Anthropic Turned Fuzz Tests Into Proofs. What Claude’s Formal Verification Work Shows, and What It Doesn’t

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Anthropic and Theorem had Claude prove that 239 fuzz-test entry points in 123 open-source libraries can never crash on inputs under 64 MB, checked by the Rocq proof checker at the RISC-V binary level. Along the way Claude found hundreds of bugs. What was proved, how fuzz tests became specifications, what it cost in tokens, and where the proofs stop.
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