Step 3.5 Flash: Open Frontier-Level Intelligence with 11B Active Parameters
Authors: name: Ailin Huang, name: Ang Li, name: Aobo Kong, name: Bin Wang, name: Binxing Jiao, name: Bo Dong, name: Bojun Wang, name: Boyu Chen, name: Brian Li, name: Buyun Ma, name: Chang Su, name: Changxin Miao, name: Changyi Wan, name: Chao Lou, name: Chen Hu, name: Chen Xu, name: Chenfeng Yu, name: Chengting Feng, name: Chengyuan Yao, name: Chunrui Han, name: Dan Ma, name: Dapeng Shi, name: Daxin Jiang, name: Dehua Ma, name: Deshan Sun, name: Di Qi, name: Enle Liu, name: Fajie Zhang, name: Fanqi Wan, name: Guanzhe Huang, name: Gulin Yan, name: Guoliang Cao, name: Guopeng Li, name: Han Cheng, name: Hangyu Guo, name: Hanshan Zhang, name: Hao Nie, name: Haonan Jia, name: Haoran Lv, name: Hebin Zhou, name: Hekun Lv, name: Heng Wang, name: Heung-Yeung Shum, name: Hongbo Huang, name: Hongbo Peng, name: Hongyu Zhou, name: Hongyuan Wang, name: Houyong Chen, name: Huangxi Zhu, name: Huimin Wu, name: Huiyong Guo, name: Jia Wang, name: Jian Zhou, name: Jianjian Sun, name: Jiaoren Wu, name: Jiaran Zhang, name: Jiashu Lv, name: Jiashuo Liu, name: Jiayi Fu, name: Jiayu Liu, name: Jie Cheng, name: Jie Luo, name: Jie Yang, name: Jie Zhou, name: Jieyi Hou, name: Jing Bai, name: Jingcheng Hu, name: Jingjing Xie, name: Jingwei Wu, name: Jingyang Zhang, name: Jishi Zhou, name: Junfeng Liu, name: Junzhe Lin, name: Ka Man Lo, name: Kai Liang, name: Kaibo Liu, name: Kaijun Tan, name: Kaiwen Yan, name: Kaixiang Li, name: Kang An, name: Kangheng Lin, name: Lei Yang, name: Liang Lv, name: Liang Zhao, name: Liangyu Chen, name: Lieyu Shi, name: Liguo Tan, name: Lin Lin, name: Lina Chen, name: Luck Ma, name: Mengqiang Ren, name: Michael Li, name: Ming Li, name: Mingliang Li, name: Mingming Zhang, name: Mingrui Chen, name: Mitt Huang, name: Na Wang, name: Peng Liu, name: Qi Han, name: Qian Zhao, name: Qinglin He, name: Qinxin Du, name: Qiuping Wu, name: Quan Sun, name: Rongqiu Yang, name: Ruihang Miao, name: Ruixin Han, name: Ruosi Wan, name: Ruyan Guo, name: Shan Wang, name: Shaoliang Pang, name: Shaowen Yang, name: Shengjie Fan, name: Shijie Shang, name: Shiliang Yang, name: Shiwei Li, name: Shuangshuang Tian, name: Siqi Liu, name: Siye Wu, name: Siyu Chen, name: Song Yuan, name: Tiancheng Cao, name: Tianchi Yue, name: Tianhao Cheng, name: Tianning Li, name: Tingdan Luo, name: Wang You, name: Wei Ji, name: Wei Yuan, name: Wei Zhang, name: Weibo Wu, name: Weihao Xie, name: Wen Sun, name: Wenjin Deng, name: Wenzhen Zheng, name: Wuxun Xie, name: Xiangfeng Wang, name: Xiangwen Kong, name: Xiangyu Liu, name: Xiangyu Zhang, name: Xiaobo Yang, name: Xiaojia Liu, name: Xiaolan Yuan, name: Xiaoran Jiao, name: Xiaoxiao Ren, name: Xiaoyun Zhang, name: Xin Li, name: Xin Liu, name: Xin Wu, name: Xing Chen, name: Xingping Yang, name: Xinran Wang, name: Xu Zhao, name: Xuan He, name: Xuanti Feng, name: Xuedan Cai, name: Xuqiang Zhou, name: Yanbo Yu, name: Yang Li, name: Yang Xu, name: Yanlin Lai, name: Yanming Xu, name: Yaoyu Wang, name: Yeqing Shen, name: Yibo Zhu, name: Yichen Lv, name: Yicheng Cao, name: Yifeng Gong, name: Yijing Yang, name: Yikun Yang, name: Yin Zhao, name: Yingxiu Zhao, name: Yinmin Zhang, name: Yitong Zhang, name: Yixuan Zhang, name: Yiyang Chen, name: Yongchi Zhao, name: Yongshen Long, name: Yongyao Wang, name: Yousong Guan, name: Yu Zhou, name: Yuang Peng, name: Yuanhao Ding, name: Yuantao Fan, name: Yuanzhen Yang, name: Yuchu Luo, name: Yudi Zhao, name: Yue Peng, name: Yueqiang Lin, name: Yufan Lu, name: Yuling Zhao, name: Yunzhou Ju, name: Yurong Zhang, name: Yusheng Li, name: Yuxiang Yang, name: Yuyang Chen, name: Yuzhu Cai, name: Zejia Weng, name: Zetao Hong, name: Zexi Li, name: Zhe Xie, name: Zheng Ge, name: Zheng Gong, name: Zheng Zeng, name: Zhenyi Lu, name: Zhewei Huang, name: Zhichao Chang, name: Zhiguo Huang, name: Zhiheng Hu, name: Zidong Yang, name: Zili Wang, name: Ziqi Ren, name: Zixin Zhang, name: Zixuan Wang
Organization: StepFun

We introduce Step 3.5 Flash, a sparse Mixture-of-Experts (MoE) model that bridges frontier-level agentic intelligence and computational efficiency. We focus on what matters most when building agents: sharp reasoning and fast, reliable execution. Step 3.5 Flash pairs a 196B-parameter foundation with 11B active parameters for efficient inference. It is optimized with interleaved 3:1 sliding-window/full attention and Multi-Token Prediction (MTP-3) to reduce the latency and cost of multi-round agentic interactions. To reach frontier-level intelligence, we design a scalable reinforcement learning framework that combines verifiable signals with preference feedback, while remaining stable under large-scale off-policy training, enabling consistent self-improvement across mathematics, code, and tool use. Step 3.5 Flash demonstrates strong performance across agent, coding, and math tasks, achieving 85.4% on IMO-AnswerBench, 86.4% on LiveCodeBench-v6 (2024.08-2025.05), 88.2% on tau2-Bench, 69.0% on BrowseComp (with context management), and 51.0% on Terminal-Bench 2.0, comparable to frontier models such as GPT-5.2 xHigh and Gemini 3.0 Pro. By redefining the efficiency frontier, Step 3.5 Flash provides a high-density foundation for deploying sophisticated agents in real-world industrial environments.