Anthropic today introduced Claude Fable 5.1 and Claude Mythos 5.1, calling them the "world's most advanced models for coding and knowledge work." Fable 5.1 sets a new standard for coding, knowledge work, and long-running problem-solving, achieving similar or better results than Fable 5 at low or medium effort and much higher performance at higher effort tiers. It outperforms Fable 5, Opus 5, and OpenAI's GPT–5.6 Sol across multiple benchmarks.

Fable 5.1 and Mythos 5.1 are the same model with different safeguard levels. Fable 5.1 is available to everyone, while Mythos 5.1 is limited to U.S. companies and individuals in Anthropic's trusted access programs. A new Life Sciences Verification Program enables Mythos 5.1's advanced biology capabilities for scientific research, with open enrollment for scientists coming soon.

Safeguards for Fable 5.1 have improved, with Claude Code users seeing around 60 percent fewer cybersecurity false positives. Fable 5.1 can discover software vulnerabilities but not develop exploits. It includes invisible watermarking for generated text, and Anthropic is rolling out a detection API in private preview for eligible organizations as required by EU law. Fable 5.1 costs approximately 25 percent less than Fable 5 for typical workloads and up to 45 percent less for highly agentic work due to reduced cache-read pricing. Base pricing remains unchanged. Both models are available today on all platforms.