AI Engineer T1 VC-backed AI Start-up London

AI Engineer | T1 VC-backed AI Start-up | Hybrid / London

The Client

Harmonic is delighted to be partnering with one of London's fastest-growing AI Tech start-ups who have already gained significant traction and have been scaling rapidly (consistent double digit growth month-on-month).

With significant backing from Tier 1 investors, exceptional early traction, and a highly technical founding team, they are likely to hit $100m ARR in the next 12 months.

This is a company operating at pace, with an uncompromising focus on shipping, learning from real customers, and iterating quickly. Engineering quality matters, but speed, ownership and outcomes matter just as much. This is an opportunity to join at a formative stage and play a defining role in shaping both the product and the technical foundations of the business.

The Role

This AI Engineer position reports directly into the CEO, sitting at the core of the product and engineering effort.

You'll be responsible for building and shipping AI-driven functionality end-to-end, from experimentation and prompt design through to production deployment and real-world usage. You'll operate across the full stack, working closely with the founding team to turn ambiguous problems into working solutions that customers can use quickly.

This is a highly hands-on role with significant autonomy and influence. You'll be expected to move fast, take ownership, and hold a high bar for both technical quality and delivery. It's an ideal opportunity for someone who wants real responsibility, enjoys solving hard problems, and is motivated by seeing their work having real world impact.

Key Responsibilities

• Develop and ship production-ready AI algorithms, building on and improving the company's proprietary approaches to significantly increase output quality.
• Lead the development of new and improved algorithms to push the limits of what is possible with large language models.
• Research, prototype and implement state-of-the-art techniques across LLMs, prompting and evaluation.
• Design and build a state-of-the-art evaluation pipeline to objectively measure output quality and guide R&D direction.
• Contribute to frontend development (TypeScript / React) to integrate AI capabilities directly into user-facing workflows.
• Build and maintain backend services and APIs, working with Postgres databases and production infrastructure.
• Research and implement advanced prompt engineering techniques to maximise the effectiveness and reliability of LLM-driven systems.
• Work closely with the founding team to translate product ideas into practical, scalable AI solutions.
• Ship quickly, gather customer feedback, and iterate relentlessly based on real-world usage.
• Clearly communicate technical decisions, trade-offs and outcomes to both technical and non-technical stakeholders.
• Help establish early best practices, documentation and technical standards as the engineering function grows.

Essential Experience

• Expert-level Python skills.
• Experience shipping algorithms and AI evaluation pipelines
• Hands-on experience with LLMs and prompt engineering.
• Familiarity with backend development and databases.
• Experience of with front-end UX (React).
• Exceptionally clear written and verbal communication skills.
• A strong sense of ownership, urgency and accountability for outcomes.
• Comfortable working in fast-moving, ambiguous environments with a bias towards action.

Salary: £150,000 - £200,000 + meaningful equity (dependent on experience).

Location: Hybrid working, London-based (flexibility offered).

Start Date: ASAP

At Harmonic, we are dedicated to fostering an inclusive and equitable workplace. We actively welcome applications from individuals of all backgrounds and assure you that every candidate will be thoughtfully considered for the roles we represent, without regard to race, religion, gender expression, disability, or sexual orientation.