An AI wrote its own TLA+ invariant and caught a real, unfixed etcd bug

I gave a tool the execution traces of a small etcd program and no properties to check. It wrote a TLA+ specification, invented its own safety invariant, ran a model checker, and produced a counterexample. That counterexample matches an open, unfixed etcd issue, filed in April 2026, after the training cutoff of the model it used, Claude Sonnet 4.5.