PricingDistributed as a free model and accessible via a free API; licensed under Apache-2.0.
MISTRAL AI · MODEL RELEASE TRACKER
Leanstral 1.5
Leanstral 1.5 is a released Mistral model for proof engineering in Lean 4. It is Apache-2.0 licensed (119B total, 6B active parameters), open-sourced and available on Hugging Face and via a free API. It was trained with mid-training, supervised fine-tuning, and RL with CISPO, uses specialized RL environments for theorem proving and code-agent interactions, and achieves state-of-the-art formal-verification benchmarks (saturates miniF2F, 587/672 PutnamBench, 87% on FATE-H, 34% on FATE-X).CURRENT SNAPSHOT3/5 DIMENSIONS WITH DATA
The dimensions that change the decision.
Not established from the available sources.
Not established from the available sources.
Benchmark resultsSaturates miniF2F; solved 587/672 PutnamBench problems; achieved 87% on FATE-H and 34% on FATE-X.
Availability and accessFully open-sourced and available on Hugging Face; also accessible via a free API for practical proof engineering in Lean 4.
VERIFIABLE FACTS
Every value stays attached to a source and date.
RELEASE · License and parameter countsDEVELOPER CLAIM
Released under the Apache-2.0 license; reported as 119B total parameters with 6B active parameters.
BENCHMARKS · Benchmark resultsDEVELOPER CLAIM
Saturates miniF2F; solved 587/672 PutnamBench problems; achieved 87% on FATE-H and 34% on FATE-X.
CAPABILITIES · Training process and RL environmentsDEVELOPER CLAIM
Trained via mid-training, supervised fine-tuning, and reinforcement learning with CISPO. Uses two RL environments: a 'multiturn environment' that provides Lean compiler feedback for iterative proving, and a 'code agent environment' where the model edits files, runs bash commands, and uses the Lean language server to work on repositories.
CAPABILITIES · Code verification and bug discoveryDEVELOPER CLAIM
Capable of verifying complex code properties in Lean 4; during tests the model uncovered 5 previously unknown bugs across 57 repositories.
AVAILABILITY · Availability and accessDEVELOPER CLAIM
Fully open-sourced and available on Hugging Face; also accessible via a free API for practical proof engineering in Lean 4.
PRICING · PricingDEVELOPER CLAIM
Distributed as a free model and accessible via a free API; licensed under Apache-2.0.
SAFETY · Verification toolingDEVELOPER CLAIM
Model outputs are verified using Mistral's fork of SafeVerify for correctness (as reported by the release).
WHAT CHANGED