On Tuesday, Anthropic launched the latest versions of its flagship Fable and Mythos models. Anthropic promises that the updated models will offer stronger performance at a lower cost, largely because the company is reducing cache read pricing.

Like before, Fable 5.1 and Mythos 5.1 are the same model under the hood, but while Fable 5.1 is now generally available, Mythos 5.1 remains restricted to Anthropic’s trusted access program because it features safeguards that Anthropic says are “specifically designed to support work in cybersecurity and the life sciences.”

Pricing remains unchanged at $10/$50 per million input/output tokens, but cache reads are now only $0.25 per million, down 75%.

Fable 5.1, like Fable 5, remains restricted to API usage and users with Max, Team Premium, and Enterprise Premium plans (at 50% of their weekly usage limits). Pro subscribers can use it via usage credits.

Fable 5.1 benchmark results

In the benchmarks Anthropic shared, Fable 5.1 easily outperforms the older model, Anthropic’s Opus 5, and OpenAI’s GPT-5.6 Sol. Nobody is bothering with comparisons to Gemini 3.1 Pro anymore.

In general, Anthropic says the new model outperforms Fable 5 in virtually all aspects at a lower cost. This depends a bit on the benchmark, but for the most part, it comes down to being able to step down the reasoning mode by one or two notches and still get the same results as Fable 5 at higher settings, all with Fable 5.1 using fewer tokens overall.

In general, Anthropic says the new model outperforms Fable 5 in virtually all aspects at a lower cost.

Many of the improvements aren’t dramatic, except for a major step-up in Fable 5.1’s performance on the Terminal-Bench-Science 0.1 benchmarks, which test the model’s capabilities around using agents for scientific research. Here, Fable 5.1 scored 52.6% versus Fable 5’s 24.7%.

In most other areas, the improvements are within 2-4% of either Fable 5 or Opus 5.

Because of this, Fable 5.1 in Claude Code defaults to the high reasoning mode setting, but medium on Claude Cowork and Claude.ai.

Claude Code safeguards loosen up

One issue with Fable 5 was that it was often too cautious and deferred to the Opus models when it sensed that the user was pushing its safeguards. Now, Anthropic says, it has made these safeguards more precise, “ensuring that they’re less likely to flag benign content (like queries about medical issues or cyber defenders using the model to make their systems safer), but still ensuring they provide robust protection against genuine threats.”

For coders who use Claude Code, the most immediate change is that the model’s safeguards will hopefully get out of the way more often.

For coders who use Claude Code, the most immediate change is that the model’s safeguards will hopefully get out of the way more often.

Anthropic says its cyber safeguards now intervene about 60% less per session than they did with Fable 5, and the model is now allowed to identify vulnerabilities in code.

Penetration testing, exploit generation, and scanning binaries for vulnerabilities remain off the table, though, and Anthropic says it will continue to redirect those requests to its Opus models.

The biology safeguards got a similar update and now intervene 85% less often on benign requests about basic biology and medical questions.

What about those watermarks?

Anthropic recently shared its technique for watermarking text generated by its models (for models released after August 2, 2026.

The watermark is an invisible numerical marker that signals how likely it is that Claude wrote a given piece of text. Anthropic says it doesn’t affect output quality and contains no data about the user, organization, or conversation. Until now, though, nobody outside Anthropic had a way actually to check for it.

With this release, Anthropic is opening a detection API in private preview “to eligible organizations as required under EU law (such as regulators, law enforcement, media, fact-checkers, independent researchers, educational organizations, and EU civil society groups).”

Enterprises that are obligated to verify watermarking to comply with the EU’s AI Act will also get access, with a wider rollout planned for the future.

Data retention

One point of contention for enterprises that wanted to use Fable 5 was Anthropic’s retention policy. While Anthropic offers zero data retention to qualifying customers on its other models, Fable 5 users had to opt into a 30-day retention period.

Now, Anthropic is rolling out what it calls Enterprise Frontier Safeguards (EFS), which it says delivers the privacy of zero data retention while preserving its ability to monitor for misuse.

“EFS works by storing data in cloud infrastructure controlled entirely by the customer, not Anthropic,” the company writes in its announcement. “It will be made available to enterprise customers in phases, beginning later this fall. Until EFS is available, eligible customers will be able to use Fable 5.1 with zero data retention.”

Anthropic says it designed this new system based on customer feedback. Businesses in regulated industries, after all, were essentially unable to use the model under those data retention rules.

“We therefore sat down with customers to design a solution that could provide the best of both worlds: the privacy of ZDR and the safety allowed by monitoring across time and accounts,” Anthropic explains.

In practice, this means EFS moves that data into the customer’s own cloud storage, such as Amazon S3, Azure Blob Storage, or Google Cloud Storage, where it remains encrypted with the user’s encryption keys. It then runs Anthropic’s automated detection against it with no human review on Anthropic’s side, and routes any alerts back to the customer.

As of now, this tooling covers Claude Code, Claude Enterprise, the Claude Platform, and Claude on Amazon Bedrock, Microsoft Foundry, and Google’s Agent Platform. The company says there will be no additional cost beyond the customer’s own cloud storage bill.

EFS will roll out in phases starting this fall. Until then, Anthropic will not retain any data from usage of Fable 5 and Fable 5.1.

Anti-distillation

Fable 5.1 also ships with what Anthropic calls strengthened mechanisms to avoid distillation. The company, of course, has been quite vocal about its suspicions that Chinese labs have been distilling its models at an industrial scale.

The first of them is a change to the API itself: New accounts can no longer “manually edit Claude’s prior context in a multi-turn conversation while preserving the transcript of Claude’s prior thinking.”

This, Anthropic says, closes off “a common, publicly documented distillation technique.” However, it’ll also trip up agent harnesses that rewrite or compact conversation history, and the company says the restriction will extend to existing accounts with future model releases.

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