Leanstral 1.5
Leanstral 1.5 is a proof-engineering model for Lean 4. It belongs to the Mistral Small 4 family and uses 119B total parameters with 6.5B active per token. Its Apache 2.0 weights remain available. The Labs API is deprecated.
Deprecated API; check access details
Treat compiler feedback as part of the workflow
The model card lists 128 experts with four active per token and text or image input. Although the context limit is 256k, its recommended setting is at most 200k. Local serving and a Lean agent setup are documented.Leanstral 1.5 official weight card
Learn from compiler and repository feedback
The release describes training in two environments. One submits proofs, receives Lean compiler feedback and revises them. The other edits a repository with shell and language-server tools. This teaches a proof workflow rather than only producing a proof-shaped answer.Leanstral 1.5 training and proof engineering
Separate the deprecated API from weights
The model page records API deprecation on 29 September 2026. The official weight repository remains available. API access is subject to Mistral's retirement schedule; a published model identifier does not guarantee continued service after retirement.Leanstral 1.5 API lifecycle pageLeanstral 1.5 official weight card
Evaluate the complete task
Practical guidance. Use the proof checker to establish whether the final proof is accepted. Also review whether the formal statement expresses the intended claim. A valid proof of the wrong specification will not establish the property that the application needs.
Specifications & access
| Detail | Value and conditions |
|---|---|
| Status | Deprecated API; check access details |
| Access | API, Downloadable |
| Model identifiers | labs-leanstral-1-5Leanstral-1.5-119B-A6B |
| Context window | 256,000 tokens Advertised 256k context. The card recommends at most 200k for use. Leanstral 1.5 official weight card |
| API status | Deprecated on 29 September 2026Leanstral 1.5 API lifecycle page |
| Weight license | Apache 2.0Leanstral 1.5 official weight card |
| Weight repository | mistralai/Leanstral-1.5-119B-A6BLeanstral 1.5 official weight card |
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