joi is used by both the library and the consumer - it is important for them to be the same version, so I believe it should be a peer dependency. I've had issues with discrepancies between versions in the past.
Coverage remained the same at 98.541% when pulling c0fb601bad1c7805f805dafeb9cae2e3ef0d0f65 on stephentuso:joi-peer-dep into f1b343a9104e48ddd8b5abaa4db715e4569238ca on clarkie:master.
joi
is used by both the library and the consumer - it is important for them to be the same version, so I believe it should be a peer dependency. I've had issues with discrepancies between versions in the past.