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Claude Fable 5 - The Most Powerful AI Model Released by Anthropic

Author: AI小集 · Original source: AI工具集

What Is Claude Fable 5

Claude Fable 5 is the most powerful AI model released by Anthropic, a safety-oriented flagship model in the Mythos-class, fully identical at the underlying level to Claude Mythos 5, with conservative safety guardrails added. The model achieves SOTA on nearly all public benchmarks and excels in software engineering, knowledge work, visual reasoning, and long-horizon autonomous tasks. The model is priced at $10 per million input tokens and $50 per million output tokens.

Claude Fable 5

Main Features of Claude Fable 5

  • Ultra-Long Autonomous Coding: Completes a full-repository migration in a 50-million-line Ruby codebase in one day, scoring 80.3% on SWE-Bench Pro.
  • Token-Efficient Reasoning: In the FrontierCode evaluation, it achieves the highest score among frontier models at 29.3% with medium effort, at a cost far lower than competitors.
  • Native Visual Understanding: Completes Pokémon FireRed using raw game screenshots without external tools, and can extract precise values from scientific charts and reconstruct webpage source code.
  • Persistent Memory and Long Context: Supports million-level token long-term tasks, uses file-level notes to improve output, and performs 3 times better than Opus 4.8 in Slay the Spire.
  • Intelligent Safety Downgrade: Built-in independent classifiers automatically route requests to Claude Opus 4.8 when cybersecurity, biosecurity, or distillation attempts are detected.

Technical Principles of Claude Fable 5

  • Model Tier Positioning: Fable 5 belongs to Anthropic's newly established Mythos-class capability tier, positioned above Opus and sharing the same underlying model as Mythos 5, made available to the public through added safety guardrails.
  • Safety Routing Mechanism: The system mounts independent classifiers to detect requests in real time. When cybersecurity, biosecurity, or model distillation is involved, queries are automatically downgrade-routed to Claude Opus 4.8 for response rather than being directly refused.
  • Frontier LLM Development Restrictions: For requests involving pretraining pipelines, distributed training infrastructure, and similar topics, invisible interventions such as prompt modification, steering vectors, and parameter-efficient fine-tuning (PEFT) are used to limit model effectiveness, affecting approximately 0.03% of traffic.
  • Long Context and Persistent Memory: Supports million-level token long-horizon tasks and is equipped with file-level persistent memory, allowing it to autonomously write notes during operation and retroactively improve output, solving the problem of amnesia in complex tasks.

How to Use Claude Fable 5

  • Web Use: Log in to Claude official website, and switch to "Fable 5" in the model selector of the chat interface to start a conversation.
  • API Access: Call the model name via the Anthropic API claude-fable-5, and integrate it into your own application through the standard REST interface.
  • Claude Code Use: After installing in the terminal Claude Code , specify Fable 5 as the default model through settings, supporting codebase-level agent operations.

Claude Fable 5 Project Official Website

  • Project Official Website：https://www.anthropic.com/news/claude-fable-5-mythos-5

Core Advantages of Claude Fable 5

  • No Guardrail Restrictions: In high-risk fields such as cybersecurity and biosecurity research, it is not subject to Fable 5's safety downgrade mechanism, with full output capabilities.
  • Scientific Research Autonomy: Can run independently for more than a week to complete complex scientific research tasks, with no human intervention needed from data integration to model training.
  • Hypothesis validation capability: The proposed molecular biology hypotheses are experimentally verifiable, and existing hypotheses about novel mechanisms of Escherichia coli proteins have been confirmed by third-party research.
  • Extreme model efficiency: On the same task, using only 1/3 of GPT-5.5's reasoning tokens, it produced physics research results approaching the latter's four days of work within 36 hours.

Comparison of Claude Fable 5 with similar competitors

Dimension Claude Mythos 5 Claude Fable 5 Claude Opus 4.8
Underlying model Same Same Different
Safety guardrails None / very few Conservative (about 5% trigger downgrade) Standard
ExploitBench 78.0% Close to Opus* 40.0%
BioMysteryBench (hard) 46.1% Close to Opus* 40.0%
Protein design validation 9/14 entered the pipeline Restricted None
Access permissions Trusted institutions/Glasswing Publicly available Publicly available
Data retention 30-day mandatory monitoring 30-day mandatory monitoring Standard policy

Application scenarios of Claude Fable 5

  • National-level cyber defense: Provide in-depth vulnerability analysis, threat modeling, and attack-defense exercises for critical infrastructure and defense systems.
  • Frontier drug development: Autonomously complete target identification, protein design, and complex validation, accelerating drug pipelines for tumors and neurodegenerative diseases.
  • Basic scientific research: Conduct long-cycle autonomous research in molecular biology, genomics, physics, and other fields, generating verifiable hypotheses.
  • AI safety red teaming: Used to test the safety boundaries of other AI systems and identify complex jailbreaks, cross-request attacks, and novel threat vectors.