I did open an issue on GitHub with the CPython implementation and it's being looked into. The crash happens for sure in the debug version of Python. However we're seeing valgrind and address sanitizer fire off for the upcoming 3.13 release in release mode when using the C API as I showed in my example. However, this behavior only appears to occur when embedding python directly, not via extending it, into an application. Because of how the (hidden because it is experimental) `_xxsubinterpreters` module works internally, it's extra internal bookkeeping seems to give enough time for the race condition to disappear.