Senior Full Stack Engineer Contractor

Who are OnBuy?

OnBuy are an online marketplace who are on a mission of being the best choice for every customer, everywhere.

We have recently been named one of the UK's fastest-growing tech companies in the Sunday Times 100 Tech list.

All achievements we are very proud of, but we don't let that go to our head. We are all laser focused on our mission and understand the huge joint effort ahead of us needed to succeed.

This is a critical role responsible for building, scaling, and evolving OnBuy’s core commerce platform across both front‑end and back‑end systems. As a Senior Full Stack Engineer, you will own engineering delivery end‑to‑end; from customer‑facing experiences to underlying services and data systems. You will operate within a cross‑functional team and be accountable for shipping high‑quality, scalable solutions that directly impact GMV, seller performance, and customer experience.

This role moves beyond “front‑end vs back‑end” you are an Engineer responsible for outcomes, not layers.

• Design, build, and maintain full‑stack solutions across UI, APIs, and backend systems
• Translate product requirements, designs, and wireframes into scalable, production‑ready systems
• Own features from concept → development → testing → release → monitoring

• Build responsive, high‑performance user interfaces using modern JavaScript frameworks
• Translate UI/UX designs into clean, maintainable code
• Optimise applications for speed, scalability, and usability

• Design and develop scalable services and APIs
• Work across monolith and microservices architecture (Laravel / PHP services)
• Maintain and evolve legacy systems while building modern, scalable architecture

• Own quality to production — testing is embedded, not a phase
• Write and maintain automated tests (unit, integration, end‑to‑end)
• Participate in and lead code reviews to ensure high engineering standards
• Debug, troubleshoot, and resolve issues across the stack

• Optimise systems for performance, reliability, and scale
• Work with large‑scale data systems (MySQL, No
  
  SQL, search platforms)
• Identify bottlenecks and proactively improve system efficiency

• Work in cross‑functional teams with Product, Design, QA, and other Engineers
• Mentor and support less experienced engineers
• Communicate technical concepts clearly to non‑technical stakeholders

• Strong full stack engineering experience, owning end‑to‑end delivery
• Proven backend expertise in PHP (Laravel)
• Strong JavaScript (ES6+) skills with modern frameworks (React/Vue/Angular)
• Experience designing and building RESTful APIs and scalable systems
• Experience with MySQL and No
  
  SQL databases (e.g. Couchbase, Mongo
  
  DB)
• Strong understanding of system architecture and microservices
• Experience with event‑driven systems (e.g. Rabbit
  
  MQ, Pub/Sub) and cloud environments (GCP)
• Familiarity with Docker and CI/CD pipelines
• Strong engineering practices: testing, code quality, and production ownership

• Experience in eCommerce or marketplace platforms
• Cloud experience (GCP)
• Experience with No
  
  SQL (e.g. Couchbase) and Search platforms (e.g. Constructor & Solr)
• Experience working in microservices architecture

This role is UK remote role you must be based in the UK and have full right to work in the UK.

The day rate is £550-£650 per day. Please confirm your outside IR35 day rate upon application. This is a 4 month contract.

OnBuy is an equal opportunities employer. We are dedicated to creating a fair and transparent workforce, starting with a recruitment process that does not discriminate on the basis of gender, sexual orientation, marital or civil partnership status, pregnancy or maternity, gender reassignment, race, colour, nationality, ethnic or national origin, religion or belief, disability, or age.