Senior Machine Learning Engineer, Model Training and Reinforcement Learning

About Nebius: Nebius is leading a new era in cloud infrastructure for the global AI economy. We are building a full-stack AI cloud platform that supports developers and enterprises from data and model training through to production deployment, without the cost and complexity of building large in-house AI/ML infrastructure. Built by engineers, for engineers. From large-scale GPU orchestration to inference optimization, we own the hard problems across compute, storage, networking and applied AI. Listed on Nasdaq (NBIS) and headquartered in Amsterdam, we have a global footprint with R&D hubs across Europe, the UK, North America and Israel. Our team of 1,500+ includes hundreds of engineers with deep expertise across hardware, software and AI R&D. The role Nebius Token Factory is building an AI training and model post-training capability for frontier model improvement. This role owns the infrastructure that makes large-scale training and RL experiments possible, reliable, reproducible, and efficient. The work sits at the intersection of distributed systems, GPU performance, model training frameworks, RL pipelines, and production engineering. A Senior Machine Learning Engineer owns substantial ML work end to end. They can translate an ambiguous capability goal into concrete experiments, implement and debug training and RL recipes, build the supporting data and systems, and deliver measurable improvements in model quality, experiment throughput, and reliability. They are deeply hands-on and can independently debug both model-behavior failures and distributed training failures. Your responsibilities: • Design and run model-training and post-training experiments, including SFT, continued pretraining, preference optimization (DPO/IPO/KTO), and RL methods such as RLHF/RLAIF, PPO, and GRPO. • Build reward functions, judge models, verifiers, task environments, and evaluation sets for reasoning, coding, tool use, and agentic workflows. • Create synthetic data and data pipelines, including teacher-student generation, self-play, rejection sampling, filtering, and quality scoring. • Analyze model-behavior failures and turn them into targeted data, reward, or algorithm improvements. • Build and maintain distributed training and RL infrastructure using frameworks such as Megatron-LM, DeepSpeed, PyTorch FSDP/DTensor, Ray, verl, slime, AReaL, or OpenRLHF. • Implement and debug parallelism strategies (tensor, pipeline, sequence/context, expert, and data parallelism) and build reliable rollout, reward-serving, checkpointing, and experiment-orchestration components. • Profile and improve GPU utilization, memory usage, communication efficiency, training throughput, and inference/serving performance. • Design rigorous evaluations and ablations for capability, instruction following, reasoning, tool use, safety, and regression risk. • Write clear experiment plans, design docs, benchmark reports, and runbooks, and partner across research and platform teams. Must-haves: • Strong Python and PyTorch engineering skills, with the ability to move quickly from idea to experiment to working system. • Hands-on experience across at least two of: model training, post-training/RL, applied modeling, data pipelines, or large-scale ML systems. • Ability to design rigorous experiments with baselines, ablations, metrics, and failure analysis. • Practical understanding of modern LLM behavior, instruction tuning, preference optimization, and evaluation challenges. • Practical understanding of transformer training bottlenecks, memory pressure, communication overhead, and checkpointing. • Ability to reason quantitatively about model quality, throughput, utilization, reliability, cost, and research velocity. • Strong communication skills and ability to collaborate with researchers, engineers, and leadership. Nice-to-haves: • Experience with LLM post-training, RL, agents, reward modeling, synthetic data, or model evaluation. • Experience with RL frameworks or pipelines such as verl, slime, AReaL, OpenRLHF, TRL, or custom PPO/GRPO/RLHF systems. • Experience with Megatron-LM, DeepSpeed, PyTorch FSDP/DTensor, Ray, Slurm, or Kubernetes on large GPU clusters. • Familiarity with NCCL, CUDA, Triton, Nsight, InfiniBand/RDMA, and H100/H200/B200 clusters, or with model serving and inference optimization. • Publications, open-source contributions, or production impact in LLM post-training, RL, reasoning, coding models, synthetic data, distributed training, or evaluation. • Experience designing agent environments, tool-use tasks, or verifier-based rewards. Key employee benefits in the US: • Health insurance: 100% company-paid medical, dental, and vision coverage for employees and families. • 401(k) plan: Up to 4% company match with immediate vesting. • Parental leave: 20 weeks paid for primary caregivers, 12 weeks for secondary caregivers. • Remote work reimbursement: Up to $85/month for mobile and internet. • Disability & life insurance: Company-paid short-term, long-term and life insurance coverage. Pay Transparency We offer competitive compensation and benefits packages. Actual compensation will be determined based on job-related factors, including experience, skills, qualifications, the level at which the candidate is hired, and geographic location, consistent with applicable law. Base Compensation Range $195,200—$262,200 USD Benefits & Perks: • Competitive compensation • Career growth and learning opportunities • Flexibility and ownership • Collaborative and innovative culture • Opportunity to work on impactful AI projects • International environment and talented teams What's it like to work at Nebius: Fast moving - Bold thinking - Constant growth - Meaningful impact - Trust and real ownership - Opportunity to shape the future of AI Equal Opportunity Statement: Nebius is an equal opportunity employer. We are committed to fostering an inclusive and diverse workplace and to providing equal employment opportunities in all aspects of employment. We do not discriminate on the basis of race, color, religion, sex (including pregnancy), national origin, ancestry, age, disability, genetic information, marital status, veteran status, sexual orientation, gender identity or expression, or any other characteristic protected by applicable law. Applicants must be authorized to work in the country in which they apply and will be required to provide proof of employment eligibility as a condition of hire. If you need accommodations during the application process, please let us know.