Mistral releases Leanstral 1.5, an Apache 2.0 Lean 4 proof agent solving 587 of 672 PutnamBench problems

The open weight 119B model with 6.5B active params saturates miniF2F and sets a new state of the art on formal math benchmarks at about 4 dollars per problem, roughly 75 times cheaper than rivals. It ships free on Hugging Face and via API.