Full Stack Software Developer - AI Agent Application in London

We are looking for a talented and energetic individual to join the AI Product team in DC Pensions at Mercer UK. This is a UK-based role with flexible working — we can support either hybrid or office-based arrangements.

The AI Product team is a small, UK-based, forward-deployed AI, tech and development team responsible for the Pensions AI Assistant — a customer-facing AI Agent. We are nimble, innovative and move quickly, using the latest technologies available to us within the group. Our tech stack includes Python and React / Next.js, makes extensive use of LLMs via API, and is deployed on OSS2 Kubernetes within Marsh Tech. Being forward-deployed means we build our products inside the business itself, so you will develop genuine domain knowledge in pensions as part of the role.

Your role is to lead on the engineering development and maintenance of the AI technology we build, spanning the backend (Python), front-end (React and Next.js) and deployment (OSS2 / Kubernetes). You will also get into the AI Agent plumbing and agent resilience. AI Agent quality and trust are vital for Mercer: because our product is customer-facing, building and maintaining reliable, trustworthy systems is central to what we do. You will work alongside the lead Data Science / AI engineer and report to the Head of AI Product — both of whom are hands-on developers, so this is very much a team effort. There are no direct reports; we are looking for a self-starter who thrives as part of a small, collaborative team.

We will count on you to:

• Lead full-stack development and maintenance of the AI Assistant tech stack across the backend (Python), front end (React/Next.js), and deployment (OSS2/Kubernetes).
• Take responsibility for design, coding, testing, and software quality across the application life cycle, ensuring user needs, performance requirements, and security standards are met.
• Improve AI agent reliability, trust, and safety, helping ensure the Assistant is dependable for customers.
• Participate in technical design and peer review, review code, and contribute to strong engineering practices across the team.
• Create and maintain documentation as part of solution delivery, and serve as a point of contact with the wider IT and Marsh Tech teams as needed.
• Diagnose issues, resolve problems, and recommend improvements and enhancements to the product.
• Alongside the core developer role, spend a portion of your time supporting AI change across the DC organization on small, ad hoc projects — helping the business use and deploy AI for process efficiencies and better client outcomes.

This is flexible work that will evolve over time.

What you need to have:

• Proven experience as a senior full-stack developer delivering production software, able to work independently and take ownership end to end.
• Strong backend engineering skills in Python (services and API design).
• Solid front-end experience with React and TypeScript.
• Hands-on experience deploying and running applications on Kubernetes.
• A track record of building and maintaining reliable, high-quality software, with a genuine interest in the quality, trust and resilience of customer-facing systems.
• The ability to collaborate closely within a small, hands-on team while also being a self-starter who can drive work forward independently.

What makes you stand out?

• Experience building AI / LLM-enabled products in production or controlled enterprise environments (e.g. agentic workflows, RAG, prompt engineering, evaluation and governance patterns).
• Experience with React / Next.js and modern web tooling.
• Familiarity with document / NoSQL stores (e.g. MongoDB) and data pipelines.
• Experience with deployment, Docker, logging and configuration.
• Understanding of the internal Marsh Tech landscape — OSS2, networking, the Voyager process, and similar — is an advantage.
• Interest in, or experience with, the UK pensions domain; relevant domain knowledge will be developed in the role.

Why join our team:

• We help you be your best through professional development opportunities, interesting work and supportive leaders.
• We foster a vibrant and inclusive culture where you can work with talented colleagues to create new solutions and have impact for colleagues, clients and communities.
• Our scale enables us to provide a range of career opportunities, as well as benefits and rewards to enhance your well-being.

Mercer is a business of Marsh (NYSE: MRSH), a global leader in risk, reinsurance and capital, people and investments, and management consulting, advising clients in 130 countries. With annual revenue of over $27 billion and more than 95,000 colleagues, Marsh helps build the confidence to thrive through the power of perspective. For more information about Mercer, visit mercer.com, or follow us on LinkedIn and X.

Marsh is committed to embracing a diverse, inclusive and flexible work environment. We aim to attract and retain the best people and embrace diversity of age background, civil partnership status, disability, ethnic origin, family duties, gender orientation or expression, gender reassignment, marital status, nationality, parental status, personal or social status, political affiliation, race, religion and beliefs, sex/gender, sexual orientation or expression, skin color, or any other characteristic protected by applicable law. We are an equal opportunities employer. We are committed to providing reasonable adjustments in accordance with applicable law to any candidate with a disability to allow them to fully participate in the recruitment process. If you have a disability that may require reasonable adjustments, please contact us at reasonableaccommodations@marsh.com.

Marsh is committed to hybrid work, which includes the flexibility of working remotely and the collaboration, connections and professional development benefits of working together in the office. All Marsh colleagues are expected to be in their local office or working onsite with clients at least three days per week. Office-based teams will identify at least one “anchor day” per week on which their full team will be together in person.