Leanstral 1.5 API Pricing
Retired September 30, 2026: Mistral deprecated labs-leanstral-1-5 on September 29 and retired it the next day, naming no replacement. Leanstral 1.5 was Mistral's model for Lean 4 formal proofs - automated theorem proving and autoformalization - and it cost nothing to call: a public preview under the Labs label, listed free from June 30, 2026, the day the earlier Leanstral 2603 was retired. The weights are Apache 2.0, so self-hosting remains an option.
Run the numbers.
Calculator pre-loaded with Leanstral 1.5's listed rate, which is zero. It is here for the comparison: switch the model to see what the same proof workload would cost on a paid Mistral or third-party model.
Real-world presets.
Lean 4 proof search
Paper lemma to Lean
Fix a broken proof
Lean tactic question
Counts use a chars-per-token calibration measured on the vendor's own published tokenizer (mistralai/Mistral-Nemo-Instruct-2407, 2026-06-10). English prose is typically within a few percent; code and non-Latin scripts tokenize heavier. For billing-exact counts use the vendor's count-tokens API.
| Model | Input /M | Output /M | Effective blended | Context | Best for |
|---|---|---|---|---|---|
| Leanstral 1.5 Current | Free | Free | Free agentic 92/8 | 256K | Lean 4 formal proofs, free until retired |
| Leanstral | Free | Free | Free retired | documented elsewhere | The Leanstral this model replaced |
| Codestral | $0.30 cache $0.03 | $0.90 | $0.144 paid | documented elsewhere — not on pricing page | Mistral's paid coding model |
| Mistral Medium 3.5 | $1.50 cache $0.15 | $7.50 | $0.962 paid | documented elsewhere — not on pricing page | Frontier agentic and coding work |
| Mistral Small 4 | $0.15 cache $0.015 | $0.60 | $0.0842 paid | 256K | Low-cost reasoning and coding |
| DeepSeek V4.1 Flash | $0.30 cache $0.006 | $1.20 | $0.15 paid | 1M | Budget coding and bulk reasoning |
Frequently asked.
Leanstral 1.5 pricing questions - what free means here, and what it does not promise.
Q · 01 Can I still call Leanstral 1.5? +
labs-leanstral-1-5 on September 29, 2026 and retired it on September 30, naming no replacement - its deprecated and retired models table leaves the alternative column empty. Mistral's API pricing page still listed the model as free on the retirement day, but the lifecycle table is what governs the endpoint. The weights are Apache 2.0, so self-hosting is the remaining route.Q · 02 Was it really free? +
Q · 03 What model IDs did Leanstral use? +
labs-leanstral-1-5 from June 30 to September 30, 2026, on chat completions, agents and conversations. The earlier labs-leanstral-2603 was retired on June 30, 2026. Both ids are now retired.