Muhammad Al-Hurr Al-Amili writes:

فعلم أن كل ما فيه صحيح بإصطلاح القدماء بمعنى الثابت عن المعصوم بالقرائن القطيعة أو التواتر

“So now it is known that everything in Al-Kafi, in accordance with the terminology of the early scholars, is authentic (Sahih), which means, that it has been proven from the Infallible through additional indisputable evidences or through a continuous route (Mutawatir).” [Wasa’il-ush-Shi’ah, volume 30, page 196]

واعلم أن إيراد الكليني له لا قصور فيه بوجه وإن ذكر في أول كتابه أن أحاديثها صحيحة فإن مراده ثبوت مضمونها بالقرائن وثبوت نقل المضمون عن المعصوم لا يوجب العلمل بظاهره مع وجود معارضه

“Know, that whatever Al-Kulaini narrates, does not belittle his status in any way and although he mentioned at the beginning of his book that his narrations are authentic (Sahih), he meant the affirmation of their contents by additional evidences and the affirmation that the contents have been narrated by the Infallible, but not that it must be acted upon according to its outward appearance if there is something that contradicts it.” [Al-Fawa’id-ut-Tusiyyah, page 87]