(2026-09-05) Zvim Claude Mythos51 And Fable51 Capabilities

Zvi Mowshowitz: Claude Mythos 5.1 and Fable 5.1: Capabilities. This is the weirdest situation in which to write a capabilities review. Introducing the world’s most powerful model, by a substantial margin. No wait, this just in, we also have someone else introducing the world’s most powerful model.

Claude Fable 5.1 and GPT-6 Astra are both excellent models. This much, we know.

Early signs are, with large error bars, that the jump from Sol to Astra is bigger and more exciting than the jump from Fable 5 to Fable 5.1. This may be similar to how the scaling move from Opus to Fable was a big deal.

Table of Contents

• The Official Pitch.
• Our Price Cheap.
• Zero Data Retention and Reduced Safeguards.
• Official Benchmarks.
• Other People’s Benchmarks.
• The System Prompt.
• The Blurb Pitches.
• The Every Review Is In and It’s Very Good.
• Positive Reactions.
• Our Price Cheap But Only Per Token.
• Negative Reactions.
• Early Whispers.
• Weapon of Choice.

The Official Pitch

The coding department thinks it is very good at coding, also Our Price Cheap.

Our Price Cheap

Anthropic is doing a strange thing with Fable 5.1 pricing. The headline price is unchanged from Fable 5, but the effective price is lower because they are reducing the pricing on cache reads from $1 to $0.25 per million tokens.

Zero Data Retention and Reduced Safeguards

*Fable 5 never got above about 11% of Anthropic dollar spend on Ramp, despite being the clearly best model out there.

Two of the big objections were the safeguards having a large blast radius that stopped ordinary work, and companies, often due to various regulatory concerns, failing to abide Anthropic’s data retention policy, where they required records be kept for 30 days.*

The individual Dude, who could abide, had huge coding edge

Anthropic has heard you, and had some time to get more comfortable, and for the government to calm down versus when it was so nervous it forced Fable offline over a supposed ‘jailbreak.’

Official Benchmarks

Anthropic shares a ton of benchmarks. Mostly we see modest improvement from Fable 5 or Opus 5 to Fable 5.1

Other People’s Benchmarks

Epoch’s ECI has both Claude Fable 5.1 and Fable 5 at 163, whereas Astra jumps ahead to 169 from Sol’s 162. This is the strongest data point for Astra.

The System Prompt

As always, Pliny the Liberator has got you. The changes all seem minor

The Blurb Pitches

I notice the blurbs this time are different. They’re talking about specifics unique to different companies

The Every Review Is In and It’s Very Good

*This is without taking Astra into consideration, but even so, this is very positive.

Dan Shipper (CEO Every): It’s friendly Fable. Fable-level intelligence, Opus-level price, Sonnet-speed. In our tests it was about twice as fast as Opus 5 and used half as many tokens, so for anyone used to using Opus as their daily driver it’s an obvious upgrade.*

Positive Reactions

Many are indeed impressed.

Our Price Cheap But Only Per Token

There were some who found it rather token hungry.

My understanding is that API costs are reduced substantially, but that subscription limits have not changed. So if 5.1 is modestly token hungrier, it could be cheaper in the API but exhaust your subscription faster.

The other explanation is, this is a model that loves doing more. Fable 5.1 will do more, and use more tokens to do it, if allowed to do so. You can also turn down effort levels.

The price is not cheap, you do have to either pay $100+ per month or pay the higher API rates, but the price is so very worth what you get.

If you want to say ‘paying $100 was off the table so only Opus counts’ and are reading this then, again, either you are highly strapped for cash or you are a fool.

Negative Reactions

There were almost no other actively negative reactions that I could see. It’s a good model, sir. This was the most negative non-comparative statement I could find that wasn’t about being token hungry or expensive:

Also, yes, a lot of people can’t tell.

Early Whispers

I will be holding the model welfare post for a bit to allow for further information to be gathered, which might become the pattern. You need more time to make that assessment

Weapon of Choice

It is very early, as most people have had Astra for less than a day, but we are seeing signs that those living near my Twitter are intrigued and impressed by Astra, as usual adjust for my region being unusually Claude-pilled. Almost no one here uses anything except Claude or GPT