Constructeur/ Engineer in London

Location: London; full in-office working as default ASAP

Reports to: Head of Engineering

Compensation: BASE: £70,000 - £90,000

At Cosine, we’re building autonomous AI engineers that plan, write, and ship code inside real development workflows. Cosine is designed for on-premise and virtual private cloud (VPC) deployments, including fully air-gapped environments. 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.

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.

A Product Engineer at Cosine works closely across research, engineering, 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 IDE extensions or 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.

Responsibilities:

• Deliver a scalable, high-quality Cosine product experience by building IDE and web application interfaces and integrations that expose the power of our models in intuitive, reliable ways.
• Build end-user products (editor extensions and 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 with 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.

Frontend & Application Development:

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

Programming Languages:

• Strong experience with TypeScript and modern frontend frameworks.
• JavaScript, Python, and Go is a plus.

Have 5+ years of experience building production-grade software, with a strong emphasis on user-facing developer tools. 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. Care deeply about developer experience, performance, and reliability—not just functionality.

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.
• 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.
• Pension.
• High-quality equipment to do your best work.

We care about focus, sustainability, and doing great work — not performative overwork. 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.

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). 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.