Claude Fable 5.1 - cheaper caching, calmer safeguards, same old Claudish

Canonical version: Claude Fable 5.1 - cheaper caching, calmer safeguards, same old Claudish.

Three months after the launch (and the ban, and the un-ban) of Claude Fable 5, Anthropic just shipped Claude Fable 5.1 and Mythos 5.1.

Fable and Mythos are one model with two safeguard levels. Fable is for everyone, Mythos is for vetted security and life sciences organizations, US only for now. If you want the full breakdown of that structure, I wrote about it in Claude 5.

In this piece, I want to share what actually changed for those of us using these models every day, and what the community discussed instead.

What changed

Terminal-Bench-Science went from 24.7% to 52.6%. AutomationBench from 17.1% to 31.4%. And on Terminal-Bench 4.0, Fable 5 was actually behind Claude Opus 5 (42.0% vs 52.3%). 5.1 is now ahead at 55.8%. So Fable 5 cost twice as much as Opus 5 and scored lower on that benchmark; 5.1 reverses that.

Anthropic's examples come from science: protein binders with 10x higher affinity than competition entries, a Venus elevation map at 2-3km resolution instead of 10-20km, deep learning models sped up by 2.5x.

The changes that affect daily use:

• Cache reads dropped to $0.25 per million tokens, down 75%. Input and output prices did not move ($10/$50). Anthropic estimates 25% savings on typical workloads and up to 45% on agentic ones. Agentic loops re-read the same context over and over, so cache reads are a large share of their cost
• Safeguards produce 60% fewer false positives in the cybersecurity domain. With Fable 5, Anthropic itself admitted its biology safeguards made the model "practically unusable"
• Anthropic claims 5.1 finds root causes before patching rather than applying shortcuts. Millennium says it found a rare crash "nobody on our team had explained in four to five years". MongoDB built a "complex prototype in about three days", mostly unattended
• Anti-distillation measures are now visible: new accounts cannot edit prior context. This replaces the 5.0 behavior, where the model silently degraded answers it suspected were distillation attempts

Every described it as "Fable-level intelligence, Opus-level price, Sonnet-speed". I would not go that far on the price, but the cache price cut does lower the cost of agentic workloads.

What the community discussed

The Hacker News thread barely mentioned benchmarks.

An Anthropic engineer opened by praising the "much more natural style" of 5.1. The thread answered with pages of complaints about what people now call "Claudish": dense, jargon-inventing, hedge-stacking prose that sounds confident while saying very little. One commenter wrote: "It's both dense and vacuous. Dense because it's full of jargon it's made up, and vacuous because even with all that it's not actually saying much."

The recurring complaints:

• You can tell it to write plainly in a system prompt or a CLAUDE.md, and it forgets within a few turns. "Training supersedes random markdown files"
• Code comments that reference files that don't exist and dates that were never real. The model writes notes for itself instead of for humans
• Workarounds commenters use: pipe the output through Haiku, use a competitor to clean it up, demand "Simplified Technical English", ban superlatives explicitly

I've been there too. I maintain a whole humanizer skill in my vault for exactly this reason, and I still catch new tics every month. The models get better with each release, but the writing habits stay, and prompt instructions don't remove them.

My take

First, point releases in this generation are where the usability fixes land: in 5.1, cheaper caching, fewer safeguard false positives and longer autonomous runs. The big version numbers are for capability. If you skipped Fable 5 because of cost or because it refused half your security questions, 5.1 is the one to re-evaluate.

Second, the Hacker News thread is a reminder that we are still responsible for what we ship, text included. Text written by the model still needs an editing pass before you sign it with your own name. As I argued in Mastering concepts matters more than ever, the final review is our job.

Prompt instructions alone won't fix the style, so plan for that editing pass.

That's it for today! ✨

References

• Anthropic announcement: https://www.anthropic.com/claude-fable-and-mythos-5-1
• Hacker News discussion: https://news.ycombinator.com/item?id=49525378

Related

• Claude Fable 5.1
• Claude Fable 5
• Claude 5
• Claude Opus 5
• Anthropic
• Claude Code Prompt Caching
• Quality non-fiction is the antithesis of AI slop
• Fable 5 ban

About Sébastien

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