EuclidLab
Lab · Tutor as gym equipment

Diagnose. Find the flaw in someone else's proof.

A tutor that explains everything is a tutor that prevents you from thinking. This one does the opposite: it presents you with proofs that are almost right — proofs that look correct, sound plausible, and end at the intended conclusion — but that quietly cheat at one specific step. Your job is to find the step, name the rule it violates, and explain how a real proof would have made the move legally.

The exercise
Five near-cases. Each contains exactly one buried error.
What's trained
Calibration. Counterexample search. The ability to be less easy to fool.
Read first

Not a chatbot

You are not asking the tutor to explain. You are diagnosing its work. The asymmetry matters: you are the active agent.

Errors are designed

Every flaw maps to a real, named failure mode — petitio principii, anachronistic dependency, unjustified existence, eyeball-equality, wrong common notion.

Wrong looks right

The figures and proofs are deliberately convincing. If they did not feel correct, the exercise would be useless — and so would the muscle it builds.

Five near-cases.

— 0 of 5 diagnosed —

The proof, as written

    Canonical diagnosis

    The rule that was violated