This contribution describes the characterisation and validation campaign of the prototype
of the CMS Readout Chip (CROC), a 65 nm CMOS pixel readout ASIC for the CMS Inner Tracker upgrade
for High Luminosity LHC. This validation campaign includes tests with single-chip and multi-chip
modules, irradiation campaigns, test beams and wafer-level tests. The main results obtained in the
testing of the CROC prototype will be outlined. Key improvements and fixes that have been
implemented in the final version of the chip before the October 2023 submission will be described.