Anthropic releases two new models in the fifth Claude generation. Claude Fable 5 claims the top spot in nearly all benchmarks, while Claude Mythos 5 (no longer in preview) is still only available to select partners. Both models share the same base model. Fable 5 ships with conservative safety guardrails for general use. Mythos 5 drops those restrictions in areas like cybersecurity and is reserved for a small group of partners. Anthropic says Fable 5 beats every generally available model the company has ever shipped and claims state-of-the-art results in nearly all benchmarks tested. The gap widens on long, complex tasks, the company states. On SWE-Bench Pro, a benchmark for solving real software engineering tasks from public GitHub repos without help, Fable 5 hits 80.3 percent. Claude Opus 4.8 lands at 69.2 percent, GPT 5.5 at 58.6 percent, and Gemini 3.1 Pro at 54.2 percent.

Full analysis : Anthropic says Fable 5 has invisible safeguards that use prompt modification, steering vectors, or PEFT to limit its effectiveness for building frontier LLMs.