Sheikh Al Kulayni believed Al Kafi to be authentic

Sheikh Al Kulayni writes:

“You said that you would like to have a book that is sufficient (Kafi), that covers all areas of religious knowledge, that is sufficient for those who seek knowledge. That serves as a reference for those who seek the true path, that is taken by those, who are acquiring the knowledge of the religion and who wants to act according to the traditions of the Imams (a.), which are authentic (Sahih).”

📚Kitab Al Kafi V1,P24(Dar Al Kutub Al Islamiyah)

Muhammad Al-Faid Al-Kashani comments:

“In accordance with the customs of the early scholars, the two authors of Al-Kafi and Al-Faqih used the terminology authentic (Sahih) for that which is supported and relied upon, and they considered all the sayings they have narrated in their two books as authentic (Sahih).” 

📚Kitab Al Wafi V1,P23