Senior UI Engineer

Department: Engineering

Location: London

Employment Type: FullTime

About StackOne:

StackOne is the AI Integration Gateway for SaaS products and AI Agents. Backed by GV and Workday Ventures ($24M raised), we help builders of SaaS platforms and AI Agents orchestrate hundreds of scalable, accurate, and enterprise-grade integrations. Our platform combines 25,000 pre-mapped actions on 200 connectors, an AI-powered integration development toolkit, plus security by design: a real-time architecture, managed authentication and permissions, and end-to-end observability.

Join us on our fast trajectory to build the future of agentic integrations.

Why this role exists

StackOne’s product experience is our brand. As our first dedicated UX Engineer you will be the in-house authority on taste, polish, and performance. You will own the component library, shape our micro-frontend architecture, and ensure that every pixel, motion, and millisecond of latency makes customers feel like they’re using the best-engineered platform on the market.

What you’ll do

• 80 % Front-end craft & design systems

• 20 % API-aware performance engineering

• Collaborate with our Designer to define & steward our design system – set the visual language, build reusable primitives, and maintain libraries that power every StackOne user interfaces.

• Ship pixel-perfect UIs – React (primarily) plus; own theming, performance, and thoughtful motion.

• Make performance a feature – instrument Core Web Vitals, budget bundle sizes, optimise hydration patterns, and chase performance where it matters.

• Provide and maintain internal tooling as well as AI rules to allow every technical team to contribute to the front-end while aligning with the best practices you will help set.

• Prototype & validate – turn product ideas into interactive proofs-of-concept that help PMs and designers converge on the right solution fast.

• Champion taste & detail – host design-quality critiques, stamp out visual debt, and be the go-to voice when “good enough” isn’t.

What you’ll bring

Must-haves

• 4+ yrs professional front-end engineering with a portfolio that focus on the details.

• React & TypeScript at production scale; comfortable with hooks, suspense, and server components.

• Deep CSS fluency (Tailwind or equivalent) and experience of modern browser APIs.

• Proven track record building or migrating component libraries/design systems.

• Strong performance tuning instincts: Lighthouse, Core Web Vitals, bundle analysis, code-splitting.

• Familiarity with monorepo tooling (NX or Turborepo) and micro-frontend patterns.

• Ability to dive into back-end code (Node, React-router, sveltekit) to remove perf bottlenecks, instrument metrics, or adjust API shape.

Nice-to-haves

• Svelte, Web Components, or other framework experimentation.

• Experience with auth flows (OAuth2, OpenID Connect) and secure front-end patterns.

• Web Motion & animation chops

• Prior work on B2B developer tools or dashboards at seed-to-Series-B stage.

Why Join Us?

• Meaningful share options (EMI) - share in the company's success as we grow

• 25 days holiday + 1 additional day per year of tenure

• Private health insurance - including dental & optical

• £15/day lunch budget when working from our London office, up to £120/month

• £1,000 for your home office set up + £500/year top-up

• Annual team offsite to sunny spots (last ones were in Spain and Portugal ☀️)

• Join one of Europe’s fastest-growing startups

• Work with a veteran team of ex-employees of Google, Microsoft, Oracle, Coinbase, JP Morgan and more

• Health, fitness and gift card discounts

• Cycle2Work and Electric Cars scheme

• Hybrid work set up - typically 2 days in the office

Ready to help us change the game for SaaS integrations? Get in touch and let's chat!

We believe diversity drives innovation. We encourage individuals from all backgrounds to apply. As an equal-opportunity employer, we celebrate diversity and are committed to creating an inclusive environment for all employees.