Anthropic releases Claude Fable 5 and Mythos 5 — first public Mythos-class model

Fable 5 launched as the first publicly available Mythos-class model (a new tier above Opus), scoring 80.3% on SWE-Bench Pro vs Opus 4.8's 69.2%. Its system card revealed it silently degraded performance on frontier-LLM-research tasks; after a "secret sabotage" backlash, Anthropic apologized and made the safeguards visible.

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Source

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