Anthropic's Claude Mythos Preview AI model uncovered a key-recovery attack against HAWK-256, a post-quantum digital signature scheme advanced to the third round of NIST's additional signature competition in May 2026, and a faster attack on a reduced-round version of AES. The findings, published by Anthropic on July 28, 2026, cut the security of HAWK-256 by roughly 67 million times in computational effort. The AI operated largely autonomously, requiring only high-level project direction and final verification from human researchers. Anthropic estimates the API cost for each of the two main research runs at approximately $100,000. The expected work factor for recovering a HAWK-256 secret key dropped from 2⁶⁴ operations to 2³⁸, and the company released a public implementation that recovers a functionally equivalent signing key in about 3 hours and 42 minutes on a 96-core server. For larger parameters, gate-count estimates fell from 2¹⁵⁰ to 2¹⁰⁸ for HAWK-512 and from 2²⁸⁸ to 2¹⁸² for HAWK-1024. Anthropic noted that fixing the vulnerability would require roughly doubling HAWK's key sizes, which "eliminates many of the reasons making the scheme an attractive PQC signature candidate."