Full-Stack Engineer - AI-Native Team

About the role

Mainloop is a new engineering team in Barcelona, built from scratch this year, that owns and runs software automating professional-services work across roughly fifteen countries. Each product is owned by two people: a Product Owner and the engineer, who holds the build, architecture, quality and release. The team is AI-native: every engineer gets a personal Claude Max subscription and a desktop orchestrator that runs several coding agents in parallel. There are four junior seats and one senior seat.

Key responsibilities

• Take over a documented product and make it yours, using code search and navigation tools to understand the codebase quickly.
• Decide with the Product Owner what to build next and ship it, with the team covering new products regularly.
• Design more than you type: turn business needs into precise specifications, define the architecture and judge the output of AI-generated code.
• Build with AI agents, learning to balance quality and speed and to spot subtle errors in generated code.
• Ensure every build passes 100% of unit, smoke and Playwright end-to-end tests, and that AI-powered features include evaluation sets.

Required profile

• Junior candidates: a working web application built front to back, serious use of a coding agent with opinions on where it breaks, and a desire to get good at it quickly; a degree, bootcamp or self-taught path is fine.
• Senior candidate: experience owning production software for years and ready to set the architecture others build on.
• Fluent English; Spanish is useful, Catalan is not required.

Required skills

• Front-end and back-end development of web applications.
• Strong TypeScript if available; other languages are considered if strong.
• Unit and end-to-end testing, with the ability to review inherited test suites.
• Ability to read and explain codebases you did not write.

What we offer

• A team that starts from scratch, with a shared scaffold and packages so new products can be picked up quickly.
• No on-call duty; alerts go to a team channel and issues are fixed on Monday morning.
• A defined body of knowledge over the first two to three years, leading to a Mainloop Barcelona certificate.
• A four-round hiring process, ending with a practical project built with the team's tools.