Why Everyone Is Freaking Out About Fable 5 (Mythos)

CFClaude Fable 5AnthropicUnverified

First Mythos-class model made safe for general use, priced at $10/$50 per million tokens. Strong on long one-shot coding tasks (Stripe 50M-line Ruby migration, game clones), but slow, token-hungry, and safety classifiers hand off to Opus 4.8. The creator tested it, including a one-shot Mega Bonk clone that worked well.

Same underlying model as Fable 5 but with safety guardrails lifted in certain areas. It remains restricted to Project Glasswing partners, cybersecurity pros and government.

CMClaude MythosAnthropicUnverified

First Mythos-class model, released in April only to a small group of cyber defenders and critical infrastructure providers via Project Glasswing.

COClaude Opus 4.8AnthropicAvailable in EU

The fallback model Fable 5 hands requests to when classifiers flag cyber, bio, chem or distillation topics. It scored 63 on the Every senior engineer benchmark.

SPSWE-bench ProAvailable in EU

Agentic coding benchmark where Fable scores just above 80%. A Data Curve audit found verifier error rates (8% false positives, 24% false negatives) and Opus 4.7 pulling answers from git history.

DEDeepSWEDatacurveAvailable in EU

Contamination-free coding benchmark with tasks written from scratch, requiring more code and output tokens than SWE-bench Pro. GPT-5.5 extra high currently leads, and Fable results aren't in yet.