Senior Product Engineer: AI Platform (LLM + RAG) — Equity

Harnham; Greater London; On-site; GBP 115,000 - 140,000; Full time

Benefits offered by this job

Salary £115,000 - £140,000

Equity

Job summary

Harnham is seeking a Senior Product Engineer to join a well-funded early-stage AI business in London, taking ownership of product development alongside senior leadership. You will help transform innovative AI capabilities into a scalable commercial platform, building production-grade APIs and backend services for enterprise integrations.

This is a rare opportunity to shape architecture, drive LLM strategies, and establish engineering best practices in a fast-growth startup environment with

Qualifications

• Strong commercial experience as a Product Software Engineer or Software Engineer.
• Experience building and scaling an LLM-enabled product from concept through to production.
• Strong backend engineering expertise, ideally within API-first products.
• Experience with Node.js and TypeScript, or a proven ability to transition successfully between technology stacks.
• Understanding of RAG architectures, evaluation frameworks, guardrails, monitoring, and model optimisation.
• Experience managing token usage, infrastructure costs, and scalability challenges in AI products.
• Comfortable working autonomously in a startup or high-growth environment.
• Strong product mindset with the ability to make technical and commercial decisions.
• Excellent communication and stakeholder management skills.
• React experience is beneficial but not essential.
• Experience within Financial Services or other regulated industries would be advantageous.

Responsibilities

• Building and scaling a production-grade AI product from the ground up.
• Designing and developing APIs and backend services that power enterprise customer integrations.
• Owning the LLM application layer, including retrieval-augmented generation (RAG) workflows.
• Improving model performance, reliability, evaluation frameworks, monitoring, and cost optimisation.
• Working across the stack to deliver customer-facing features and platform capabilities.
• Collaborating with stakeholders to translate customer feedback into product improvements.
• Establishing engineering best practices across architecture, testing, deployment, and delivery.
• Contributing to technical strategy and product roadmap decisions.

Skills

Product Software Engineer

LLM-enabled product

Backend engineering

Node.js

TypeScript

RAG architectures

Token usage optimization

Autonomy in startups

Product mindset

Communication skills

React experience

Regulated industries experience

Job description

Harnham is seeking a Senior Product Engineer to join a well-funded early-stage AI business in London, taking ownership of product development alongside senior leadership. You will help transform innovative AI capabilities into a scalable commercial platform, building production-grade APIs and backend services for enterprise integrations.

This is a rare opportunity to shape architecture, drive LLM strategies, and establish engineering best practices in a fast-growth startup environment with