Senior Research Engineer, LLM Training & Post-Training

Who We Are Lightning AI is the company behind PyTorch Lightning. Founded in 2019, we build an end-to-end platform for developing, training, and deploying AI systems-designed to take ideas from research to production with less friction. Through our merger with Voltage Park, a neocloud and AI Factory, Lightning AI combines developer-first software with cost-efficient, large-scale compute. Teams get the tools they need for experimentation, training, and production inference, with security, observability, and control built in. We serve solo researchers, startups, and large enterprises. Lightning AI operates globally with offices in New York City, San Francisco, Seattle, and London, and is backed by Coatue, Index Ventures, Bain Capital Ventures, and Firstminute. The Way We Work The people who thrive here are builders who move fast, communicate openly, take ownership, and continuously improve themselves, their teams, and our company. Here's what that looks like in practice: Move with Urgency: We move quickly, make thoughtful decisions, and keep momentum. We value action over perfection and learn by shipping.Take Ownership: We own outcomes, not just our individual work. We make decisions that move the company forward and follow through.Communicate Openly: We communicate directly, seek to understand, and create clarity for others. Honest conversations help us move faster together.Build Great Teams: We lead by example, empower others, and create healthy teams where people can do their best work.Raise the Bar: We're always improving ourselves. We learn from feedback, consistently challenge ourselves to grow, and focus on the work that matters most.Think Long-Term: We design for what's next. We create scalable systems, simplify complexity, and use AI and automation to amplify our impact. What We're Looking For We're are looking for an experienced Senior Research Engineer who has built, trained, and optimized modern transformer-based language models to join our Research Engineering function at Lightning. This role will focus on advancing how large language models are trained, fine-tuned, evaluated, and deployed across Lightning AI's platform and real-world customer workloads. It will work across model training, post-training, PyTorch, distributed systems, and AI systems engineering to improve model quality, training efficiency, and developer productivity while collaborating closely with researchers, infrastructure engineers, and customers. We're looking for someone who enjoys turning cutting-edge research into production systems. You have deep experience training and improving transformer-based language models, strong software engineering fundamentals, and a passion for solving difficult problems across model training, evaluation, and AI systems. Rather than building applications on top of existing models, you're motivated by improving the models themselves and the systems that power them. Our work spans models that power the Lightning AI platform, customer-specific model workloads, and research that translates into reusable training and platform capabilities. This role is hybrid with a minimum of 2 in-office days per week in San Francisco, Seattle, NYC, or London, with fully remote work considered for candidates outside of our office hub locations. All employees participate in occasional team and company offsites. What You'll Do Design, build, and optimize training and post-training pipelines for large language models.Improve model quality through supervised fine-tuning, continued pretraining, preference optimization, reinforcement learning, evaluation, and experimentation.Build and improve PyTorch-based training infrastructure, tooling, and developer workflows.Optimize distributed training across multi-GPU environments by improving throughput, memory efficiency, scalability, and GPU utilization.Investigate model training issues, including convergence, instability, communication overhead, and performance bottlenecks.Design evaluation methodologies, benchmark models, analyze failure modes, and acheive model improvements through experimentation.Collaborate directly with customers to understand real-world workloads and translate those learnings into improvements across Lightning AI's research platform.Partner closely with research, infrastructure, and platform engineering teams to build production-ready AI systems.Contribute to open-source projects through new features, tooling improvements, documentation, and community engagement What You'll Need Required Qualifications Significant experience training, fine-tuning, evaluating, and/or optimizing transformer-based language models using PyTorch.Experience with modern LLM training and post-training techniques such as continued pretraining, SFT, RLHF, preference optimization (DPO, PPO, GRPO), reward modeling, or similar approaches.Strong understanding of distributed training and multi-node systems, with experience improving training performance, scalability, and/or