Anthropic just launched Claude Fable 5, their most powerful model ever and the first of the Mythos class to reach the public. The benchmarks are genuinely impressive. The real-world results are extraordinary. But there are things Anthropic isn't shouting about. Here's the full picture.

TLDR:

• The Benchmarks Are Real 📊

• What It Actually Does 🚀

• The Controversies Nobody Is Talking About ⚠️

• From Tasks to Responsibilities 🔄

• The Bro's Take 🤔

The Benchmarks Are Real 📊

Fable 5 is not an incremental update.

On SWE-bench Pro, Fable scores 80.3% against GPT-5.5's 58.6%. On Frontier Code, which tests whether AI code is actually mergeable into production, it more than doubled the previous best.

Stripe compressed months of engineering into days on a 50 million line codebase. One developer replicated a competitor's mobile app in four prompts.

What It Actually Does 🚀

The real shift isn't benchmarks. It's behaviour.

Fable runs complex multi-step tasks with almost no babysitting. It pushes back on bad assumptions without immediately caving.

It updates its position when given new information but doesn't abandon it entirely. For long-running agentic tasks and complex codebases, it's in a different league.

The Controversies Nobody Is Talking About ⚠️

Here's what the launch post didn't lead with.

• Biology guardrails automatically route chemistry and cybersecurity questions to Opus 4.8 mid-conversation. Legitimate researchers are getting blocked asking about cancer mutations and DNA transcription.

• Buried on page 13 of the system card, Fable is deliberately restricted for AI research tasks to stop competing labs from benefiting. Legitimate open researchers get caught in the same net.

• All Fable sessions are retained for 30 days including human review. Enterprise users with memory turned on may have already pulled sensitive historical conversations into sessions without realising it.

From Tasks to Responsibilities 🔄

The most interesting framing came from inside Anthropic.

Felix Ryberg described it as a third era. First we asked AI questions. Then we gave it tasks. Now we give it responsibilities.

Not fix this crash. Keep our app from crashing. The new skill this requires isn't prompting. It's task imagination. Figuring out what you can hand to an agent that runs for hours or days.

The Bro's Take 🤔

Fable 5 is genuinely the most capable model we've ever had. The coding results alone are hard to argue with.

But the guardrails controversy matters. The biology filters are arguable. The invisible AI research restrictions affecting legitimate work are not. If Anthropic is making those calls, they should say so upfront. Not on page 13.

The model is extraordinary. The communication around it could have been better..

And that's it for today! Thanks for reading ♥️

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