Anthropic Launches Claude Fable 5.1 With Lower Costs and Fewer False Positives

Anthropic has unveiled Claude Fable 5.1 and Claude Mythos 5.1, touting them as the world's most advanced models for coding and knowledge work. According to Anthropic, Fable 5.1 excels at coding, knowledge work, and long-running problem-solving tasks, matching or surpassing Fable 5 performance at lower or medium effort levels and delivering superior results at higher effort tiers. Moreover, Fable 5.1 outperforms Fable 5, Opus 5, and OpenAI's GPT–5.6 Sol across various benchmarks.

Claude Fable 5.1 and Mythos 5.1 are essentially the same model, differing only in their safety settings. Fable 5.1 is accessible to all users, while Mythos 5.1 is restricted to U.S. companies and individuals participating in Anthropic's trusted access programs. Anthropic has also launched a Life Sciences Verification Program, allowing Mythos 5.1's advanced biology capabilities to be employed for scientific research, with open enrollment for scientists expected soon.

To bolster security, Anthropic has strengthened safeguards for Fable 5.1, with users of Claude Code expected to experience around 60 percent fewer cybersecurity false positives. Furthermore, Fable 5.1 can now assist in identifying software vulnerabilities, though it cannot be used to develop exploits.

In a bid to preserve the originality of text generated by Claude, the company has introduced invisible watermarking. Additionally, a detection API has been rolled out in a private preview, available to eligible organizations as required by EU law. Fable 5.1 is priced around 25 percent lower than Fable 5 for standard tasks and up to 45 percent cheaper for highly agentic work due to reduced cache-read pricing. Base pricing remains unchanged and the model is now available across all platforms.