Coding with Mythos Fable 5 is actually insane.

Claude 4 'Fable 5' (a safeguarded version of Anthropic's Mythos-class model) is reviewed through hands-on coding experience. Key findings: it's the best coding model tested, excelling at goal-driven multi-agent tasks and deploying parallel agents to tackle complex work. However, it's 10x slower and 5x more expensive than Opus 4.8 or GPT 5.5 for equivalent tasks. Major caveats include aggressive safety guardrails that auto-switch to Opus 4.8 for any cybersecurity, biology, or ML research queries — even benign ones. A real-world security audit of a TypeScript TanStack Start URL shortener found one genuine vulnerability (exposed password hashes via dead code), one false positive, and one real bug with an incomplete fix. The model will be removed from subscription plans on June 23rd, moving to API-only usage credits. Enterprise users with zero data retention policies cannot use it due to a new mandatory 30-day data retention requirement.