C.UTF-8 is a glibc built-in only from 2.35 (Debian 12+) onward; on an older Debian runner setlocale() silently falls back to POSIX, which is exactly the "main argv conversion failed!" crash from before. Install locales and generate en_US.UTF-8 explicitly instead, which works on any Debian version. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>