StepFun: Step 3.5 Flash

StepFun's Step 3.5 Flash, released on January 29, 2026, is a highly efficient open-source foundation model leveraging a sparse Mixture of Experts (MoE) architecture. It consistently ranks in the top tier for speed, performing in the 62nd percentile across nine benchmarks, and offers exceptionally competitive pricing, placing in the 81st percentile across eight benchmarks. The model demonstrates outstanding reliability with a 99% success rate, indicating minimal technical failures. Step 3.5 Flash exhibits perfect accuracy (100%) across a remarkable seven out of nine benchmarks, including Hallucinations, Instruction Following, General Knowledge, Email Classification, Reasoning, Ethics, and Mathematics. This highlights its robust capabilities in critical areas such as understanding uncertainty, complex instruction execution, and logical problem-solving. Notably, it achieves perfect accuracy in these categories while often being the most accurate model at its price point and speed. While its Coding performance is strong, achieving 94.9% and 91.9% accuracy, the duration for one Coding benchmark was significantly higher (33,404,092ms), suggesting potential variability or specific challenges in certain coding tasks. Overall, Step 3.5 Flash stands out for its exceptional accuracy, cost-efficiency, and speed, making it a powerful and reliable choice for a wide range of applications.