Anthropic ships Claude Fable 5 and Mythos 5, its first public Mythos-class model with Opus fallback safeguards

Anthropic positioned Fable 5 as a 'major-version-bump-deserving step change,' claiming SOTA across coding, knowledge work, and vision benchmarks including SWE-Bench Pro, FrontierCode, and GDPval. The headline architectural novelty is a two-model permission split: Fable 5 serves 95%+ of sessions, while queries flagged for cyber, bio, or autonomy risk are routed to Opus 4.8 — the first frontier release that splits permissions by risk class. Mythos 5, the unsafeguarded sibling, is gated to vetted cyber defenders and critical-infrastructure operators, reflecting Anthropic's claim that the model can find thousands of critical OS vulnerabilities. Pricing of $10/$50 per million tokens is roughly 2x Opus, and free access ends June 22 before credit-based billing begins.

The release arrived a week after a Trump executive order asking frontier developers to submit models for voluntary federal review, and CEO Dario Amodei used the moment to call for mandatory third-party testing for cyber, bio, and autonomy risks with the power to block or revoke deployment. Anthropic separately launched Claude Corps, pairing 1,000 early-career professionals with US nonprofits and paying them to deploy Claude on host missions.

Competitively, Fable 5 lands as Anthropic preps an IPO following a $65B Series H at a reported ~$965B valuation, racing OpenAI's own filing. Early developer reaction was polarized: Karpathy and Anthropic's Boris Cherny called it the biggest step up since Opus 4.5, while LlamaIndex's Jerry Liu found it merely on par with Gemini 3 Flash on document parsing at 10-15x the cost. Skeptics on r/MachineLearning allege the Mythos safeguards deliberately handicap AI-research workloads, and many question whether classifiers touching <5% of sessions reflect genuine misuse risk or simply shift liability.