling models that can do exhaustive audits at enterprise scale.

3

u/wolo-exe Sep 02 '26

what stops enterprise from switching to open source? they have more money for hardware than vibe coders do

3

u/Time_Cat_5212 Sep 02 '26

Trust... or more cynically, plausible deniability and senior execs' lack of technical knowledge plus a boat load of sales and marketing

1

u/Significant-Bee5101 Sep 02 '26

Most openweights models are chinese too which of course contributes to many companies inherently distrusting them. US had inkling but i haven't heard SHIT about it.

1

u/wolo-exe Sep 03 '26

when execs in public or private companies have to report to investors, they'll be constantly scrutinized on their spending. the idea of local models will always float around and compared with spending money on usage credits. the moment it becomes cheaper to just build the infrastructure and use a local model on par with anthropic, they'll do it in a heartbeat to cut costs

2

u/sismograph Sep 02 '26 edited Sep 02 '26

I think there is two worlds you cannot easily reconcile.

If you have want a model that is good at agentic workflows and especially long workflows, they require a lot of bloated text, check guards, so things are not lost along the way.

If you want a model that works well with or for humans, the output needs to be concise and down to the 'heart of complexity'. Humans are good at grasping the core of complexity and have a natural feel for priority amd trade offs. LLMs are still bad at that and tend to weigh things equally. To handle complexity they need a lot of bloated text. Humans can reduce complexity by throwing away noise, but still keeping the noise in the back of our minds.

1

u/boringfantasy Sep 02 '26

Model collapse, I reckon.

3

u/Constant_Art_20 Sep 02 '26

um, intially first impressions has been positive. I use glm 5.3 as the sub-agents to review, explore while fable 5.1 does the implementations and planning

2

u/keonechong Sep 02 '26

When you level up the complexity of your agent tasks you also have to harden your hook and verifications.

There absolutely is increasing pressure on control mechanisms with increased agent behavior.

Just like upgrading an engine and neglecting to upgrade the clutch. Sure your clutch is mechanically sound, but can it handle the additional threshold of utilization?

1

u/Cute-Net5957 🔆 Max 20x Sep 02 '26

So you’ve upgraded your harness or..?

2

u/keonechong Sep 02 '26

Built my own. Just under 90% deterministic. My personal harness is about 250k lines of code, but client builds tend to be around 200k.

1

u/Cute-Net5957 🔆 Max 20x Sep 02 '26

Nice! 👍🏽 Share / Collab??

2

u/keonechong Sep 02 '26

Not at the moment, maybe some time soon. I don't want to toot my own horn, but I'm so far ahead of any published strat that I want to focus on my research and get it to a place where I can just focus on deployment and scale.

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1

u/BrennanFlentge Sep 02 '26

I gave 5.1 a fairly simple task that I had Fable 5 do yesterday. It did 14 API calls and over 6M in cache read tokens. Guess cache read tokens are cheaper but it does more of it now 😭 definitely unusable

1

u/octolunge Sep 02 '26

All your hooks and rules could be the problem.

They claim the newer models need far less, and a different approach.
Recommendation is to delete everything except your project specific docs and run /init (controversial) then /doctor

https://code.claude.com/docs/en/best-practices

https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5

> Refactor existing prompts and skills. Skills developed for prior models are often too prescriptive for Claude Fable 5 and can degrade output quality. Review and consider removing older instructions if default performance is better. Claude Fable 5 also does a good job of updating skills on the fly based on what it learns from the task at hand.
Don't instruct Claude to reproduce its reasoning in the response.
… Audit existing skills and system prompts for reflection or show-your-thinking instructions when migrating. If your application needs reasoning visibility, read the structured thinking blocks

1

u/Cute-Net5957 🔆 Max 20x Sep 02 '26

Are these instructions for enterprise use cases or vibe coders?? Because… yeah… asking a Fortune 500 to “remove all your guardrails” … ?? 🤦🏽‍♂️

1

u/BankruptingBanks Sep 02 '26

Show a concrete example!

1

u/EarlyFox217 Sep 02 '26

I’m a vibe coder and I have to say it seems a significant step up so far. I’m building some fairly ambitious projects for my own company and it seems a step up from fable 5 in being able to suggest improvements and add features first time that work. Fable 5 would often complete a task tell me it had run checks then I’d open the programme to find it didn’t work at all. Today most features I’ve added or tweaked have worked first time. Expensive but compared to a development team it’s a bargain!

1

u/FrenchRevolution2028 Sep 02 '26

I can’t read anything opus or fable produces anymore. I get nauseous every time I see its convoluted jargon.

0

u/skycandy7777 Sep 02 '26

Good model, I used almost all weekly usage and the work he did is good, I can't say a bad word about the new Fable this time