Claude Fable 5.1
Anthropic's most intelligent generally available model, built for demanding reasoning and long-horizon agentic work that fixes root causes rather than symptoms
Claude Fable 5.1
Anthropic • September 2026
Training Data
Up to June 2026
Claude Fable 5.1
September 2026
Parameters
Not disclosed
Training Method
Advanced Constitutional AI with always-on adaptive thinking, safety classifiers, and Opus fallback routing
Context Window
1,000,000 tokens
Knowledge Cutoff
June 2026
Key Features
Mid-conversation Effort Control • Content Provenance Watermarking • 1M Context / 128K Output
Capabilities
Agentic: Outstanding
Reasoning: Outstanding
Coding: Outstanding
Vision: Outstanding
What's New in This Version
Outperforms Fable 5, Opus 5, and GPT-5.6 Sol on coding, knowledge work, and long-running problem-solving; matches Fable 5 at low or medium effort, cuts cache-read pricing 75% (about 25% cheaper on typical workloads, up to 45% on agentic ones), and produces around 60% fewer cybersecurity false positives
Anthropic's most intelligent generally available model, built for demanding reasoning and long-horizon agentic work that fixes root causes rather than symptoms
What's New in This Version
Outperforms Fable 5, Opus 5, and GPT-5.6 Sol on coding, knowledge work, and long-running problem-solving; matches Fable 5 at low or medium effort, cuts cache-read pricing 75% (about 25% cheaper on typical workloads, up to 45% on agentic ones), and produces around 60% fewer cybersecurity false positives
Technical Specifications
Key Features
Capabilities
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