Mistral's open Leanstral 1.5 model solved 587 of 672 Putnam math problems and is already finding real bugs in open-source code.

I was doing the do out on the internet, and you do, and came across Mistral AI Leanstral 1.5. Which calls itself: > An updated Lean 4 formal proof engineering model optimised for automated theorem proving and autoformalization. 119B total parameters, 6.5B active. I was like, wow, thats a thingymabob of the first order. The last time someone said "formal methods" to me was in 1998, when a bloke called in a computer lab in Surrey. I had not a scooby doo what that was, and here we are more than a quarter of a century later, and solving something cool with formal methods is still on my bucket list. Boo hoo. I had to check the blog to ask what-the-actual-fluffs this is all about Leanstral 1.5: Proof Abundance for All. They are letting you use it for free. So you can go wild having it formally prove you don't have bugs in your mission-critical systems code! Yipee! Well, it turns out I need a formal proof that my Unbounded Viewstamped Replication Revisited code doesn't have many problems. So I thought I'd ask Fable 5.1, GPT 5 and GLM-5.3-Flash to kick the tyres on getting the Leanstral model to write out formal proofs in pretty Greek characters and debug it. Huh you did what!? Well, lets say a model was not good at, I dunno, coding in a rare language, like Felienne Hermans’ Hedy. You could fine-tune a model on that and make a language server that lints the code so that a special model trained on a fast set of examples can happily write code in Arabic and right-to-left with all the different and rare Arabic numerals. That would be so darn cool. Well, Mistal AI have done that for the LeanProver language Lean. They have a VS Code Plugin. On their blog, they show Claude can find real solutions to real problems in that system, but at a very high cost. I told Claude to call Leanstral if it needed help in writing Lean proofs that could be solved. Which it duly did when it got a bit stuck. Here is what it wrote: So there you go! So, like, you know you could, I dunno,

Mistral AI veröffentlicht Leanstral 1.5 unter Apache-2.0-Lizenz. Das Modell löst laut Mistral 587 von 672 Putnam-Aufgaben und findet automatisiert Bugs in Code.

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First open-source code agent for Lean 4.