The Depth Engine is an AI tutor that refuses to make learning feel easy. Up to five topics a segment, each explored five ways: what it is, why it works, where it came from, what breaks it, how to use it. An agreement meter scores its own replies; above 70% it stops flattering you and pushes back with real contradictions, edge cases and named teachers. Your phrases are quoted back, coined terms build a personal glossary, and each day closes with one line worth remembering.