Full Stack Product Engineer

Job title: Full Stack Product Engineer

Location: London; full in-office working as default

Start date: ASAP

Reports to: Head of Engineering

Compensation: BASE : £70,000 - £90,000

EQUITY : £70,000 - £90,000

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Cosine is the UK's frontier AI lab for autonomous software engineering — not another coding assistant, but our own models, agent stack, eval harnesses, and product layer. We build our agent tooling entirely in-house and post‑train open‑source models to deliver reliable, enterprise‑grade coding performance in security‑critical settings.

Our offerings can be used as SaaS, deployed on VPC, on‑prem or fully air‑gapped. Through this flexability, we are able to service large-scale, highly regulated enterprises and industries including defence, financial service and healthcare.

We’re backed by YC and were recently selected for the UK’s sovereign AI programme. The company is at a rare stage: deep technical work, real enterprise pull, and a chance to shape how AI is deployed inside defence, banking, government, and other high‑trust environments.

This role is based in our Hoxton office, five days a week, because close collaboration, fast feedback, and shared context matter for the problems we’re solving.

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A Full Stack Product Engineer at Cosine works closely across research, engineering, infrastructure, product, and design to turn advanced AI capabilities into intuitive, reliable tools for developers.

You will be responsible for building and evolving Cosine’s user‑facing product surfaces, including dedicated apps and web applications that expose the power of our models through well‑designed workflows and interfaces. Your work sits at the intersection of product, platform, and research, translating emerging model capabilities into concrete experiences developers can use every day.

We focus on rapid product iteration without compromising quality. This means building systems that are performant, observable, and robust, while remaining flexible enough to evolve alongside fast‑moving research and user needs. You will help define how developers interact with Cosine, from first use through advanced, production‑grade workflows.

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Push the boundaries of what an autonomous agentic AI engineer can do by building scalable, high‑quality web application interfaces and integrations that expose the power of our models in intuitive, reliable ways.

• Build end‑user products (command line interfaces, web applications and platform capabilities) that integrate with the Cosine platform APIs
• Enable developers to use advanced language models to augment their workflows and problem‑solving capabilities
• Collaborate closely research to explore and validate new product experiences and capabilities
• Translate emerging model capabilities into concrete, user‑facing features
• Maintain a high bar for performance, reliability, and usability across platforms
• Be opinionated about and bring your own ideas to the table, we’re building for engineers, as an engineer you have a say in what you want to use.

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• Experience building reactive, highly interactive applications (React, Svelte, Vue)
• Experience with cli application development or local software development is a plus

• Experience with backend development
• Experience designing and consuming REST APIs

• Experience working with developer tooling
• Familiarity with CI/CD pipelines
• Experience with containerization

• Strong experience with Type Script and modern frontend frameworks
• Knowledge of Node.js, Python, and Go is a plus

• Git, Docker, Webpack, npm, Kubernetes, GCP
• Monitoring and alerting tools (e.g. Grafana, Datadog)
• Feature flagging systems

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• Have 5+ years of experience building production‑grade software.
• Have shipped and maintained complex frontend or full‑stack applications used by technical users.
• Ar…