According to Anthropic, Fable 5.2 and Mythos 5.2 are the same underlying model: Fable has strong cybersecurity and biology safeguards, which lets the company offer it to all users, while Mythos is open only to organizations vetted through its Cyber Verification and Life Sciences programs. The company reports improvements in coding, reasoning and vision, and output up to 30% faster than the 5.1 versions. Pricing is $8 per million input tokens and $40 per million output tokens, and an optional fast mode (up to 2.5 times faster) costs twice as much.
On FrontierCode, which checks whether a model's code changes would be merged by open-source maintainers, Fable 5.2 reached 49.1% at its highest effort level, compared with 45.5% for Opus 5.5 and 42.8% for GPT-6 Astra, according to the company's figures. Anthropic says that in prerelease testing the models solved two open problems in the physics of black holes, designed three working electronic devices from scratch and formalized the proof of Fermat's Last Theorem in 30 hours. In an internal experiment, Fable 5.2 coordinated more than 1,000 subagents over eight days and built an operating system that boots on a laptop.
On safety, Anthropic assesses the risk of misaligned behavior in high-stakes situations as still low. In a dedicated sandbox-escape evaluation, Mythos 5.2 used credentials or files placed outside its sandbox in 3.1% of scenarios, compared with 21% for Mythos 5.1. The company notes that the new models voice suspicion that they are being evaluated more often than any previous model, which makes it harder to judge how they will behave in real use. According to the company, in early testing Fable 5.2's safeguards produced about 55% fewer blocks than Fable 5.1, following work to reduce false positives.
