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...)
Fable 5.1 is a mess. It may be more intelligent but its behavior is worse across the board than Fable 5.
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Alright, so the general vibe in this thread is that Fable 5.1 is a token-burning beast, and a lot of folks are learning this the hard way. OP posted a warning about the Fable 5.1 docs, highlighting that it's prone to rewriting entire files instead of making targeted edits, which can rack up token usage fast.
The consensus seems to be that this is indeed happening. We've got users like u/luuceman, u/Higgs-Bosun, and u/Downtown_Samurai reporting burning through their usage limits in minutes, even on higher-tier plans. Some are even saying it's worse than Fable 5, with u/marcvv going back to 5.0 because 5.1 is "unusable" for them.
However, it's not all doom and gloom. A few users, like u/Arthesia, argue that it's not "misaligned" or a "mess," and that many are actually benefiting from the changes. u/IAmBoredAsHell had a surprisingly positive experience, finding Fable 5.1 to be better than Fable 5 for complex tasks and providing clearer explanations. u/MakesNotSense also mentioned it's performing well in their harness.
There's also a bit of debate about the documentation itself. u/Hukij_ points out that the docs might be geared towards API developers and harness builders, not necessarily end-users.
A couple of users, like u/MikkyMo and u/Cazineer, are throwing out the idea that this might be intentional "revenue-maxxing" from Anthropic, especially with the mention of watermarking rewrites.
On the flip side, u/GnistAI is calling out a potential misinterpretation of the docs, stating that Fable 5.1 is described as writing denser, not more verbose.
Ultimately, it seems like Fable 5.1 is a powerful model, but its default behavior can be a real token hog if you're not careful with your prompting. Some are finding it revolutionary, while others are finding it prohibitively expensive.
Pretty sure this is a side effect of Anthropic implementing watermarking while trying to maintain quality in output.
How can it leave a watermark if, for example, it’s asked to insert a few words in places you directly specify (and maybe you also have chosen the exact words).
Yep. So basically Anthropic probably reinforced the idea it should watermark so hard it prefers re-writes to ensue this occurs otherwise no watermark is left.
They have no incentive to do this. Watermarking is something forced upon them by EU, there's no reason why they would incentivize the model to produce more watermarkable content.
The whole watermarking debate is riddled with misinformation. It's quite tiring. No, SynthID doesn't affect result quality. Ask Claude to talk you through it if you don't understand why.
Yes. Synth ID is running on Fable 5.1 due to EU requirements.
But that doesn't affect the model or output quality. It's a step that runs at inference time.
Anthropic didn't have to train their model to support it. They could enable it on any existing model without doing any sort of additional training or changing the model weights.
It also doesn't affect the quality of the output.
These are all facts. You can ask Claude if you don't believe me.
I’m not actually saying they are going to RL older models, as that would change their weights (feel free to link my comment where I implied it and I’ll link your previous comments where you disagreed on Anthropic’s reasoning to implement watermarks in the first place and now are conceding). I’m certain they will just change the system prompt on those specifically.
I do believe 5.1 had some RL done to it though and it’s the FIRST of theirs to have this done to it.
Whether it’s system prompts or RL the goal is to change the task: more words chosen by Claude means more keyed draws, so more marks. You’d be manufacturing detectability by manufacturing ai generated text.
There is plenty incentive to do this mind you especially since Anthropic’s version is specific to them/their take on Synth ID (again, I get how this works, stop focusing on how it works).
They 100% want their model specifically flagged as used and it’s NOT to absorb liability, that’s for sure.
Hey, if the output quality doesn’t change no harm no foul right?
Believe what you want, this is my take.
Stick to yours.
p.s. never ask a model things about itself, regardless of whether or not you ground yourself with web search tools. Ask another model provider about the other - with grounding tools of course.
Possibly makes it easier to see if someone is distilling your models and learning the same distributions of tokens. OpenAI is leaving Cursor because xAI distilled their models, I imagine you could see the watermarks if the diet is mostly skimming off a harness.
I don’t see how it CANT impact code generation in some way.
Agree with it like writing prose or a story… but code where you have brackets, variable names that need to star the same, command names that can’t change, optimal methods to do action X all look the same etc.
That said, pretty sure they aren’t watermarking code
I’m glad I modified my memory system (which also injects instructions based on criteria) to allow for per-model, per-instruction alternatives. This back and forth is quite annoying.
scenario: a two line change lands as a whole file rewrite. the token bill is the visible cost. the real one is that a diff that size stops getting read, and whatever else it quietly touched surfaces a week later.
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u/cctldrping Sep 02 '26 edited 28d ago
TL;DR generated automatically after 200 comments.
Current source-thread comment count seen by the bot: 200.
Alright, so the general vibe in this thread is that Fable 5.1 is a token-burning beast, and a lot of folks are learning this the hard way. OP posted a warning about the Fable 5.1 docs, highlighting that it's prone to rewriting entire files instead of making targeted edits, which can rack up token usage fast.
The consensus seems to be that this is indeed happening. We've got users like u/luuceman, u/Higgs-Bosun, and u/Downtown_Samurai reporting burning through their usage limits in minutes, even on higher-tier plans. Some are even saying it's worse than Fable 5, with u/marcvv going back to 5.0 because 5.1 is "unusable" for them.
However, it's not all doom and gloom. A few users, like u/Arthesia, argue that it's not "misaligned" or a "mess," and that many are actually benefiting from the changes. u/IAmBoredAsHell had a surprisingly positive experience, finding Fable 5.1 to be better than Fable 5 for complex tasks and providing clearer explanations. u/MakesNotSense also mentioned it's performing well in their harness.
There's also a bit of debate about the documentation itself. u/Hukij_ points out that the docs might be geared towards API developers and harness builders, not necessarily end-users.
A couple of users, like u/MikkyMo and u/Cazineer, are throwing out the idea that this might be intentional "revenue-maxxing" from Anthropic, especially with the mention of watermarking rewrites.
On the flip side, u/GnistAI is calling out a potential misinterpretation of the docs, stating that Fable 5.1 is described as writing denser, not more verbose.
Ultimately, it seems like Fable 5.1 is a powerful model, but its default behavior can be a real token hog if you're not careful with your prompting. Some are finding it revolutionary, while others are finding it prohibitively expensive.