Senior Fullstack Engineer (Java/Kotlin + React)

senior

EngineeringFullstack Developer

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Quick Summary

Key Responsibilities

API design and strategic partner integrations - Designing flexible API contracts (API-first) for external B2B partners, writing developer documentation and creating mock environments,

Requirements Summary

Must have: Deep, hands-on experience in Java and/or Kotlin, including designing,

Technical Tools

EngineeringFullstack Developer

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

~1 min read

✓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.

Requirements

~1 min read

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

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

Nice to Have

~1 min read

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