Fullstack developer

Experis España; Barcelona; On-site; EUR 70,000 - 95,000; Full time

Job summary

Experis, a ManpowerGroup company, seeks a Senior Software Engineer to join Kompanion as a core early member in Engineering. You will build the foundations of an AI-native platform for critical tax, accounting, and financial workflows, owning features end to end and shaping product and engineering standards.

You’ll work with Java, Spring Boot, Next.js, and PostgreSQL, contributing across backend and platform-level concerns while collaborating with Tax, Accounting, Growth, and Operations teams.

Qualifications

• 5+ years of experience building and operating production software.
• Strong backend engineering experience.
• Proficiency in Java and Spring Boot.
• Solid PostgreSQL knowledge and data modelling.
• Ability to work with React or Next.js when needed.
• Experience owning systems in production.
• Strong fundamentals in clean code, testing, security, and system design.
• High autonomy and ability to turn ambiguous problems into shipped products.

Responsibilities

• Owning features end to end, from problem definition to production and validation.
• Building across the stack, with a stronger focus on backend systems and business logic.
• Operating the platform in production and improving its reliability, performance, observability, and security.
• Working closely with Tax, Accounting, Growth, and Operations.
• Building integrations, automations, internal tools, and AI-powered features.
• Improving our architecture, developer experience, testing, and engineering standards.
• Reviewing code, mentoring engineers, and participating in hiring.

Skills

Backend development

Java

Spring Boot

PostgreSQL

React/Next.js

Production ownership

System design

Testing

Security

Education

Computer Science / Software Engineering degree welcome

Job description

At Experis, a ManpowerGroup company, we specialise in recruiting the best tech talent and matching them with all kinds of organisations, from large corporations to start-ups. We are a company specialising in IT consultancy and the recruitment of professionals in the technology sector across our three practice areas: Business Transformation, Cloud & Infrastructure, and Enterprise Applications. We have a workforce of over 1,800 IT specialists in Spain and an international presence in 54 countries. If you’re a technology professional, we’d love to help you find that new opportunity you’re looking for – the one that will propel you to the next level in your career. Find your next opportunity with us. Take your career to the next level with Experis.

About the Opportunity

We’re looking for a Senior Software Engineer to join Kompanion as the third person in Engineering and one of the first eight employees in the company.

You’ll help build the foundations of a robust, AI-native platform for critical tax, accounting, and financial workflows. This is a hands-on role with broad ownership. You’ll lead features end to end, work closely with Tax, Accounting, Growth, and Operations, and help shape our product, engineering standards, hiring, and technical culture. Our stack and initial architecture are in place. Most of what comes next is still open to influence.

How We Build

Our stack includes Java, Spring Boot, Next.js, and PostgreSQL.

Engineering owns almost the full product loop: refining problems, researching the domain, writing PRDs, designing solutions, building, shipping, and validating them.

We use AI across development and inside the product, from document processing and assistants to autonomous agents.

We move fast where we can and carefully where reliability, security, and compliance matter. We avoid unnecessary architecture, excessive meetings, and working from tickets without understanding the problem.

What You’ll Be Doing

• Owning features end to end, from problem definition to production and validation
• Building across the stack, with a stronger focus on backend systems and business logic
• Operating the platform in production and improving its reliability, performance, observability, and security
• Working closely with Tax, Accounting, Growth, and Operations
• Building integrations, automations, internal tools, and AI-powered features
• Improving our architecture, developer experience, testing, and engineering standards
• Reviewing code, mentoring engineers, and participating in hiring

What You’ll Need

• 5+ years of experience building and operating production software
• Strong backend engineering experience
• Experience with Java and Spring Boot, or the ability to become productive with them quickly
• Solid PostgreSQL knowledge, including data modelling and query performance
• The ability to work with React or Next.js when needed
• Experience owning systems in production
• Strong fundamentals in clean code, testing, security, and system design
• High autonomy and the ability to turn ambiguous problems into shipped products

A degree in Computer Science, Software Engineering, or a related field is welcome, but not required.

Nice to Have

• Experience building regulated, auditable, or high-stakes software
• Experience designing a platform from its early stages
• Experience with security, privacy, or sensitive financial data
• Experience with event-driven or asynchronous systems
• Experience building AI-powered products or using AI-assisted development tools
• Previous early-stage startup experience

Who You Are

• You’re a hands-on, low-ego engineer who wants to build. You balance speed with rigor, challenge decisions constructively, and seek the source of truth. You care about outcomes, communicate clearly, and help others improve.