Claude Fable 5: Anthropic Releases Its Most Powerful Model to the Public, With a Safety Leash

Anthropic just handed the public a version of the model that spooked governments worldwide. Claude Fable 5, launched today, brings Mythos-class power to every paying user, but with a firm safety leash that blocks the most dangerous capabilities.

Claude Fable 5: Mythos without the fangs

Back in April, Anthropic unveiled Mythos Preview to a tiny group of cyber defenders through Project Glasswing, a collaboration with AWS, Microsoft, Apple, and CrowdStrike. The model could autonomously discover and chain zero-day exploits across major operating systems.

Fable 5 shares the same architecture but adds classifiers that block high-risk queries in cybersecurity, biology, chemistry, and model distillation. When triggered, responses fall back to Claude Opus 4.8. Anthropic says this happens in fewer than 5% of sessions.

Benchmarks that leave Opus 4.8 behind

On some evaluations, Fable 5 scored over 10% higher than Opus 4.8. Stripe reported that a codebase-wide migration on 50 million lines of Ruby, normally a two-month job, was completed in a single day.

Analytics firm Hex said Fable 5 is the first model to hit 90% on its core benchmark for complex, long-running analytical tasks. The pattern is clear: the harder and longer the problem, the wider Fable 5's lead.

Pricing and the 30-day data retention catch

Fable 5 costs $10 per million input tokens and $50 per million output tokens. That's double Opus 4.8, but less than half the price of the original Mythos Preview. Rerouted queries to Opus 4.8 are not billed at Fable rates.

Here's the trade-off worth watching: Anthropic now mandates 30-day data retention on all Fable traffic, even for enterprises with previous zero-retention agreements. The company says the data won't be used for training, only jailbreak detection. Whether competitors follow suit could set a new industry norm.

An IPO-ready power move

The timing is no coincidence. Anthropic confidentially filed its IPO prospectus last week, riding a $47 billion revenue run rate and a $965 billion valuation that now tops OpenAI. Fable 5 isn't just a model release. It's a statement to investors, developers, and rivals alike: Anthropic can build the most powerful AI in the world and still ship it responsibly.