Term
Claude Fable 5
Claude Fable 5 is Anthropics first publicly available Mythos-class model, released in June 2026. It is built for the most demanding reasoning and long-horizon agentic work and offers a one-million-token context window.
Claude Fable 5 — explained in detail
Claude Fable 5 was released by Anthropic on 9 June 2026 and is the first publicly accessible model of the so-called Mythos class — the providers most capable model generation. Fable 5 is the variant cleared for general use; released alongside it was Claude Mythos 5, a more restricted version for a select group of cyber defenders.
The model is designed for especially demanding reasoning and long-horizon agentic tasks and, according to the provider, reaches state-of-the-art results on nearly all tested capability benchmarks — including software engineering, knowledge work, vision and scientific research. It handles text, image and file inputs, produces text output, supports explicit reasoning and offers a context window of 1,000,000 tokens. Pricing is 10 US dollars per million input tokens and 50 US dollars per million output tokens.
To guard against misuse, Fable 5 includes safety mechanisms: in high-risk areas such as cybersecurity, biology or chemistry the model blocks responses and falls back to the weaker Claude Opus 4.8. Shortly after launch, Fable 5 was temporarily taken offline by a US export directive and redeployed on 1 July 2026.
Example / practical relevance
Fable 5 is used for the most demanding use cases: long-horizon coding agents that work through entire projects autonomously, complex scientific analyses, or tasks that require consistent reasoning across many steps. Its large context window also allows the processing of extensive document and code collections in a single pass.
Distinction
Fable 5 sits above the Opus line and is the public expression of the Mythos class, while Claude Mythos 5 has only restricted access. Compared with Opus 4.8, Sonnet and Haiku, Fable 5 is more capable but more expensive and more heavily safeguarded. In blocked high-risk domains it deliberately falls back to Opus 4.8.
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