Claude Fable 5 hard-refuses cybersecurity, bio/chem, and 'distillation' prompts - silently routing them to the older Opus 4.8
Anthropic launched Claude Fable 5 on June 9, 2026 - a new 'Mythos-class' flagship 'made safe for general use' at $10/M input and $50/M output. The catch for security and science users: when Fable 5 detects a request in three domains - offensive/exploitation cybersecurity, most biology and chemistry, and prompts that try to 'distill' Claude's capabilities - it does not answer; the request is automatically handled by the older Claude Opus 4.8 instead. Anthropic says >95% of sessions hit no fallback and users are told when it happens. The unrestricted version, Claude Mythos 5, is gated to authorized cybersecurity partners and selected biology researchers only.
Claude (Anthropic) has not publicly addressed this change. 20 days since filing · 10 days remaining in standard response window · we will mark this receipt as uncontested on day 30.
This is not a silent paid-tier removal - Anthropic announced it openly and the fallback model is still capable - so for most users Fable 5 is simply a new model.
This is not a silent paid-tier removal - Anthropic announced it openly and the fallback model is still capable - so for most users Fable 5 is simply a new model. But for the security-research and scientific audience, the new flagship won't do their core work: it refuses and routes them down to a prior-generation model, and the unrestricted Mythos 5 is access-gated. Logged as a MINOR capability restriction, not a price/tier nerf. The $10/M-in, $50/M-out figure is Fable 5's launch pricing; we have not compared it to a prior SKU as a hike.
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