Claude Fable 5.1 is Anthropic's most capable generally available model, aimed at demanding reasoning, research and long running agentic work. Anthropic released it on September 1, 2026, together with Claude Mythos 5.1, which is the same model with looser safeguards for vetted cybersecurity and life science organizations.
Key facts
| Item | Claude Fable 5.1 |
|---|---|
| Maker | Anthropic |
| Released | September 1, 2026 |
| API model ID | claude-fable-5-1 |
| Context window | 1 million tokens |
| Max output | 128,000 tokens |
| Knowledge cutoff | June 2026 |
| Default effort | high, with adaptive thinking always on |
| API price per million tokens | $10 input, $50 output, $0.25 cache reads |
Sources: Anthropic's models overview and pricing page, plus the Fable 5.1 and Mythos 5.1 announcement, checked October 24, 2026.
Pricing details
Fable 5.1 keeps Fable 5's price of $10 per million input tokens and $50 per million output tokens, but Anthropic cut cache reads by 75 percent to $0.25 per million. Anthropic estimates this lowers typical costs by about 25 percent against Fable 5, and by up to about 45 percent for highly agentic work. Batch API requests are half price.
Where to use Claude Fable 5.1
- API and clouds: on the Anthropic API, Amazon Bedrock, Claude Platform on AWS, Google Cloud and Microsoft Foundry.
- Claude apps: on Claude Pro through usage credits, on Max with up to half of the weekly limits, and on Team premium seats and Enterprise.
- Mythos 5.1: only through Anthropic's Cyber Verification Program and Life Sciences Verification Program, currently for a set of US organizations.
Safeguards
Fable 5.1 can help find software vulnerabilities but not build exploits. It routes several dual use cyber tasks, such as penetration testing and exploit generation, and life science research questions to Opus models. Anthropic says its new cyber safeguards produce about 60 percent fewer interventions per Claude Code session than Fable 5's.
How it compares
Anthropic itself recommends Claude Opus 5.5 first, since it performs at Fable 5.1's level on most work for less than half the price. At the top of the market, Fable's rival is GPT-6 Astra at the same token price. Read Claude Fable vs Opus vs Mythos and GPT-6 Astra vs Claude Fable 5.1, or see the release timeline.