Three years of formal validation: AI now writes the Rust and proves it in Lean

carlk22 · reddit · 2026-09-23

Over three years the author's formal-validation workflow evolved from manual Rust + Dafny, to manual Rust + AI-written Lean proofs (three weeks, hundreds of prompts), to AI writing both — with Codex Sol 5.6 now writing algorithms, translating to Lean, constructing machine-checked proofs, and refactoring slop, sometimes in minutes. Not a general fix for trusting AI software, but for precisely specifiable algorithms formal validation is becoming practical; lessons written up as "Nine Rules for Vibe Validation."

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