Ultrahigh intensity contrast and short pulse laser–solid interactions offer an attractive platform for investigating high-energy-density matter, particularly in the context of structured and ultra-thin targets that form hot, dense plasma conditions. Harmonic generation can improve the contrast of laser pulses by several orders of magnitude. In this study, we present the characterization of extreme contrast, relativistic intensity second-harmonic pulses at 400 nm, using the self-diffraction frequency-resolved optical gating technique. The 400 nm pulses were generated at various input intensities using potassium dihydrogen phosphate and lithium triborate crystals. Our observations reveal the presence of spectral broadening, pulse compression, and complex structures at higher input intensities. We see that extreme contrast, few tens of femtosecond pulses can have multiple “prepulses” at the 100s femtosecond scale as large as ten percent of the peak value. These can preionize a solid significantly and may influence the interaction. Simulations based on nonlinear pulse propagation equations reinforce our findings.