The article analyzes the key role of the effective tax rate (ETR), supported by artificial intelligence (AI), in reducing the corporate income tax (CIT) gap, along with a comparative analysis between Poland and the Czech Republic. In the era of AI and Pillar Two, ETR is becoming a key objective indicator enabling a shift from subjective assessment of taxpayer intent to data-driven analysis. The article verifies the hypothesis that ETR using AI can play a key role in reducing the CIT gap through actual monitoring of ETR and comparison with the profiles of similar entities for early detection of base erosion; quantitative measurement of the compliance gap using reference ETR levels and deviations; and behavioral impact through algorithmic risk scoring, early warning, and integration with the minimum tax, encouraging voluntary reduction of aggressive optimization. A comparative analysis of Poland and the Czech Republic has allowed us to draw universal conclusions about CIT compliance systems using AI under Pillar Two. The added value lies in showing how AI transforms ETR from a simple reporting indicator into a proactive tool for systematically and sustainably reducing the CIT tax gap.