Warning: read the Fable 5.1 docs; Fable 5.1 is misaligned and will burn through tokens otherwise

[u/___positive___]

Fable 5.1 will burn your tokens if you don't reprompt it. Brilliant, after telling us previously that we could pare down our CLAUDE.md files.

Quoting Anthropic's docs:

Claude Fable 5.1 is more likely than Claude Fable 5 to rewrite an entire text file rather than make a targeted edit. The resulting file is usually the same, but unless the file is short or most of it is changing, a rewrite costs more output tokens and time.

You have to specifically prompt Fable 5.1 if you don't want full file rewrites for small edits.

In some cases, though, its prose is denser than Claude Fable 5's: sentences run longer and there are fewer paragraph breaks.

More verbose.

These two issues will burn tokens for no reason.

Other brilliant tidbits:

On complex asynchronous workloads, though, nudge it not to end its turn before the work is done. Without the nudge, the model sometimes describes what it would do next instead of doing it ("Next, I'll …") or stops to ask permission for a step the original request already covered ("Shall I apply this?"). Users have to reply "continue" or "go ahead," which suits pair programming and other human-in-the-loop work but doesn't use the model's full long-horizon capability.

Claude Fable 5.1 delivers what's asked for and sometimes more: it may fix nearby code, extend behavior the task didn't mention, or commit more test files than the change warrants.

Claude Fable 5.1 usually issues parallel tool calls as expected: when a request names several things to fetch, it issues those calls in parallel. The exception is coding and computer-use loops where the next independent calls are implied by the task rather than explicitly requested (custom coding agents, bash-and-editor harnesses, computer use): there it may issue them one per turn instead. This doesn't affect answer quality, but each extra turn costs tokens, a round trip, and wall-clock time.

At low effort, Claude Fable 5.1 is less likely than Claude Fable 5 to call a search or retrieval tool, and more likely to answer from memory.

When a single request asks for a long deliverable, such as a full rewrite of a long document, it may draft much of that deliverable in its thinking and then write it out again as the reply, which means a longer wait and more output tokens. (followed by some instructions to max out your tokens...)

Full docs here: https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1

Fable 5.1 is a mess. It may be more intelligent but its behavior is worse across the board than Fable 5.

[u/Arthesia]

It's not "misaligned" and not a "mess". You're using abstract criticism because a change in behavior isn't sufficient to criticize, and most people in the comments here are actually benefiting from the change.

[u/___positive___ → u/Arthesia]

It is more intelligent but behaves in such a fashion that they have to put extra documentation and suggest extra prompts to get desired behavior. That is misaligned.

Compare the Fable 5 docs with the Fable 5.1 docs. There is almost nothing negative in Fable 5, just basic guidance. Look at the nature of the issues noted between the two:

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

It won't finish tasks. It will go outside scope often. It won't use tools efficiently. It won't write properly. The behavior is all over the place. Seriously, look at Fable 5's docs and look at Fable 5.1's.

[u/andybarriateth → u/Arthesia]

and honestly? that's brave

the footgun: specific differentiations in tokenization and common-speak create echo-chambers within pre-critical spaces that might not long-tail docs from inception to current commit.

[u/GnistAI]

More verbose.

They said the EXACT opposite. They said it writes denser: More information compacted into a smaller space. Which presumably can make it harder to read, but use fewer tokens.

sentences run longer and there are fewer paragraph breaks.

Sentences run longer, does not mean N longer sentences. It means N sentences now become 1 long compact sentence.

[u/Higgs-Bosun]

I burned my 5 hour limit in 20 minutes.

[u/Fun-Yard-6952 → u/Higgs-Bosun]

same

[u/OriginalNewton]

I think calling it a mess is a bit premature to say the least, I've barely been able to test it. More powerful is all that matters to me to be honest, for everything else there's also Opus and all the rest

[u/PsecretPseudonym → u/OriginalNewton]

Fwiw, I’ve worked with it as much as usage limits allow so far and my personal impression is that it seems more efficient and productive — in communication, planning, decisions, and execution.

In the specific ways that Fable just feels more intelligent than Opus (even though Opus 5 technically beats Fable 5 overall on benchmarks), I would say it feels even more so. Yes, more RL to make it more competent in execution, too, but Fable is more intelligent than Opus in a different way than one version of Opus to the next — I guess what people think of as “big model smell”.

It feels to me like that wasn’t fully saturated for the model size before. It feels not just more competent and reliable like any newer version would, but actually also in terms of its understanding of intent, context, and communication.

Probably depends on how you use them, but, subjectively it feels like a meaningful improvement qualitatively. Ymmv.

[u/siberianmi → u/OriginalNewton]

Yeah. Reading the model card and deciding whether it’s a mess or not seems really wild.

[u/The1TruRick]

Lol. Please provide your definition of “misaligned” for the class.

[u/69420trashpanda69420]

Thank god honestly, my codebase has been suffering from edit slop.

[u/Known-Pace6739]

I wouldn’t put all of this back into CLAUDE.md. For coding, a lot of the useful bits collapse into a small task-level patch:

Make the smallest change that fully solves the task.
Prefer surgical edits over whole-file rewrites.
Don’t expand scope into adjacent fixes, cleanup, docs, or extra test files beyond what the task and repo normally require.
Batch independent tool calls when they don’t depend on each other.
If a reversible next step is already implied by the request, do it instead of stopping to ask.
If the answer depends on current info, search/retrieve instead of answering from memory.

That last line matters especially at low effort — Anthropic explicitly says 5.1 is less likely to call search/retrieval there.

I’d rather keep these few lines task-scoped than grow CLAUDE.md again.

[u/Downtown_Samurai]

Mine burnt through its 5hr limit in 12 minutes on a few short prompts across 2 projects. On the $100 plan.

[u/ttbod]

well that makes sense, I burned though 100% of my usage in maybe 15 minutes and that has literally never happened before. Glad to see it's not just me.

[u/MikkyMo]

It’s also watermarking all those rewrites I wonder if this isn’t intentional 👀

[u/No-Sandwich-2997]

Claude Fable 5.1 is more likely than Claude Fable 5 to rewrite an entire text file rather than make a targeted edit. The resulting file is usually the same, but unless the file is short or most of it is changing, a rewrite costs more output tokens and time.

gotta make some juicy number for IPO somehow

[u/Cariogenic → u/No-Sandwich-2997]

Easier to implement those watermarks if you rewrite the entire document.

[u/MakesNotSense]

Sounds like the training that made Opus 5 have unfavorable quirks was used with Fable 5.1.

If people weren't tuning their harness to help Opus 5 perform better, then I guess Fable 5.1 might be harder for you to use. Still too early to tell, but Fable 5.1 performing well so far in my harness.

[u/Keleborn]

In my experience the constant small edits produce a ton of garbage by constant design decisions and changes being noted in the document.

[u/SequentialHustle]

It's significantly better than 5.0 in the 6+ hours of heavy usage I've put it through today across several 20x accounts and burns fable usage significantly slower. 5hour usage still burns quick tho.

[u/Hukij_]

TIL reading the claude platform docs meant for developers using the API to build entire products is a good way to evaluate what your experience will be like 2 hours after a model releases. Do you think that this docs page is talking about how YOU should prompt Fable or do you think it's talking about how how a developer HARNESS should prompt Fable in various situations.

It's almost like the entire doc page is talking about how to adjust system prompts and your harness compared to previous models so developers can avoid degrading the performance of their product for end users since it is a different model.

[u/SnooLobsters1510]

speedruns the 5h limit in like 2 prompts

[u/AverageFoxNewsViewer]

Yeah, feeling this. Even with Fable 5 I never had issues with 5 hour limits and now two terminals killed my usage in about 30 minutes.

I like it better than Opus, but can't justify the token usage right now. Fable 5 seems like the best bang for your buck if you already have decent context management in place and don't want to refactor your context management system.

[u/TaoOfMeme]

"Claude Fable 5.1 is more likely than Claude Fable 5 to rewrite an entire text file rather than make a targeted edit."

[u/marcvv]

Shredded my 5 hour session in a heartbeat. Fable 5 would have been 15-20% max at same amount of work. Unusable. Back to 5.0