The limitation is the debug adapter, not VSCodium itself. The FOSS C# adapter, netcoredbg, can't feed stdin through the integrated terminal. Only Microsoft's vsdbg does that, and its licence ties it to official VS Code and Visual Studio. The way round it that's worked for me is to skip launch mode and attach instead: start the program yourself in a normal terminal, then use an attach configuration to hook netcoredbg onto the running process. You get breakpoints and inspection, and since it's a real terminal the stdin behaves. Not as smooth as launch, but it stays fully FOSS.