The default is the same as for CMake, which is CMAKE_BUILD_TYPE=None. Build type of None is neither release nor debug. It does not use the -g flag, but also does not define the NDEBUG definition and does not use any -O flags if I remember correctly. You need to explicitly set a build type if you want Release or Debug or RelWithDebInfo or something else. For example: ament.py build --cmake-args -DCMAKE_BUILD_TYPE=RelWithDebInfo --