Claude Fable 5.1 Vs Claude Mythos 5.1: What's The Difference?

Fable and Mythos are the same Claude model with different levels of safeguard. A couple of months after the introduction of Fable 5 and Mythos 5, Anthropic's latest models, the company released their 5.1 versions. Fable 5.1 is available to all Claude Pro and Max users, while Mythos 5.1 is part of a trusted access program designed for scientists and cybersecurity researchers — regular users can't access it through public subscriptions.

Fable 5.1 is full of new capabilities and cheaper to use. For example, it was able to build a new high-resolution terrain map of Venus using 30-year-old NASA radar data, sharpening pictures from 20 km blurs down to 2 km. Mythos 5.1 rewrote GPU kernels for widely-used genomics models in computational biology, speeding up their execution by up to 2.5 times. Anthropic says that all of that was achieved while loosening restrictions so legitimate scientists and security researchers weren't blocked by false alarms, keeping the truly dangerous capabilities of these models inaccessible to anonymous users — if an amateur hacker was able to hack companies using the weaker Claude Opus, chances are they'd be more dangerous with Mythos.

In addition, these models are built on a new Enterprise Frontier Safeguards system, giving business customers privacy from the ground up, as they're the ones controlling the information. Anthropic says it will retain zero data from enterprise customers until this advanced system fully rolls out this fall.

Fable 5.1 isn't bundled the same way for every user. First of all, it's not available in the free plan, and even Pro users (those paying $20/month) don't have the new model included in their monthly usage allowance. Every message sent from Pro draws from a separate pay-as-you-go credits balance.

Max subscribers have Fable 5.1 included in their quota, but only up to half of their weekly allowance ––which is 5 times the Pro limits for $100/month plans and 20 times for $200/month plans. That said, if you use Fable 5.1. a lot, you'll burn through your weekly capacity twice as fast than using a standard model like Opus. Once you hit the ceiling, you can buy more credits or use another model.

What's important to note, however, is that Fable 5.1 isn't cheap to run on pure token pricing: $10 per million input tokens and $50 per million output tokens, which is the same as Fable 5. The price cuts come on cached tokens, which are now 75% cheaper, from $1.00 to $0.25 per million. In real-world experience, such as giving Claude a 5,000-token article draft (around 3,750 words), and getting back a 2,000-token revision, will cost about $0.15. By doing that a few times a day, the API costs can add up pretty fast compared to a flat $20/month Pro subscription.

With LLMs, companies are always playing catch-up. While Claude's latest 5.1 upgrades were leading several categories compared to other companies, the recent release of OpenAI's GPT-6 Astra shows some things have already changed. For example, according to OpenAI, Astra can beat Fable 5.1 on Terminal-Bench 4.0, Terminal Bench Science, FrontierMath Tier 4, and GPQA Diamond, while often using fewer tokens and running faster. OpenAI also claims a major leap in cybersecurity capability and reports big improvements in alignment — meaning the model never tries to circumvent restrictions. Still, Fable 5.1 continues to lead on a few specific evaluations, like Humanity's Last Exam with tools.

Ultimately, choosing an AI depends on your needs. Google Gemini, for example, can offer a few terabytes of Google Drive storage in addition to YouTube Premium Lite, making the AI subscription more interesting for the bundled features. OpenAI offers very generous usage limits, so casual paying customers may never use them as an argument to ditch ChatGPT. On Claude, which is often the preferred platform for developers and writers, users still face several token limitations even on a premium tier.

Anthropic has been dealing with its capacity as Fable 5.1 cuts costs by a considerable amount while still delivering top-tier memory and capabilities of analyzing huge documents all at once. BGR recently reviewed Claude's capabilities, which might give you a better overview whether or not it's worth making the switch to this AI.