Usooli scholar Abdul Husain Al Musawi believed Al Kafi is authentic

Abdul Husain Al Musawi writes:

“The best that has been collected from these, are the four books which serve as a reference (Marja) for the followers of the Imams (as) in terms of their principles and branches from the early times until this time and these are Al-Kafi, At-Tahdhib, Al-Istibsar and Man La Yahduruh-ul-Faqih, which have been continuously transmitted (Mutawatir) and their contents leave no doubt that they are authentic (Sahih).”

📚Al-Murajat P334