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-form problem-solving tasks. It achieves similar or better results than Fable 5 at low to medium effort, and delivers much better performance at higher effort levels. It outperforms OpenAI’s Fable 5, Opus 5 and GPT-5.6 Sol on several benchmarks.
Fable 5.1 and Mythos 5.1 are the same model, but with different save levels. Fable 5.1 is available to everyone, but Mythos 5.1 is limited to U.S. businesses and individuals participating in Anthropic’s Trusted Access programs. There is a new Life Sciences Verification program that allows Mythos 5.1’s advanced biological capabilities to be used for scientific research, with open registration for upcoming scientists.
Fable 5.1’s safeguards have been improved, and Claude Code users should see approximately 60% fewer false positives when it comes to cybersecurity. Anthropic says Fable 5.1 can now be used to discover software vulnerabilities, but not to develop exploits for them. Fable 5.1 includes an invisible watermark for Claude-generated text, and Anthropic deploys 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% less than Fable 5 for typical workloads, and up to 45% for highly agentic work, due to a reduction in cache read price. The base price remains the same. It is available today on all platforms.
