Research question: What is the reliability of Geri® Assess 2.0 software time-lapse technology for annotating kinetic events and identifying abnormal phenotypes in preimplantation human embryos? Design: Embryos were annotated using Assess 2.0 for the appearance and fading of pronuclei, and for progression to the 2-, 3-, 4-, 5- and 6-cell stages and to three blastocyst stages. Identification of reverse cleavage and direct cleavage phenotypes was also recorded. Manual annotation was undertaken after these events in a blinded fashion. Embryo scores were compared between Assess 2.0 and manual annotation. Results: A total of 513 oocytes from 34 women were included. Detection rates for Assess 2.0 versus manual annotation among the 10 kinetic events and including direct cleavage and reverse cleavage ranged between 0% and 94.4%. The percentage of discordant pairs was significantly different for all 12 events analysed (P-value range 0.036 to <0.0001). The sensitivity of Assess 2.0 ranged from 68.2% to 94.4% and specificity ranged from 63.8% to 97.3%. Assess 2.0 called for verification by the embryologist for at least one event in 55.2% of oocytes assessed. Of the 297 embryos scored by manual annotation, Assess 2.0 assigned the same score for only 125 (42.1%), although after manual corrections, concordance with manual annotation scores was raised to 66.0%. Conclusions: The results reveal striking differences between Assess 2.0 and manual annotation for kinetic annotations. Failure of Assess 2.0 to detect direct cleavage events and the low detection rate of reverse cleavage are further limitations. These collective findings highlight the importance of validating time-lapse annotation software before clinical implementation. Manual verification of Assess 2.0 outputs remains essential for accurate data interpretation.