What is Claude Mythos 5.1?

Claude Mythos 5.1 is a variant of Claude Fable 5.1 with modified safety features. It is provided only to Anthropic-certified partners for cybersecurity and life sciences support.

How much cheaper is the API with Fable 5.1?

Cache-read prices are reduced by 75%. Overall API costs drop about 25% for typical tasks and up to about 45% for complex coding.

Large Language ModelsAI Business & IndustryITmedia AI＋Sep 2, 2026Sep 2, 2026, 13:00 JST

Anthropic resets Claude usage limits with Fable 5.1 launch

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3 Key Points

What happened
Anthropic reset the 5-hour and 1-week usage limit windows for its AI service Claude on September 1, in connection with the release of the Claude Fable 5.1 model.
Why it matters
Claude Fable 5.1 shows improved performance on research and coding tasks compared with the previous model Claude Fable 5. It also cuts cache-read prices by 75%, reducing token-based API costs by about 25% for typical tasks and up to about 45% for complex coding.
What to watch
Anthropic also released Claude Mythos 5.1, a safety-tuned variant of Fable 5.1, available only to certified partners for cybersecurity and life sciences support.

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Context & Analysis

The reset of usage limits for Claude follows the release of Claude Fable 5.1, which Anthropic announced on the same day. This model builds on Claude Fable 5 by improving autonomous performance in scientific research and coding. The cost reductions are notable: cache-read prices fall by 75%, and API costs drop by about 25% for typical tasks and up to about 45% for complex coding. These changes could make Claude more accessible and affordable for businesses that rely on token-based APIs.

Anthropic also introduced Claude Mythos 5.1, a model with adjusted safety features, restricted to certified partners in cybersecurity and life sciences. This selective availability suggests a strategy to offer specialized capabilities while maintaining control over high-risk applications. The usage limit reset likely aims to smooth the transition for existing users as they adopt the new model, but the article does not specify whether the reset applies to all users or only those on certain plans.

FAQ

What is Claude Mythos 5.1?: Claude Mythos 5.1 is a variant of Claude Fable 5.1 with modified safety features. It is provided only to Anthropic-certified partners for cybersecurity and life sciences support.
How much cheaper is the API with Fable 5.1?: Cache-read prices are reduced by 75%. Overall API costs drop about 25% for typical tasks and up to about 45% for complex coding.

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