Anthropic Releases Claude Fable 5, Its Most Capable Model Ever, After Months of Safety Holdback
Anthropic's new Mythos-class model claims massive leaps in agentic reasoning, coding, and biology — and was reportedly delayed for months over cybersecurity concerns before being patched and released to the public.
Anthropic on Tuesday released Claude Fable 5, a model it describes as "Mythos-class" and the most capable system it has ever made generally available. As the company's official account announced, the model's capabilities "exceed those of any model we've ever made generally available," a framing that underscores both its power and the deliberate caution Anthropic exercised before shipping it. The release ends what appears to have been a months-long internal hold: according to details surfaced by @BlueJay87476298, Anthropic had previously withheld the full Mythos-class version from the public after internal testing revealed extreme cybersecurity vulnerabilities — specifically, the model's ability to discover thousands of software vulnerabilities. Anthropic says those issues have now been patched "for defenders," enabling the broader release.
The early benchmarks and user reports are staggering, if taken at face value. @MaaSonder claimed that Fable 5 completed two months of Stripe engineering work in 24 hours, calling it "the most capable AI ever released to the public." @Phoenixyin13 described a "cognitive paradigm shift," claiming the model is 6x better than Opus 4.7 on complex tasks and pointing to breakthroughs in what they call "Loop Engineering" and a "Memory Substrate" architecture. Whether these numbers come from controlled benchmarks or anecdotal testing is unclear, but the directional signal is consistent across multiple early adopters: Fable 5 represents a significant step function, not an incremental update.
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