Mid-Senior Backend Engineer (ML integrations)

Cleo; City of London; Hybrid; GBP 65,000 - 85,000; Full time

Job summary

A dynamic tech startup is looking for a Mid-Senior Backend Engineer to integrate ML-driven models into their Ruby on Rails monolith. You’ll work closely with ML engineers and PMs to enhance decision-making systems, focusing on observability and reliability. The ideal candidate is proficient in Ruby and has experience with production systems that interface with ML models. Offering competitive compensation, flexible work options in London, and a supportive growth environment.

Benefits

Competitive compensation package

6% employer-matched pension

Private medical/dental/life insurance

Enhanced parental leave

Regular socials

Mental health support

Qualifications

• Proficient in Ruby with experience in building/maintaining production systems.
• Experience with integrating ML models into production environments.
• Focus on observability and reliable deployments in production.

Responsibilities

• Integrate ML-driven credit models into Ruby on Rails.
• Work closely with ML engineers and PMs to expose model outputs.
• Contribute to improving the Ruby monolith.

Skills

Ruby

ML models

Production systems

Data pipelines

Python

Tools

PostgreSQL

Heroku

React Native

CircleCI

Job description

Overview

Mid-Senior Backend Engineer (ML integrations) at Cleo. You’ll join the team responsible for building and maintaining the decisioning systems that power Cleo’s core product. This is a business-critical domain directly driving user experience, growth, and profitability.

You’ll focus on integrating ML-driven credit and eligibility models into our Ruby on Rails monolith. Models are written in Python by ML engineers and data scientists and 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 stakeholders have the tooling to understand how decisions are made. You’ll also be part of the Ruby chapter, improving the Ruby monolith for users and fellow engineers.

What We’re Looking For / Responsibilities

• Backend engineer comfortable with technically complex domains, particularly those involving machine learning, financial decisioning, or large-scale data systems.
• Proficient in Ruby and experienced building/maintaining production systems that interface with ML models, data pipelines, or high-throughput decision engines. Exposure to Python and ML concepts is a plus; deep ML knowledge isn’t required.
• Focus on observability, reliability, deployment best practices, and robust, maintainable integrations that operate correctly in production.
• Data-aware, able to reason about data lifecycle, ownership, transformations, and downstream consumer needs. Strong cross-functional collaboration with product managers, analysts, and ML engineers.

Our Tech Stack

Cleo is built as a Ruby on Rails monolith with a single React Native app frontend (TypeScript). We also use Python for ML services and PostgreSQL on AWS. CI/CD is fully automated; production deployments occur on every merge via Heroku. Backend engineers deploy multiple times a week; frontend releases to Google/Apple weekly.

We emphasize quality: code is peer-reviewed; automated testing uses Minitest and CircleCI. We’re working toward a more modular architecture while maintaining a monolith, with ongoing refactoring as features are built. Everyone contributes to driving our technical strategy, and all voices are valued.

What You Get / Benefits

• A competitive compensation package (base + equity) with bi-annual reviews; progression framework and salary bands available publicly.
• Work at a fast-growing tech startup backed by Balderton & EQT Ventures.
• Clear progression plan with support for growth, leadership, and impact.
• Flexibility and healthy work-life balance; distributed team with hybrid options in London and travel coverage for in-person collaboration.
• Other benefits include: 6% employer-matched pension (UK), private medical/dental/life insurance, enhanced parental leave, sabbatical after 4 years, regular socials, OpenAI subscription, mental health support, and 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.

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 for a while. This shift focuses on regions where we can provide the most value while learning to support UK users in the future.