LLM Comparison
Leanstral 1.5 vs Step 3.5 Flash
Side-by-side specs, pricing & capabilities · Updated August 2026
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2/6 modelsSame tier:
| Organization | ||
| OpenTools Score | ||
| Family | Leanstral | Step |
| Status | Current | Current |
| Release Date | Jun 2026 | Jan 2026 |
| Context Window | 256K tokens | 262K tokens |
| Input Price | Free | $0.10/M tokens |
| Output Price | Free | $0.30/M tokens |
| Pricing Notes | Mistral model card lists Leanstral 1.5 price as $0. Changelog says the Labs endpoint is scheduled to retire on 2026-09-30. | — |
| Capabilities | textreasoningcodingformal-verificationtool-usestructured-output | textcode |
| Max Output | 128K tokens | 66K tokens |
| API Identifier | labs-leanstral-1-5 | stepfun/step-3.5-flash |
| Benchmarks | ||
| miniF2F | 100mistral | — |
| PutnamBench | 587mistral | — |
| FLTEval pass@8 | 43.2mistral | — |
| View Leanstral 1.5 | View Step 3.5 Flash | |
Cost Calculator
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| Model | Input | Output | Total / mo | vs Best |
|---|---|---|---|---|
| Leanstral 1.5Cheapest | $0.00 | $0.00 | $0.00 | — |
| Step 3.5 Flash | $0.10 | $0.15 | $0.25 | +0% |
Mistral AI
Leanstral 1.5
Leanstral 1.5 is Mistral AI free Lean 4 formal proof-engineering model for automated theorem proving, autoformalization, and real-world code verification. Official Mistral docs list it as a Labs model with 119B total parameters, 6.5B active parameters, a 256k context window, 128k max output, $0 pricing, and the labs-leanstral-1-5 API identifier.
StepFun
Step 3.5 Flash
Step 3.5 Flash is a large language model from StepFun. Supports up to 262,144 token context window. Available from $0.10/M input tokens.
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