Mistral AI · 2026-07-02 · major
Leanstral 1.5 — Mistral's updated Lean 4 formal-proof model
Leanstral 1.5 saturates miniF2F at 100%, solves 587/672 PutnamBench problems, and finds 5 previously unreported bugs across 57 open-source repos. Apache-2.0 weights on Hugging Face and a free labs-leanstral-1-5 API.
Mistral's Lean 4 theorem-prover saturates miniF2F, tops PutnamBench, and ships as an Apache-2.0 119B/6.5B MoE.
Key specs
| Parameters | 119B |
|---|---|
| Active params | 6.5B |
| Mini f2 f | 100% |
| Putnam bench | 587/672 |
Quick facts
| Maker | Mistral AI |
|---|---|
| Version | 1.5 (Jul 2, 2026) |
| Params | 119B total / 6.5B active (MoE) |
| License | Apache-2.0 |
| Access | Open weights on Hugging Face + free API endpoint labs-leanstral-1-5 |
| Context window | 256K tokens |
| Focus | Lean 4 theorem proving and autoformalization |
Benchmarks
What is it?
Leanstral 1.5 is Mistral's mixture-of-experts model tuned for Lean 4 formal proofs and autoformalization — writing machine-checkable proofs and turning informal math into Lean statements. Released under Apache-2.0 with 119B total and 6.5B active parameters, weights are on Hugging Face and the same model runs for free as labs-leanstral-1-5 on Mistral's API.
How does it work?
A sparse expert routing keeps compute low while giving the model access to a wide bank of specialised weights for tactics, term-mode edits, and autoformalization patterns. Training combined mid-training, supervised fine-tuning, and reinforcement learning with CISPO across two environments: multi-turn Lean proof interaction and a code-agent workflow that reads repos, writes properties, and drives the Lean kernel.
Why does it matter?
Formal proof is one of the few coding tasks with a hard ground-truth checker, so a stronger prover directly reduces verification busywork. Leanstral 1.5 saturates miniF2F, solves 587/672 PutnamBench problems, and in a 57-repo sweep uncovered 5 previously unreported bugs — including an integer-overflow in datrs/varinteger — at roughly $4 per PutnamBench problem versus $300+ elsewhere.
Who is it for?
Formal-methods researchers, Lean 4 users, and teams building verified libraries or protocols.
Frequently asked questions
- Is Leanstral 1.5 open source?
- Yes. Leanstral 1.5 ships under Apache-2.0 with full weights hosted at huggingface.co/mistralai/Leanstral-1.5-119B-A6B. Anyone can download, fine-tune, and self-host the 119B-parameter MoE, and Mistral also runs a free API endpoint at model id labs-leanstral-1-5 for users who prefer not to run it themselves.
- How much does Leanstral 1.5 cost to use?
- Leanstral 1.5 is free through Mistral's Labs endpoint (model id labs-leanstral-1-5). Mistral says a full PutnamBench proof search costs about $4 per problem on their infrastructure, versus an estimated $300+ per problem on comparable frontier models — the pricing gap is central to the 'proof abundance' framing.
- Which benchmarks does Leanstral 1.5 lead on?
- Leanstral 1.5 saturates miniF2F at 100% on both validation and test sets, solves 587 of 672 PutnamBench problems (seven more than Seed-Prover 1.5), and reaches 87% on FATE-H. On FLTEval it lands Pass@1 28.9% and Pass@8 43.2%, competitive with Claude Opus 4.6 at roughly one-seventh the compute cost.
- Has Leanstral 1.5 found real bugs in open-source code?
- Yes. Applied to 57 Rust and Go repositories, Leanstral 1.5 flagged 47 violated properties, of which 11 were genuine bugs and 5 were previously unreported on GitHub. One concrete find: an integer-overflow crash in the datrs/varinteger zigzag-decoding sign function on input Std.U64.MAX.
- How is Leanstral 1.5 different from the June model card release?
- Leanstral 1.5 first appeared as a Labs model card on June 30, 2026. The July 2 'Proof Abundance for All' announcement is the same 119B/6.5B MoE with the full benchmark disclosure, real-world bug-hunting results, and an Apache-2.0 open-weights drop on Hugging Face — turning a quiet playground preview into a public open-source release.
Try it
API: model id labs-leanstral-1-5. Weights: https://huggingface.co/mistralai/Leanstral-1.5-119B-A6B.