13M LoC, are we sure it didn't exploit any latent issues in the lean proof system?
The proof system is relatively easy to verify. I am not entirely sure about lean, but the core algebras for systems like lean are in the 100s of lines of code. You can likely convince yourself it is correct in a weekend or less - especially with an Ai to help you understand it.
https://leodemoura.github.io/blog/2026-3-16-who-watches-the-...
...and for those who are looking to roll-their-own:
https://ammkrn.github.io/type_checking_in_lean4/title_page.h...
...and some thoughts on putting stuff in the kernel: