A single ag run should work, but: https://github.com/ggreer/the_silver_searcher/issues/964#issuecomment-1625721711

reuse tries to do too much IMHO, including enforcing a specific project structure and locale: https://github.com/fsfe/reuse-tool/pull/794

Something like grep -rL SPDX is a nice and simple approach, but it will traverse into target and .git.

Oh, fd | xargs grep -rL SPDX works. Just need to ignore the obvious exception: LICENCE (or rather, check that it's the only result).