Senior Machine Learning Engineer

🌍 UK or Poland (Remote or Hybrid, it's up to you!) 💰 Dependent on experience 📈 Be part of our success with the opportunity to join our company equity scheme Our Perks ❤️ Monthly Health & Wellness budget, increasing with length of service 📚 Annual Learning and Development budget, increasing with length of service 🤸🏽‍♂️ Flexible working in a choice-first environment - we trust the way you want to work! 🖥️ Work From Home Allowance 🌴 25 Holiday Days + your local bank holidays, plus an extra day for every year of service 🎂 Your birthday off 🍼 Enhanced Family Leave (UK Only), Fertility Leave, and Neonatal Leave ⚕️ Optional Healthcare Plan 🛟 Life & income protection (Location dependent) 🤝 Employee Assistance Programme (UK Only) 📈 The opportunity to share in the company's success through options 🌆 If you're in London, a dog-friendly office with great classes, events, and a rooftop terrace 🦸‍♀️ The Role 🦸‍♀️ Our mission is to help large successful brands like Uber, Amazon, Wise, HelloFresh (and more!) put their customers at the centre of everything they do. Using best-in-class tech in a fast-developing AI space, our Customer Experience Intelligence platform continuously analyses explicit and implicit feedback to enable our clients to identify what they should do next. We're hiring a Senior Machine Learning Engineer to join the team and help build and ship the next generation of that stack. 👉 What You'll Be Doing Unlike many companies, we use our own custom models, specialised for customer feedback, across various parts of the stack: extraction, retrieval, reranking, summarisation, and sentiment analysis. We are also pragmatic and understand that the right solution can be a combination of off-the-shelf LLMs, bespoke fine-tuned models, and sometimes techniques that utilise no LLM at all. This Means You Will

• Train, evaluate, and iterate on ML models and agentic systems for customer feedback, including owning our custom fine-tuning pipelines. Run experiments end-to-end, track results rigorously, and make clear recommendations on what to ship, iterate, or retire.
• Build and maintain LLM-powered features: retrieval pipelines, reranking systems, insight agents, data mining agents, and automated taxonomy generation.
• Design and run robust evaluation frameworks: build test sets, define metrics, evaluate non-deterministic systems, handle class imbalance, and automate checkpoint comparisons.
• Improve and extend semantic search and retrieval, evolving from embedding-based approaches toward more advanced methods.
• Write production-quality code and collaborate closely with Engineering on productionisation, model serving, data pipelines, and monitoring.
• Work with Product and Commercial teams to translate business needs into practical ML solutions, and support client evaluations and accuracy benchmarking.
• Mentor team members, review code and research, and bring relevant advances from the literature into the product. 🧰 What You'll Need
• A deep working knowledge of transformer architectures.
• Strong PyTorch skills, with the ability to write custom training loops, modify model architectures, and debug issues at the tensor level. Ideally, experience with parameter-efficient fine-tuning techniques such as LoRA
• Extensive experience working with large-scale, messy real-world text data, including classification, extraction, embeddings, re-rankers, clustering, and search.
• Experience in instruction fine-tuning and serving language models, familiarity with frameworks such as vLLM, DeepSpeed, or similar tools
• A solid grounding in classical ML and statistics, and the judgement to choose simpler methods when they're the right solution.
• Practical experience building with GenAI and agentic patterns.
• Excellent communication skills and confidence translating complex technical concepts for non-technical audiences (and vice versa!).
• Technical curiosity and a keen interest in AI – a love of experimenting to make the most of available technology.
• High ownership and initiative, with the ability to identify problems, prioritise effectively, and drive solutions forward. ➕It Would Be a Bonus If You
• MSc/PhD in Computer Science, Machine Learning, Artificial Intelligence, Data Science, Computational Linguistics or a closely related STEM field.
• Experience with reinforcement learning techniques, such as with verifiable reward (RLVR) 🔎 Our Hiring Process
• Let's introduce ourselves – you'll complete an introductory asynchronous interview - we'd love to learn more about you, your ambitions, and what you're looking for in your next step.
• Get to know your would-be manager – you'll have a call with Aji, our Chief Scientist, to learn more about the role and show off your experience.
• Show us how you work – you'll complete a short take home assignment
• Get to know your would-be team – You'll meet a mix of people who you'll be working closely with from the Data Science, Engineering and Product teams.
• How our values and your career goals align – you'll have a call with our cofounder to learn more about life at Chattermill and ensure we're the right place for your next stage of growth. 💖 Our Values
• We are obsessed with experience – We take our mission to rid the world of bad Customer Experience seriously, and we practice what we preach.
• We believe in the power of trust – Whether it's with each other, our customers, partners, or other stakeholders, we always communicate with openness and trust.
• We act as responsible owners – Whether it's about the company, a team, a project, or a task, having the freedom to make decisions in our area of responsibility is a crucial driver for us.
• We share a passion for growth & progress – On every level, we're motivated by taking on new challenges – even if they seem out of reach. We recognise that we are learning machines and we always seek to action feedback and improve collectively.
• We set our ambitions high but stay humble – We've come together to build a product and a category that's never been seen before. While we're an ambitious bunch with lofty goals, we don't approach this goal carelessly.
• We believe the right team is the key to success – At Chattermill we've learned that all our important achievements have been the result of the right people collaborating together – that's why we need you to apply today! 🌈 Diversity & Inclusion 🌈