Senior Fullstack Engineer (Java/Kotlin + React)

What you work on today shapes where you’ll be tomorrow.

Looking for your next challenge as a Senior Fullstack Engineer ? We may have the right opportunity for you.

At B3, we start with people and their experience. We match projects to your skills and professional growth path. Our consultants work across a wide range of industries and technologies, delivering IT projects both in Poland and internationally. This allows you to build your career in a deliberate way - not by chance.

We are part of an international organization listed on Nasdaq Stockholm, operating in 11 countries. In Poland, we operate with full autonomy. We choose our technologies, processes, and ways of working while maintaining the highest professional standards.

Join us today.

What we offer

• Work in a Scandinavian culture built on logic, trust, and open dialogue.

• Health and sports benefits tailored to your needs.

• Speed Coaching – short sessions designed to provide new perspectives and solutions.

• Sports training, an internal team, and opportunities to take part in competitions.

Key areas of the role:

• API design and strategic partner integrations - Designing flexible API contracts (API-first) for external B2B partners, writing developer documentation and creating mock environments, supporting the onboarding of partners integrating with the platform via APIs

• Building new large-scale services in a startup-like model - Developing new services (e.g., from other domains) in a greenfield environment, outside the core market but integrated with platform services, moving towards a super app concept; developing web interfaces (checkout paths, landing pages, discovery hubs) and integrating the frontend with the backend and the mobile app using agentic development tools

• Technical leadership within the team - Acting as the technical leader and "architectural anchor" for integrations, bridging Java backend design with modern frontend development, defining coding standards, leading technical discussions, supporting the Engineering Manager in organizing the work and helping team members

What you need to join us:

Must have:

• Deep, hands-on experience in Java and/or Kotlin , including designing, building and scaling RESTful APIs with an API-first approach (OpenAPI/Swagger)

• Good frontend skills: JavaScript/TypeScript and a modern framework ( React preferred ) - at a level that allows you to support other team members

• Solid understanding of distributed systems, integration patterns and secure authentication (OAuth 2.0, OpenID Connect)

• Experience in technical leadership - defining coding standards, leading technical discussions, designing API contracts for external B2B partners - combined with a full-stack perspective and the ability to translate product requirements into feasible solutions within a complex technical environment

• Advanced use of AI in engineering work (beyond code autocompletion), a problem-solver mindset (instead of narrow&deep specialization) , flexibility in greenfield projects

• English at min. B2+ level

• Right to work in Poland (citizenship, residence card, visa, etc.)

Nice to have:

• Experience with data isolation and security standards (e.g., GDPR in healthcare integrations)

• Experience with Agentic AI tools (e.g., GitHub Copilot, LangGraph) used to accelerate team workflows

• Knowledge of mobile development - understanding what happens on the mobile app side

• Experience integrating with services such as payments, login (SSO) or search

• Experience in maintaining production solutions and participating in on-call / on-duty rotations