Leanstral 1.5 finds five unknown code bugs

Mistral’s Leanstral 1.5 found five previously unreported bugs while checking 57 open-source repositories, according to the company’s Leanstral 1.5 release. The Apache-2.0 model activates 6 billion of 119 billion parameters, solved 587 of 672 PutnamBench problems, and is available through both open weights and a free API endpoint. For teams weighing formal verification alongside probabilistic coding agents, the result is worth filing: proof-oriented models can now surface production defects, but four failed proof attempts still require a separate negation pass before a finding deserves trust.