Machine Learning Engineer – Lumen Enterprise Models (SWE-focused LLMs)

Job Title: Machine Learning Engineer – Lumen Enterprise Models (SWE-focused LLMs) Company: Cosine UK Job Category: Machine Learning Roles & Responsibilities: Our Lumen Enterprise models are designed to work across real codebases. They need to understand unfamiliar architectures, make changes across multiple files, use tools, run tests and produce software that engineers can maintain and trust. You’ll work across the full training stack, including pre\-training, supervised fine\-tuning and reinforcement learning. This is a truly unique role that will provide exposure across all areas of the model training lifecycle. WE’re looking for someone who is just as happy to profile the training loop, identifying performance bottlenecks and memory leaks, as implementing new RL algorithms. This is a hands\-on role for someone who enjoys working close to the model and cares about the quality of the software it produces. The role ------------ * What should Lumen learn next? * Which data and environments will teach it most effectively? * How should we reward correct, maintainable and useful software? * How do we train models to work across large repositories and long contexts? * How do we know when a model is genuinely better for engineers? Why this role matters ------------------------- * Work across the full training lifecycle, running pre\-training, supervised fine\-tuning and reinforcement\-learning experiments. * Optimize training methods at scale on frontier models. * Diagnose and fix performance bottlenecks and communication overhead in distributed training environments. * Train models on long\-context, multi\-file and repository\-level tasks. * Develop RL and RLVR loops using signals from execution, tests, linters, static analysis and other engineering feedback, turning fresh ideas into scalable, reproducible code. * Investigate model failure modes such as poor planning, brittle fixes, reward hacking and regressions. * Collaborate with infrastructure, platform and data teams. If this sounds like a fit, this is a role where you can meaningfully push the frontier of open\-source–based software engineering models. This role won’t suit everyone. But if you’re looking for a fast\-moving environment, close collaboration, and a team that takes both the work and work\-life balance seriously, it’s a great place to be. Work Location: London, ENG, UK Availability: Full time

Compensation: Salary and Equity options available for discussion We encourage applications from anyone who meets the role requirements, even if you don’t meet every single qualification. If you need reasonable adjustments at any stage of the hiring process, we’re happy to discuss them. Compensation, Benefits \& Ways of Working --------------------------------------------- We’re an in\-office team, five days a week, by design. We believe the work we’re doing benefits from being together, collaborating closely, and building shared context. * Competitive salary, benchmarked to the market * Equity / share options, so you share in the upside you help create * 30 days’ holiday \+ bank holidays * Work hard in the office, collaborate closely, and switch off properly * Dog\-friendly office — bring your dog to work * Weekly team breakfast \& lunch * Monthly socials * Pension * High\-quality equipment to do your best work Key Skills: Machine Learning, PyTorch, Model Training Salary: Not disclosed