Claude Fable 5 - Anthropic's most powerful AI model

What is Claude Fable 5?

Claude Fable 5 is Anthropic's most powerful AI model, a flagship model in the Mythos-class security version. Its underlying architecture is identical to Claude Mythos 5, but with added conservative security safeguards. The model achieves state-of-the-art (SOTA) performance in almost all public benchmarks, demonstrating exceptional performance in software engineering, knowledge work, visual reasoning, and long-cycle autonomous tasks. The model is priced at $10 per million input tokens and $50 per million output tokens.

Claude Fable 5's main functions

• Ultra-long custom codingCompleted a full migration of a 50 million-line Ruby codebase in one day, achieving a SWE-Bench Pro score of 80.3%.
• Efficient Token ReasoningIn the FrontierCode benchmark, it achieved the highest score of 29.3% for a cutting-edge model with moderate effort, at a cost far lower than its competitors.
• Native visual understandingA player can complete Pokémon FireRed without using cheats by simply viewing original game screenshots. The player can also extract precise values from scientific charts and reconstruct webpage source code.
• Persistent memory and long contextSupports long-term tasks with millions of tokens, improves output with file-level notes, and delivers 3x better performance than Opus 4.8 in Slay the Spire.
• Intelligent security downgradeIt has a built-in independent classifier that automatically routes requests to Claude Opus 4.8 for processing when it detects cybersecurity, biohazard risks, or distillation attempts.

Technical principles of Claude Fable 5

• Model hierarchical positioningFable 5 belongs to the newly established Mythos-class capability level in Anthropic, which is above Opus. It shares the same underlying model as Mythos 5 and is made available to the public by adding security barriers.
• Secure routing mechanismWhen the system mounts an independent classifier to detect real-time requests, and these requests involve cybersecurity, biochemical risks, or model distillation, the query will be automatically downgraded and routed to the Claude Opus 4.8 response instead of being directly rejected.
• Frontier LLM Development LimitationsFor requests to pre-training pipelines and distributed training infrastructure, invisible intervention methods such as prompt modification and efficient fine-tuning of steering vectors and parameters (PEFT) are used to limit model effectiveness, affecting approximately 0.03% of traffic.
• Long context and persistent memorySupports long-cycle tasks with millions of tokens, equipped with file-level persistent memory, and can autonomously write notes and backtrack to improve output during operation, solving the forgetting problem in complex tasks.

How to use Claude Fable 5

• Web-based useLog in to the Claude website and switch to "Fable 5" in the model selector in the dialogue interface to start the dialogue.
• API Access: Call the model name via the Anthropic API claude-fable-5Integrate it into your own application using the standard REST interface.
• Claude Code usageAfter installing Claude Code in the terminal, you can set the default model to Fable 5 through settings to support codebase-level proxy operations.

Claude Fable 5's core advantages

• No guardrail restrictionsIn high-risk fields such as cybersecurity and biochemical research, it is not constrained by Fable 5's security degradation mechanism and has complete output capabilities.
• Research autonomyIt can run independently for more than a week to complete complex scientific research tasks, and no human intervention is required from data integration to model training.
• Hypothesis testing capabilityThe proposed molecular biology hypothesis is experimentally verifiable, and the hypothesis regarding the new mechanism of E. coli protein has been confirmed by third-party research.
• Model efficiency to the extremeUnder the same task, using only 1/3 of the inference tokens of GPT-5.5, it produced physical research results in 36 hours that were close to those of the latter in four days.

Comparison of Claude Fable 5 with similar competitors

Application scenarios of Claude Fable 5

• National-level network defenseProvides in-depth vulnerability analysis, threat modeling, and attack and defense drills for critical infrastructure and defense systems.
• Cutting-edge drug developmentIt can independently complete target identification, protein design, and complex validation, accelerating the pipeline of drugs for tumors and neurodegenerative diseases.
• Basic scientific research: To conduct long-term autonomous research in fields such as molecular biology, genomics, and physics, and generate verifiable hypotheses.
• AI Security Red TeamUsed to test the security boundaries of other AI systems and identify sophisticated jailbreaks, cross-request attacks, and new threat vectors.