Senior Product Engineer

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

This is a rare opportunity to join a well-funded early-stage AI business as one of the first senior engineering hires. You will play a pivotal role in shaping a greenfield AI product, owning technical decisions, influencing the roadmap, and helping build an enterprise-grade platform used by leading organisations.

THE COMPANY

They are an AI startup developing advanced behavioural intelligence technology for enterprise customers. Their platform helps organisations better understand customer behaviours, preferences, and decision-making patterns to improve communications, customer engagement, and product experiences.

Having secured significant early-stage funding, they are now working with major enterprise clients and are focused on scaling their product from proof of concept into wider commercial deployment. With a small, highly experienced team, they offer the opportunity to make a meaningful impact from day one.

THE ROLE

As a Senior Product Engineer, you will take ownership of product development alongside senior leadership, helping transform innovative AI capabilities into a scalable commercial platform.

Specifically, you can expect to be involved in the following:

• 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 AND EXPERIENCE

The successful Senior Product Engineer will have the following skills and experience:

• 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

BENEFITS

The successful Senior Product Engineer will receive the following benefits:

• Salary between £115,000 - £140,000 - depending on experience
• Equity