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: £100,000 - £110,000 + EQUITY

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Cosine at a glance

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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The role

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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Your Mission

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.

Responsibilities

• 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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Skills & Experience

Frontend & Application Development

• Experience building reactive, highly interactive applications (React, Svelte, Vue)

• Experience with cli application development or local software development is a plus

Backend & Systems

• Experience with backend development

• Experience designing and consuming REST APIs

Developer Experience

• Experience working with developer tooling

• Familiarity with CI/CD pipelines

• Experience with containerization

Programming Languages

• Strong experience with TypeScript and modern frontend frameworks

• Knowledge of Node.js, Python, and Go is a plus

Tooling & Infrastructure

• Git, Docker, Webpack, npm, Kubernetes, GCP

• Monitoring and alerting tools (e.g. Grafana, Datadog)

• Feature flagging systems

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You Might Thrive in This Role If You:

• Have 5+ years of experience building production-grade software.

• Have shipped and maintained complex frontend or full-stack applications used by technical users.

• Are comfortable building highly interactive web or IDE-based experiences using modern frameworks (e.g. React, TypeScript).

• Have experience designing and consuming APIs, and understand how frontend and backend systems evolve together.

• Enjoy working close to the product surface, translating ambiguous ideas and model capabilities into intuitive developer workflows.

• Care deeply about developer experience, performance, and reliability—not just functionality.

• Are comfortable iterating quickly, running experiments, and refining products based on real user feedback.

• Can operate effectively in a fast-moving environment with evolving priorities and incomplete information.

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Cosine is an equal opportunity employer.

We value diverse backgrounds, perspectives, and ways of thinking, and we’re committed to creating an inclusive and respectful workplace.

We encourage applications from anyone who meets the role requirements, even if you don’t meet every single qualification. If you need reasonable adjustments at any stage of the hiring process, we’re happy to discuss them.

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Compensation, Benefits & Ways of Working

We’re an in-office team, five days a week, by design. We believe the work we’re doing benefits from being together, collaborating closely, and building shared context.

What you can expect:

• Competitive salary , benchmarked to the market

• Equity / share options , so you share in the upside you help create

• 30 days’ holiday + bank holidays

• Genuine 9–5 working hours - we don’t expect late nights or weekend work

• Work hard in the office, collaborate closely, and switch off properly

• Dog-friendly office - bring your dog to work

• Weekly team breakfast & lunch

• Monthly socials

• Pension

• High-quality equipment to do your best work

We care about focus, sustainability, and doing great work — not performative overwork. We value people who show up, contribute thoughtfully, collaborate well with their colleagues, and then go home.

This role won’t suit everyone. But if you want structure, clarity, strong collaboration, and a team that takes both the work and work-life balance seriously, it’s a great place to be.

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Agency & Data Protection Notice

To comply with UK GDPR and our internal data-protection and equal-opportunity obligations, we only accept candidate applications and agency submissions via our Applicant Tracking System (ATS). This ensures appropriate privacy notices, lawful processing, auditability, and consistent retention controls.

Any CVs or candidate details received outside the ATS (including via email, Slack, or direct message) will be treated as unsolicited, will not be considered as part of the recruitment process, and will not give rise to any fee or payment obligation.