Depth-first opening trainers bury you in one long line. Hard Lines imports any PGN (Lichess studies, your own files, course exports) and drills your early choices across the whole tree first, then goes deeper as you're ready, using spaced repetition to keep it all fresh. Bring your own repertoire, no marketplace lock-in.
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Hey! Made this because every opening trainer I tried made me an expert in one line and blind to everything else. Hard Lines flips that: breadth first, depth later, all scheduled with spaced repetition. Import any PGN and try it at hardlines.app — would love feedback, especially from anyone who's used other opening trainers and can compare the feel.