Light curve of the main episode of GRB 221009A prompt emission as observed from INTEGRAL/ACS (in count rate unit). In black, we represent the original data (saturated), the red line shows the model calibrated against the BepiColombo detector, and in blue we plot the restored light curves using the INTEGRAL/IBIS-VETO detector. Our earliest optical observations closely resemble the epoch of the precursor activity and are the earliest optical observations ever reported for this historic burst.