Anthropic today introduced Claude Fable 5.1 and Claude Mythos 5.1, which the company says are the "world's most advanced models for coding and knowledge work."
According to Anthropic, Claude Fable 5.1 sets a new standard for coding, knowledge work, and long-running problem-solving tasks. It achieves similar or better results than Fable 5 at low or medium effort, and has 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, but with different safeguard levels. Fable 5.1 is available to everyone, but Mythos 5.1 is limited to U.S. companies and individuals participating in Anthropic's trusted access programs. There's a new Life Sciences Verification Program that allows Mythos 5.1's advanced biology capabilities to be used for scientific research, with open enrollment for scientists coming soon.
Safeguards for Fable 5.1 have been improved, and Claude Code users should see around 60 percent fewer cybersecurity false positives. Anthropic says Fable 5.1 can now be used to discover software vulnerabilities, but not develop exploits for them. Fable 5.1 includes invisible watermarking for text generated by Claude, and Anthropic is rolling out a detection API in a private preview. The API is available to 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 for highly agentic work, because of a reduction in cache-read pricing. Base pricing remains the same. It is available today on all platforms.
