Sr. Software Engineer, Fullstack (UK)

Who We Are

At Firstup, our mission is to improve the employee experience at every moment that matters, large and small. As the communication pipeline for the world's workforce, we now serve 40 of the Fortune 100 companies, reaching and connecting more than 17 million employees daily.

Our employees are experts in the employee experience, workforce communications and technology.

Joining Firstup means joining a movement to make work better for every worker. As the world’s first intelligent communication platform, Firstup meaningfully engages employees at every moment from hire to retire, and delivers engagement insights to help companies support, promote and retain their talent. Our movement has taken root and is evident in our world-class customer base. Now we need your help. Ready to make a difference in the world?

Job Summary

We're looking for a Senior Full Stack Engineer who thrives at the intersection of craft and scale. You'll design and deliver large-scale, multi-tenant cloud services — with a relentless focus on availability, fault-tolerance, and elegant architecture. You'll set the bar for engineering quality and help elevate those around you.

Responsibilities

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Core Engineering & Ownership: Build and own scalable SaaS applications end-to-end — frontend, backend, and data layers. Write production-grade code that is reliable, readable, efficient, and testable. Raise the bar through thoughtful code reviews and comprehensive documentation.

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System Architecture: Design future-ready architectures and proactively close gaps before they become problems.

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Performance & Reliability: Drive system availability and performance — investigate, analyze, and resolve reliability issues across the stack. Recognize opportunities and advocate for better performance and efficiency of software, systems, and processes across teams.

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Shift Left Testing & Quality: You champion a rigorous 'shift-left' quality culture, embedding comprehensive testing—unit, integration, and contract—early and continuously in the development lifecycle, not as an afterthought.

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AI Integration: Leverage and advocate for AI-assisted tools (e.g., GitHub Copilot, Claude) to accelerate development while maintaining rigorous engineering judgment and accountability for code correctness.

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Leadership: Drive team-wide best practices, conduct quality code reviews, and mentor engineers to elevate overall engineering quality.

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Collaboration (Product & Design): Partner with Product and Design to translate requirements into intuitive UI, making informed technical trade-offs in the face of ambiguity.

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Collaboration (Engineering Standards): Champion engineering best practices across teams and collaborate on solving complex, high-impact problems.

Minimum Qualifications

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Bachelor's degree in Computer Science or related field, or equivalent experience.

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5+ years building reliable, performant applications and microservices in production environments. Proven full stack fluency - you're comfortable across frontend, backend, and the database layer.

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A polyglot mindset — you evolve with the tech. Familiarity with Ruby on Rails, TypeScript, Node.js, or React.js is a strong plus.

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Proven full-stack experience across frontend (React.js), backend, and database layers.

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Technical depth in at least one engineering domain: distributed systems, multi-tenant SaaS, application databases, networking, security, serverless, event streaming, or similar.

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A genuine drive to build beautiful, intuitive products with the customer always in focus. Innate curiosity — you love learning and have a desire to teach and lift others. Strong communication skills and openness to collaborative idea-sharing across teams.

Preferred Qualifications

• Core Stack Experience: Familiarity with Ruby (including Ruby on Rails), TypeScript, Node.js, or React.js is a strong plus.