Principal Backend Engineer

The systems thinker

The one who sees failure modes six months before they happen. Who designs for scale before scale is real. Who knows the difference between a decision that is reversible and one that is not, and treats them accordingly. You might be a staff engineer at a growth-stage company. You might be a principal at a larger tech firm who has been quietly building the infrastructure others will depend on for years. There are engineers who built the core banking layer at Monzo, the dispatch system at Deliveroo, the data pipelines that now feel inevitable, who started exactly where you are: Staring at a hard problem with no playbook and real consequences. We do not care about the title. We care about whether you can design a backend system from first principles, defend every trade-off under pressure, and own the outcome when the requirements change at 11pm on a Thursday.

Healf is Europe's fastest-growing health platform

m.£100M+ revenue run-rate. 700k+ customers. A high-retention, high-margin marketplace. Strong brand and distribution. We curate the world's best wellbeing products across EAT, MOVE, MIND, and SLEEP. That is the first chapter.The next chapter is harder and more interesting. Today, wellbeing decisions are primitive. Customers guess. They follow influencers. They try products and hope. We are building the system that changes that. A personalised wellbeing intelligence engine that understands behavioural patterns, captures longitudinal health data, and builds smart AI agents that help customers solve their wellbeing needs. A closed feedback loop. A decision engine, not just a marketplace.If we get this right, we do not just sell products. We become the operating system for how the world lives well.

Why this role?

This is a greenfield build. There is no existing codebase to inherit. The Founding Backend Engineer has defined the vision. You are the person who makes it real at the systems level. Every architectural decision you make in the next twelve months determines what is possible in year three. The data model you choose. The pipeline architecture you put in place. The infrastructure bets you make early. These compound. The right calls save years of rework. The wrong ones do the opposite. We do not see this as a conventional senior engineering role. Somewhere right now there is a staff engineer at a Series B company who has spent three years making the right calls quietly. Who said no to things that looked fast but would have cost the company two years of rework. Who has never heard of Healf. Who would not click on a standard InMa

il. But if someone who understood what they actually cared about reached them with the right story at the right moment, they would listen.

What you will own

→ The backend architecture. You will design the core services powering Healf's Wellbeing Intelligence platform from first principles. The API layer, the data pipelines, the async processing, the infrastructure decisions. The choices you make in the first six months will define what is possible in year three. You will own those choices.

→ The integration layer. You will own how the backend connects to the AI and ML systems the AI Engineer is building. Real-time personalisation, user behaviour pipelines, health data inputs. The seam between backend and intelligence is yours to define.

→ The build vs buy decisions. Every platform team reaches these inflection points. You will own them with authority and with the context to make them well. What to build, what to buy, what to defer.

→ The observability layer. Monitoring, alerting, and the instrumentation that tells you when something is wrong before users do. You will build this from day one, not as an afterthought.

→ The engineering principles. Not a style guide. The actual principles that govern how the team thinks about quality, velocity, and long-term consequences in this part of the stack.

What you will deliver in the first 12 months→ A production-grade backend powering personalised decisioning for hundreds of thousands of customers→ A scalable data architecture designed for international growth from the start→ An API and pipeline layer that the AI Engineer can build on without constraint→ Full observability across every critical system from day one→ Infrastructure decisions that compound in the right direction.

What we are looking for:

System design is the primary bar. This is the single most important signal in our process. Not your CV. Not the company on your profile. Whether you can pick up an unfamiliar problem, identify the right building blocks, surface the correct trade-offs, and adapt the design fast when the requirements change mid-conversation. If you cannot defend your decisions under pressure, that is a disqualifier regardless of background.

Extreme ownership is non-negotiable.

Once you have a problem, it is yours. If something is broken, you fix it. If something is missing, you build it or escalate with a proposed solution. You pull others along when needed. You do not let blockers sit. You take responsibility for outcomes, not just outputs.

Technical depth

We expect:→ Production-grade backend systems built at scale. Python is our primary language; Go is considered for performance-critical services.→ API design and service architecture: RESTful and event-driven patterns, microservices trade-offs understood from experience, not textbooks.→ Real-time data pipelines and async processing: Kafka, Celery, or equivalent. You have built these in production.→ Infrastructure and DevOps fluency: Docker, Kubernetes, AWS or GCP, CI/CD. You make these decisions, not just execute them.→ Database design across relational and non-relational stores. Strong data modelling at the service layer.→ Experience integrating LLM or AI features into production backend systems is a meaningful plus.

How you work:→ You use AI coding tools natively: Claude, Cursor, Copilot, or equivalent. You can talk concretely about where they accelerate you and where they introduce risk.→ You are comfortable making infrastructure decisions with incomplete information and being accountable for them.→ You thrive when the brief is a vision, not a specification.→ You deliver complex systems fast. Speed and quality of execution both matter here.

What we are not looking for:

We want to be direct. It will save everyone time.→ Engineers who have operated within a narrow, well-defined scope and struggle when the brief becomes ambiguous.→ Candidates who execute tasks well but wait to be told what the next task is.→ People managers whose primary signal is leadership experience rather than hands-on technical depth. A strong background at a large tech company is not a disqualifier. The engineers we have seen thrive here combine genuine independence with rigorous system design instinct. What we are screening for is whether that independence actually exists, not where someone has worked.

The deal

Competitive base. Meaningful equity. Direct access to the co-founder from

day one. We ask a great deal of the people who work here. Full ownership. Genuine commitment. Everything you have, for this chapter. In return: the resources to build properly, a team worth being part of, and the creative authority to do work that defines this part of your career. We have built the fastest-growing health company in Europe with a team small enough that every person in it shapes the outcome. That is still true today.If the most important backend work of your career is ahead of you, this is the place to do it.