Senior ML Engineer (AI Research/ Portability)

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 This role is for Nebius AI R&D, a team focused on applied research in AI. Our Portability research aims to make intelligent agent systems work reliably as models, providers, harnesses, skills, memory systems, and deployment environments change. We build and evaluate portable layers that preserve capability, context, identity, provenance, and user control across heterogeneous systems. Research areas include: • Per-turn model routing across quality, cost, latency, capability, cache state, and reliability objectives • Provider and protocol portability across frontier models, open-source models, local inference, and compatible APIs • Agent and harness interoperability, including transferable skills, capability profiles, actions, tools, and trajectories • Portable, user-owned memory and context with scoped identity, provenance, retrieval, feedback, and reviewable compaction • Agent interchange standards, conformance testing, tool and MCP access, and agent-to-agent communication • Agent and harness optimization through evaluation, distillation, customization, and multi-agent learning You will design and build research prototypes and robust systems at the seams between models, providers, and agent runtimes. You will formulate research questions, develop evaluation methods, test ideas in realistic agent workflows, and turn promising results into reusable components. The work will often involve collaboration with adjacent research, infrastructure, security, product, and engineering teams, where findings are validated and applied in practice. We are currently looking for senior- and staff-level ML engineers to work on research in areas such as: • Learned, rule-based, and hybrid model routing, cascading, and candidate-ranking systems • Quality-cost-latency trade-offs, uncertainty estimation, exploration, and outcome-aware routing • Multi-provider gateways, protocol translation, catalog normalization, and fail-closed execution contracts • Portable agent skills, harness capability discovery, package adaptation, and cross-harness conformance • Memory, identity, context, trajectory, and outcome representations that remain portable across agents and models • Retrieval, context selection, context compaction, and feedback systems with explicit provenance and trust boundaries • Agent interoperability standards, including metadata, action formats, plugins, tools, MCP, and agent-to-agent interfaces • Agent optimization, teacher-student distillation, skill generation, harness customization, and multi-agent learning • Benchmarking and evaluation infrastructure for model, router, memory, skill, and harness changes Some examples of what your responsibilities might include are: • Designing, implementing, training, and evaluating model routers that select an appropriate model or reasoning profile for each turn • Developing portable provider and protocol abstractions that preserve authentication, telemetry, cache and context signals, and execution provenance • Defining versioned schemas and contracts for models, provider offers, agents, workspaces, skills, actions, tools, memories, and trajectories • Building systems that discover, package, adapt, and validate agent skills across coding agents, editors, and other harnesses • Researching user-owned memory, scoped identity, trajectory checkpoints, terminal outcomes, retrieval quality, and reviewable context compaction • Creating benchmark suites and evaluation protocols for quality, cost, latency, reliability, safety, and portability • Designing held-out, out-of-domain, and change-impact evaluations that test new or removed models, providers, skills, and harness versions • Investigating distillation, self-improving harnesses, multi-agent training, agent factories, and automated skill creation • Writing robust research software, APIs, integration layers, and test infrastructure that enable rapid but reproducible experimentation • Collaborating across research and engineering teams to translate promising ideas into secure, reversible, and reliable systems • Communicating results through technical reports, demonstrations, open-source releases, benchmarks, and research publications We expect you to have: • A profound understanding of machine learning, large language models, or statistical decision-making • Deep expertise in at least one relevant area, such as model routing, recommender systems, agent systems, retrieval and memory, model evaluation, distributed systems, or protocol and API design • Experience building and evaluating modern language-model or agentic systems, including tool use and multi-turn workflows • Experience designing, executing, and analyzing machine learning experiments with appropriate statistical rigor • Ability to formulate meaningful research questions, design experiments that test clear hypotheses, and draw defensible conclusions • Understanding of evaluation leakage, held-out testing, out-of-domain generalization, uncertainty, and reproducibility • Strong software-engineering and algorithm-design skills; excellent Python skills and the ability to work across production systems • Experience with APIs, data schemas, distributed services, testing, observability, code review, and CI/CD • Ability to reason about security, privacy, provenance, permissions, failure modes, and user control in agent systems • Experience implementing research ideas and iterating quickly across modeling, data, systems, and evaluation • Strong communication and technical leadership abilities, including collaboration across research and engineering disciplines and clear documentation of findings in technical reports or research publications Nice to have: • Experience with model routers, cascades, mixture-of-experts systems, recommenders, or cost-aware inference • Experience integrating multiple model providers or inference stacks, including OpenAI-compatible APIs, Anthropic-style APIs, local inference, or open-source serving systems • Familiarity with agent harnesses, coding agents, editor integrations, function calling, tool execution, MCP, or agent-to-agent protocols • Experience with retrieval systems, vector search, knowledge graphs, temporal data, memory architectures, or context management • Experience with benchmark suites for coding, reasoning, factuality, instruction following, tool use, or multi-turn agent workflows • Experience with teacher-student distillation, reinforcement learning, preference learning, reward modeling, or automated skill generation • Proficiency in TypeScript, Go, Rust, or another systems language in addition to Python • Experience with secure authentication, sandboxing, privacy-preserving telemetry, provenance, or policy-enforced execution • Experience building distributed data-processing, evaluation, model-training, or inference systems • A PhD in Computer Science, Machine Learning, Artificial Intelligence, or a related technical field, or equivalent practical experience • A track record of impactful publications, open-source contributions, or deployed AI systems • A record of building and delivering products or research prototypes in a dynamic, startup-like environment • Passion for making advanced AI systems composable, inspectable, user-controlled, and resilient to changing models and platforms • Excellent command of English, with strong technical writing, presentation, and communication skills • Proficiency in contemporary software-engineering practices, including version control, testing, code review, and CI/CD 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.