LLMs are making formal software verification feasible at a much larger scale

Anthropic researchers, in collaboration with Theorem, demonstrate that large language models like Claude can autonomously produce formal correctness proofs for real-world software at an unprecedented scale. Working at the RISC-V assembly level, Claude generated proofs for 239 entry points across 123 open-source OSS-Fuzz libraries—roughly 180 million lines of Rocq proof code—showing that these binaries never crash on any input under 64 MB. The process also uncovered hundreds of latent bugs that had evaded years of fuzzing, including a counter underflow requiring a rare ~20 MB input layout and a rediscovery of a deep-nesting CVE in libexpat. The authors describe additional case studies (proving zlib's Linux kernel implementation against RFC 1950, verifying a Rust Python interpreter against CPython semantics, and auditing a PDF reader) and note falling costs as newer models need ~40% fewer tokens and proof reuse cuts token use by ~17%. They stress that "formally verified" does not mean "bug-free," since proofs only guarantee conformance to a spec and its machine model, and outline limitations around threading, architectures, and incomplete coverage. 

https://www.anthropic.com/research/formal-verification

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