Claude Fable 5.1
Claude Fable 5.1 is designed for difficult reasoning and work that spans many steps. Its migration rules are particularly important for applications that store and replay an agent’s conversation.
Thinking history is not interchangeable
Only Fable 5.1 and Mythos 5.1 can read Fable 5.1 thinking blocks. Editing an earlier turn invalidates those blocks. Forced tool use also returns an error. These are integration changes, not prompt-writing preferences.Claude Fable 5.1 reference
Mythos is a separate access decision
Anthropic describes Mythos 5.1 as having the same capabilities, specifications and pricing, but limits it to verified organizations. Access to Fable does not establish access to Mythos.Claude Fable 5.1 reference
Preserve the evidence behind a long task
Practical guidance. For a research assignment, keep source notes and a clear completion condition outside the model’s narrative. Ask the final answer to separate confirmed findings from unresolved issues. This helps a reviewer judge the result without treating the length of the work as proof of accuracy.
Specifications & access
| Detail | Value and conditions |
|---|---|
| Launch date | |
| Access | API |
| Model identifiers | claude-fable-5-1 |
| Context window | 1,000,000 tokens Context limit Claude Fable 5.1 reference |
| Maximum output | 128,000 tokens Maximum output Claude Fable 5.1 reference |
| Input price | 10 USD per million tokens Standard input rate Claude Fable 5.1 reference |
| Output price | 50 USD per million tokens Standard output rate Claude Fable 5.1 reference |
| Thinking | Adaptive thinking is always on.Claude Fable 5.1 reference |
| Default effort | highClaude Fable 5.1 reference |
| Input and output | Text and images in; text outClaude model comparison |
Related models
- Claude Opus 5.5
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Launch date API - Claude Fable 5
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Claude Fable 5 is the earlier Fable release for extended reasoning.
Launch date API - Claude Mythos 5.1
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Claude Mythos 5.1 gives verified organizations access to the Fable 5.1 model for defensive cybersecurity.
Launch date Restricted accessAPI