Claude Fable 5 is Anthropic's first publicly available Mythos-class model, released June 9, 2026 (source: Anthropic announcement). It beats Opus 4.8 across the board—coding, vision, analysis, long documents. Priced at $10 per million input tokens and $50 per million output tokens.
It also comes with safety classifiers that intercept requests related to cybersecurity, biology/chemistry, and model distillation (less than 5% of sessions on average). Flagged requests are automatically downgraded to Opus 4.8.
Should you switch? If you want the strongest general-purpose AI available and can live with roughly 5% of topics being restricted, go for Fable 5. If your work depends on cybersecurity research or biology/chemistry, stick with Opus 4.8 or apply for trusted access to Mythos 5.
Model Tier
Mythos is a new model tier above Opus. The first one, Claude Mythos Preview, was released in April 2026 through Project Glasswing exclusively for cybersecurity defenders. Fable 5 is the first publicly accessible Mythos-class model—same base model as Mythos 5, but with safety classifiers added.
Mythos 5 has partial safety restrictions lifted (Glasswing partners bypass the cybersecurity classifier, biology researchers bypass the biology/chemistry classifier), but is currently limited to Project Glasswing partners and select biology researchers.
Published Evaluations and Cases
The following section summarizes public customer cases, partner benchmarks, and attributed third-party reports. It is not AIGCDev hands-on testing, and each result applies only to the task and conditions described by its source.
Software Engineering
Stripe used it to migrate a 50-million-line Ruby codebase in a single day—work that would have taken a team over two months by hand. On Cognition's FrontierCode benchmark, Fable 5 scored highest among frontier models at medium effort.
Vision
It extracts precise numbers from scientific figures and rebuilds web app source code from screenshots alone. Anthropic demonstrated it beating Pokémon FireRed using only raw game screenshots—no maps, navigation aids, or extra game-state info.
Knowledge Work
Fable 5 scored highest on Hebbia's finance benchmark, with substantial leads in document reasoning, chart interpretation, and problem solving. IMC reported near-perfect scores across their trading analysis evaluations—factual lookup, conceptual reasoning, root-cause analysis, and expected-value analysis, all close to perfect.
Memory and Long Context
It stays focused across millions of tokens in long-running tasks and improves outputs using persistent file-based notes. In Slay the Spire, Fable 5 with persistent memory improved three times more than Opus 4.8 and reached the final act three times more often.
Autonomous Task Execution
University of Pennsylvania professor Ethan Mollick tested it independently and said Fable 5 "consistently outperformed basically every other public model I have used by a considerable margin," working autonomously on multi-page specs for hours. He generated multiple playable games from a single prompt in Claude Code—Snake, Strata, and a Rilke-based Duino.
Safety Restrictions
Fable 5 is the first publicly available Mythos-class model with content classifiers. When the system detects any of the following three request types, it automatically downgrades the response to Opus 4.8:
- Cybersecurity: vulnerability discovery, exploit development, defense evasion, automated hacking. Anthropic's internal evaluations show the cybersecurity classifier prevents the model from making any progress on these tasks. External red teams also found no universal jailbreaks on long-form agentic tasks.
- Biology and chemistry: including dual-use capabilities like viral vector design for gene therapy. Anthropic found that Mythos-class models outperformed dedicated protein language models on this task.
- Model distillation: large-scale extraction of model capabilities to train competing models.
Over 95% of sessions don't trigger any classifier. When triggered, users are notified and the response comes from Opus 4.8 instead—much better than an outright refusal. Anthropic ran over 1,000 hours of external bug bounties with no universal jailbreak found, though the UK AISI made some progress within a short testing window.
Fable 5 also adds a 30-day data retention policy to defend against novel attacks and reduce classifier false positives. Anthropic promises not to use this data for model training or non-safety purposes.
Pricing
API pricing is $10 per million input tokens and $50 per million output tokens—roughly half the price of Mythos Preview.
Subscription timeline:
| Period | Access |
|---|---|
| June 9 – June 22 | Free for Pro/Max/Team/Enterprise users |
| Starting June 23 | Removed from subscription plans, requires usage credits |
| Later (when capacity allows) | Goal to restore as standard subscription feature |
Anthropic will extend the free window if capacity allows, and will communicate changes in advance.
Developers can access it via the Claude API using the model name claude-fable-5. Demand is expected to be high, so rollouts are staged.
FAQ
What's the difference between Fable 5 and Mythos 5?
Same underlying model. Fable 5 has safety classifiers (cybersecurity, biology/chemistry, distillation). Mythos 5 lifts the corresponding restrictions (Glasswing partners bypass cybersecurity; biology researchers bypass biology/chemistry). Mythos 5 is currently limited to Project Glasswing partners and trusted biology researchers.
What if the classifier blocks legitimate requests?
The classifiers are currently tuned to be conservative and may block harmless requests. Anthropic says it will keep reducing false positives after launch.
Can it handle coding and game development?
Yes. Stripe used it for a large-scale code migration. Ethan Mollick generated multiple playable games from a single prompt in Claude Code. Fable 5 also tops CursorBench, FrontierCode, and other coding benchmarks.
How does it compare to GPT-5.5?
Anthropic partner Matthew Pines (CEO) independently tested it and said Fable 5 "got nearly to where GPT-5.5 landed after four days in 36 hours using a third of the reasoning tokens." This is a third-party user report, not an official comparison.
What if my work gets blocked by the classifiers?
Switch to Opus 4.8—it's still highly capable and not affected by the classifiers. Cybersecurity professionals can apply for Project Glasswing to get Mythos 5 access. Biology and chemistry researchers can apply for the trusted biology access program in the future.
How is Fable 5 data handled?
All Fable 5 and Mythos 5 traffic is retained for 30 days, used only for security and defense purposes. Data won't be used to train new models. All human access is logged, and data is deleted after 30 days.