Full-Stack Product Engineer

Own a product end to end — the users, the roadmap, the build and the release — with nobody standing between you and the people who use it.

Mainloop is anew engineering team inside an established international group. We are in Barcelona, we are being built from scratch this year, and we have the backing, the customers and the product portfolio of a company that has been around for decades. You get the interesting part of a new team without the part where you check whether payroll clears.

Two more things about where this comes from and where it goes. The group has been running a tech lab in Casablanca for more than eight years— Barcelona is its second, built to sit closer to the teams we build for, and AI-native from the first commit, because it starts from a blank page. And the plan does not stop at internal work:within about a year and a half we intend to externalise— external clients, our products sold on the market, products developed for it. The first eighteen months are deliberately spent building as much as possible for the group while the machine gets set up. You would arrive at the start of that curve, not after it.

What we build is the software that automates professional-services work — across roughly fifteen countries, for businesses that are drowning in process. We work in two halves. One half goes into a business, works out what actually happens there, and proves fast whether an idea is worth having. The other half — this half — takes what is proven and owns it for years.

This is the seat where the product and the engineering are the same job. Most of our products are run by a pair: a Product Owner who holds the business, and an engineer who holds the build.



Some of them do not need two people. They are smaller, their users sit close enough to talk to directly, and a separate PO would be a relay rather than a help. Those products are run by one person, and that person is you.

What the job actually is

You own a product end to end. Not a slice of it, and not the tickets someone else wrote about it. You decide what it should do next, you build it, you ship it, and you find out whether it worked.

- You talk to the people who use it. In person when you can. You read the feedback, you watch how the product is really used, and you notice the workaround nobody thought to mention — because the workaround is usually the next feature.

- You decide what comes next. There is no one above you writing the roadmap for your product and no one below you writing the stories. You choose what moves the outcome, what waits, and what you decline — with a reason the person asking can respect.

- You build it, front to back. The interface, the logic underneath, the data it keeps. You decide whether a feature makes sense on the screen and whether it holds up behind it, which is exactly the judgement that gets lost when those two calls belong to different people.

- You ship it and you measure it. A feature is not done when it merges; it is done when people use it and the number moves.



You own that number — time saved, errors avoided, uptake — and you report it whether it flatters us or not.

- You carry more than one. Our products are built on the same scaffold and arrive the same shape, so one person can own several small ones well. New ones keep arriving from the other half of the team.

How this differs from the Full-Stack Engineer seat

Both seats sit in the Tech Lab, both build with agents, and both own products for years. The difference is where the product decisions live. A Full-Stack Engineer works beside a Product Owner, who holds the business relationship and the roadmap, and owns the build, the architecture and the release. A Full-Stack Product Engineer holds both halves — on products small enough, and close enough to their users, that splitting them would only slow the product down.

If you want to go deep on architecture and let someone else hold the room, the Full-Stack Engineer ad is the better fit. If you care as much about whether a feature should exist as about how to build it, this one is.

You will get seriously good at building with AI agents

We are an AI-native team, genuinely — not a team that added a Copilot licence. Every engineer gets their own Claude Max subscription and a desktop orchestrator that runs several coding agents in parallel. In this seat that changes the arithmetic of the job: when building is fast, the expensive part is deciding — and you are the person who does both.

- Every change is reviewed by an agent before a person signs it off.

- Every build ships tests, and the gate is 100% pass. Unit, smoke against the real running thing, and browser-level end-to-end.

📌 Full-Stack Product Engineer (San Cugat del Vallés)
🏢 Leyton
📍 San Cugat del Vallés