Senior AI Platform Engineer - LLM & Scale

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

Benefits offered by this job

Equity

Job summary

Harnham is seeking a Senior Product Engineer to own a greenfield AI product, shaping the roadmap and building an enterprise-grade platform with a small, experienced team.

You will lead the LLM application layer, design robust APIs, and drive cost-efficient, scalable solutions while collaborating with leadership to translate customer feedback into product improvements. Equity is included.

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 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

LLM product

Backend engineering

Node.js

TypeScript

RAG architectures

Cost optimisation

Token usage management

Startup experience

Communication skills

Product mindset

Job description

Harnham is seeking a Senior Product Engineer to own a greenfield AI product, shaping the roadmap and building an enterprise-grade platform with a small, experienced team.

You will lead the LLM application layer, design robust APIs, and drive cost-efficient, scalable solutions while collaborating with leadership to translate customer feedback into product improvements. Equity is included.