Hi, I have a strange issue when trying to bootstrapping boost itself. I have installed MSYS2 (msys2-x86_64-20251213.exe), then installed the UCRT64 toolchain via "pacman -S mingw-w64-ucrt-x86_64-toolchain". I've downloaded and extracted Boost 1.90.0. Afterwards I executed these commands inside the normal Windows command shell: bootstrap.bat --toolset=gcc b2 toolset=gcc variant=release link=static runtime-link=static threading=multi address-model=64 --stagedir=./stage But when I try to execute the following command to see if everything worked b2 --show-libraries then b2 restarts the compilation: D:\...\Boost>b2 --show-libraries Performing configuration checks     - default address-model    : 64-bit [1]     - default architecture     : x86 [1] Building the Boost C++ Libraries. warning: Graph library does not contain MPI-based parallel components. note: to enable them, add "using mpi ;" to your user-config.jam. note: to suppress this message, pass "--without-graph_parallel" to bjam.     - std_wstreambuf builds    : yes [2]     - std_wstreambuf           : yes [2]     - has pthread_cond_clockwait : no [2] I've cleaned everything and restarted the build, but no change. I tried also the minimum bjam files to see if this works: test.jam: "exe hello : hello.cpp ;" hello.cpp: "int main() { return 0; }" b2 -f test.jam which yields the following output: D:\...\Boost>b2 -f test.jam test.jam:1: in module scope ERROR: rule "exe" unknown in root module. So apparently something went entirely wrong, but I don't know what exactly I'm missing here. Any ideas? Best regards, Johannes