Mid-Senior Backend Engineer (ML integrations)

About Cleo

At Cleo, we're not just building another fintech app. We're embarking on a mission to fundamentally change humanity's relationship with money. Imagine a world where everyone, regardless of background or income, has access to a hyper-intelligent financial advisor in their pocket. That's the future we're creating.

Cleo is a rare success story: a profitable, fast-growing unicorn with over $200 million in ARR and growing over 2x year-over-year. This isn't just a job; it's a chance to join a team of brilliant, driven individuals who are passionate about making a real difference. We have an exceptionally high bar for talent, seeking individuals who are not only at the top of their field but also embody our culture of collaboration and positive impact.

If you’re driven by complex challenges that push your expertise, the chance to shape something truly transformative, and the potential to share in Cleo’s success as we scale, while growing alongside a company that’s scaling fast, this might be your perfect fit.

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About the Role

You’ll join the team responsible for building and maintaining the decisioning systems that power Cleo’s core product. This is one of the most business-critical domains at Cleo and one of our most impactful - directly driving user experience, growth, and profitability.

Your primary focus will be integrating ML-driven credit and eligibility models into our Ruby on Rails monolith. These models, written in Python by ML engineers and data scientists, require robust, scalable integration patterns; you’ll own the productionisation of those services: from data ingestion and interface design to monitoring, alerting, and runtime observability.

You’ll work closely with ML engineers, analysts, and PMs to expose model outputs cleanly, help debug behavioral anomalies, and ensure that stakeholders have the tooling they need to understand how decisions are made.

You’ll also be part of our wider Ruby chapter, working across our Ruby monolith to improve it for both our users as well as your fellow engineers.

What We’re Looking For

You’re a backend engineer who’s comfortable working in technically complex domains - particularly those involving machine learning, financial decisioning, or large-scale data systems.

You're proficient in Ruby and have experience building and maintaining production systems that interface with machine learning models, data pipelines, or high-throughput decision engines. Exposure to Python and ML concepts is a plus, but deep ML knowledge isn’t required.

You care about observability, system reliability, and deployment best practices. You know how to make integrations robust and maintainable, and how to ensure they operate correctly in production over time.

You’re data-aware - able to reason about data lifecycle, ownership, transformations, and how downstream consumers rely on it. You’re also a strong cross-functional collaborator, able to work closely with product managers, analysts and ML engineers to debug issues and iterate quickly.

Our Tech Stack

Cleo is built as a Ruby on Rails monolith with a single React Native app frontend, utilising TypeScript. We also leverage Python for machine learning services and PostgreSQL for our database, all hosted on AWS. Our CI/CD pipeline is fully automated, with production deployments happening on every merge via Heroku. Our backend engineers deploy multiple times a week, and we release our frontend app to Google and Apple for review at least once a week.

While we take a pragmatic approach, we place a strong emphasis on quality. Our code is peer-reviewed, and we maintain automated testing using Minitest and CircleCI. We're also actively working towards a more modular architecture, focusing on separating concerns to achieve all the benefits of microservices within a monolith, while progressively refactoring our code as we build new features. Everyone in the engineering team contributes to driving our technical strategy, voices & ideas from all levels are valued: we are all owners at Cleo.

What do you get for all your hard work?

• A competitive compensation package (base + equity) with bi-annual reviews, aligned to our quarterly OKR planning cycles. You can view our public progression framework and salary bandings here: https://cleo-ai.progressionapp.com/
• Work at one of the fastest-growing tech startups, backed by top VC firms, Balderton & EQT Ventures
• A clear progression plan. We want you to keep growing. That means trying new things, leading others, challenging the status quo and owning your impact. Always with our complete support.
• Flexibility. We can’t fight for the world’s financial health if we’re not healthy ourselves. We work with everyone to make sure they have the balance they need to do their best work
• Work where you work best. We’re a globally distributed team. If you live in London we have a hybrid approach, we’d love you to spend one day a week or more in our beautiful office. If you’re outside of London, we’ll encourage you to spend a couple of days with us a few times per year. And we’ll cover your travel costs, naturally.
• Other benefits;
• Company-wide performance reviews every 6 months
• Generous pay increases for high-performing team members
• Equity top-ups for team members getting promoted
• 25 days annual leave a year + public holidays (+ an additional day for every year you spend at Cleo, up to 30 days)
• 6% employer-matched pension in the UK
• Private Medical Insurance via Vitality, dental cover, and life assurance
• Enhanced parental leave
• 1 month paid sabbatical after 4 years at Cleo
• Regular socials and activities, online and in-person
• We'll pay for your OpenAI subscription
• Online mental health support via Spill
• Workplace Nursery Scheme
• And many more!

We strongly encourage applications from people of colour, the LGBTQ+ community, people with disabilities, neurodivergent people, parents, carers, and people from lower socio-economic backgrounds.

If there’s anything we can do to accommodate your specific situation, please let us know.

UK App access: The Cleo app is no longer downloadable in the UK (but only until next year). If you’re an existing user, you’ll still have access to the app. But some features won’t be available (just for a little while). Why? 99% of our users are based in the US – where financial health is often overlooked. We’ve decided to shift our focus to where we can provide the most value and make the greatest impact for users who need it most. Then we’ll be able to apply what we learn to better support our UK users in the future.