Anthropic released two models on June 9, 2026: Claude Fable 5 and Claude Mythos 5. Both are under the so-called “Mythos-class.” This category sits above the Opus category in strength. Myth 5 is a version that is said to be safe for general use. The Mythos 5 is the same model with some enhanced protections, kept in limited release.
Legend of Claude 5 and Mythos 5
Mythos class models are a class of Claude models. They stay above the Opus category knowingly. The first was the Claude Mythos Preview, released in April by Project Glasswing.
Mythos 5 and Mythos 5 share the same basic model. The difference is the defenses. 5 legendary ships with safety separators for general use. Mythos 5 has some spoilers removed and remains in limited release.
The words reflect this separation. The word “myth” comes from the Latin fabula“that’s what it says.” This is similar to the Greek legends. The protections separate the two models, so they have different names.
The Anthropic team calls the Fable 5 its most capable model that has been widely released. It directs hard thinking work and long horizon work. Anthropic states Fable 5’s capabilities exceed any model that has made it generally available.
Both models support a 1M token context window by default. They allow up to 128k output tokens per request. The price is $10 per million input tokens and $50 per million output tokens. That’s less than half the price of the Claude Mythos Preview.
The Case of Power
The Anthropic Legend 5 reports high scores in almost every power benchmark tested. It shows strong results across software engineering, information work, theory, and scientific research. The longer and more difficult the task, the greater its lead over other Anthropic models.
For software developers, Stripe tested Fable 5 during early access. Model performed an extensive codebase migration to the Ruby codebase of 50 million lines. According to Stripe: this took one day. By hand, the team would have needed more than two months.
The Legend 5 also performs better than Claude’s previous models. In Cognition’s FrontierCode test, the Fable 5 scores the highest among frontier models. This holds even in medium effort. eval evaluates complex coding tasks below production-codebase standards.
For information work, Anthropic cites Hebbia’s Finance Benchmark for high-level thinking. The Fable 5 posts the highest score of any model out there. Benefits come from document-based reasoning, chart and table interpretation, and problem solving.
In theory, Anthropic calls Fable 5 a new state-of-the-art. It can extract precise numbers from detailed scientific calculations. It can reconstruct the source code of a web application from screenshots alone. It also requires less scaffolding than previous models. Legend 5 beat Pokémon FireRed with a small harness, which only has vision.
In memory and long-term content, Fable 5 remains rooted in millions of tokens. It improves the output using its notes. In the game Kill Spirepersistent file-based memory is three times faster than Opus 4.8.
Mythos 5 carries scientific claims. In-house protein design experts have sped up parts of the drug’s design nearly ten times. Anthropic also claims that the Mythos 5 is its first model to consistently produce new scientific ideas. Scientists prefer its molecular biology predictions about 80% of the time in unblinded comparisons.
Mythos 5 also ran a new genomics study during a week of highly independent work. It trained a custom model on single-cell data covering 138 species. Anthropic says that model performed better than the latest model it published Sciencedespite being 100 times smaller.
How Defenses Work
Modeling what you know carries risk. Without protections, Fable 5’s cybersecurity capabilities can be misused to cause serious damage. So Anthropic introduced Fable 5 with a new set of splitters.
Classifiers are different AI systems. They recognize potential abuses, including jailbreak attempts. They prevent the main model from responding to flagged requests.
When the Fable 5 parsers flag a request, the response is handled by Claude Opus 4.8 instead. Areas covered are cybersecurity, biology and chemistry, and distillation. Users are notified whenever an error occurs.
For biology and chemistry, Fable 5 goes back to Opus 4.8 in most applications at the moment. Anthropic cites concerns that similar dual-use queries may encourage malicious players. It organizes a trusted access program for biology, giving authorized researchers Fable 5 without those protections.
Anthropic has tuned these defenses carefully. Sometimes they will hold harmless requests. On average, they start in less than 5% of sessions. Anthropic says that more than 95% of fantasy moments don’t involve going back at all. In those moments, the performance of Fable 5 is effectively the same as Mythos 5.
Anthropic red-team has included classifiers widely. The external bug bounty has not produced a worldwide jailbreak in over 1,000 hours. The universal jailbreak allows the user to interact with the model as if its protections were not there. Anthropic notes the UK AISI has made progress towards one in a short testing window.
The Mythos 5 is the same model with enhanced internet protection. Anthropic describes it as the strongest cybersecurity capability of any current model. It is operated through Project Glasswing in collaboration with the US government.
Use Cases
These capabilities map the workflows of several strong technical teams:
- Large scale code migration: Long horizon coding suits big refactors and cross-repo migrations. The Stripe example shows this at a scale of 50 million lines.
- Agent code pipelines: Few turns and efficiency of tokens helps to work more steps. GitHub has reported independence and reliability for complex, long-horizon coding tasks.
- Financial work and analysis: Strong document and chart thinking suited to high-level financial analysis. Hebbia and IMC cited benefits in business-analytical consulting services.
- Vision-to-code functions: The reconstruction source from the screenshots is suitable for the reconstruction of the end and the extraction of the figure. A vision-only harness reduces tool performance.
- Research agents work for a long time: Persistent memory in millions of tokens is suitable for multi-day research loops. Mythos 5 used novel genomics working over a week of highly independent work.
Comparison table: Legend 5 vs. Mythos 5 vs. Opus 4.8
| Attribute | The legend of Claude 5 | Claude Mythos 5 | Claude Opus 4.8 |
|---|---|---|---|
| Model section | Mythos-class | Mythos-class | Opus category |
| Basic model | Same with Mythos 5 | Same with Fable 5 | Opus 4.8 |
| Availability | It is generally available | Limited (Project Glasswing) | It is generally available |
| Safety categories | Functional (cyber, bio/chem, distillation) | Cyber defenses have been removed | Opus-level protections |
| The target for going back | It goes back to Opus 4.8 | It’s not worth it | It’s not worth it |
| API model ID | claude-fable-5 |
claude-mythos-5 |
(existing Opus ID) |
| Content window | 1M default tokens | 1M default tokens | According to Opus details |
| High output | 128k tokens/request | 128k tokens/request | According to Opus details |
| Input price (per 1M) | $10 | $10 | (at Opus prices) |
| Exit price (per 1M) | $50 | $50 | (at Opus prices) |
| Thinking mode | Adaptive only, always on | Adaptive only, always on | It is fixable |
| Data retention | 30 Days (Covered Model) | 30 Days (Covered Model) | General options |
Note: Opus 4.8 specifications and pricing are not specified in the Fable 5 launch sources. The table marks those cells accordingly.
Key Takeaways
- Mythos 5 and Mythos 5 share the same basic model; defenses are the only difference.
- The Anthropic Legend 5 reports high scores in almost every power benchmark tested.
- Legendary 5 splitters go back to Opus 4.8 and launch less than 5% of the time.
- Both models offer a total window of 1M tokens for $10 deposits and $50 withdrawals for a million tokens.
- Mythos 5 remains limited to Project Glasswing; Legend 5 is usually available on all major platforms.
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