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Anthropic Launches Claude Fable 5.1 and Mythos 5.1: Same Model, Different Safety Guardrails

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Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026, describing them as identical models that differ only in their safety guardrails. Both versions share the same weights, pricing ($10/$50), 1M token context window, and June 2026 knowledge cutoff. Fable 5.1 is available to all customers, while Mythos 5.1 is restricted to approved organizations under the Project Glasswing initiative, accessible via Cybersecurity or Life Sciences verification programs. The key distinction lies in safety classifiers: Mythos 5.1 allows approved users to explore exploitation paths for defensive research, whereas Fable 5.1 can identify vulnerabilities but not develop exploits. A published benchmark gap — 60.9% versus 55.8% on Terminal-Bench 4.0 — suggests a measurable performance cost associated with the stricter guardrails in Fable 5.1.

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