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:

• 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:

• Complete an introductory asynchronous interview.
• Have a call with Aji, our Chief Scientist, to learn more about the role.
• Complete a short take home assignment.
• Meet a mix of people from the Data Science, Engineering and Product teams.
• Have a call with our cofounder to learn more about life at Chattermill.

Our Values:

• We are obsessed with experience.
• We believe in the power of trust.
• We act as responsible owners.
• We share a passion for growth & progress.
• We set our ambitions high but stay humble.
• We believe the right team is the key to success.

Diversity & Inclusion:

We want to enable exceptional experiences for everyone, and to achieve this we need everyone’s voice in our team. We are on a mission to bring more diversity into the business and to give everyone (from all backgrounds and abilities) a chance to join us, even if they may not fit all of the requirements set out in this job spec.