Anthropic releases Fable 5, a safer version of Mythos, with a big jump over Opus 4.8

New Claude model - Fable

Fable 5 is the safer version of Anthropic's unreleased Mythos model, which is restricted to select companies due to cybersecurity risks. It scores much higher than Opus 4.8 on benchmarks, though the gap vs GPT-5.5 is smaller. Its standout feature is the ability to work longer and reliably spawn dozens of subagents without losing context. Fable medium already beats Opus xhigh while being cheaper. It's available in Claude subscriptions only until June 22, then moves to paid credits at 2x the cost of Opus. Anthropic also introduced a policy where Fable would secretly sabotage ML/AI-related work, sparking backlash and a partial walkback of the 'secretly' part. Ben finds Fable less chatty than Opus—a sweet spot between GPT's directness and old Claude's verbosity—but notes it's slow.

Why it matters: Fable 5, a derivative of Anthropic's undisclosed Mythos model, leaked with a significant benchmark jump over Opus 4.8 and the ability to reliably spawn dozens of subagents without losing context. This is a substantive new capability signal from Anthropic with cross-source buzz...