Senior Fullstack Engineer (Kotlin&React)

Engg; Poland; Hybrid; PLN 150,000 - 240,000; Full time

Job summary

Xebia is seeking a technically skilled engineer to design and develop APIs for partner integrations. You will lead architectural decisions, define API contracts with OpenAPI/Swagger, and implement backend services in Java and Kotlin, while building modern React/TypeScript frontends.

You will participate in on-call rotations, promote coding standards, and collaborate across distributed teams to deliver scalable data-driven solutions for EU clients.

Qualifications

• Strong commercial experience with Java and Kotlin.
• Proven experience designing, building, and maintaining REST APIs.
• Hands-on experience with API-first design and OpenAPI/Swagger.
• Very good knowledge of React and TypeScript.
• Solid understanding of distributed systems and system-to-system integrations.
• Practical knowledge of OAuth 2.0 and OpenID Connect.
• Experience using AI-powered assistants to improve software delivery.
• Experience making architectural decisions and leading technical discussions.
• Experience defining development standards and providing technical leadership.
• Strong communication and collaboration skills.
• English at least level B2; Polish at least level B1.
• Willingness to visit the client’s office in Warsaw 1–2 times per quarter.

Responsibilities

• Designing and developing APIs enabling integrations with external partners.
• Taking a technical lead in integration architecture and key decisions.
• Defining API contracts and API-first approach using OpenAPI/Swagger.
• Developing backend services using Java and Kotlin.
• Building modern web applications using React and TypeScript.
• Defining and promoting software development standards and best practices.
• Preparing and maintaining technical documentation.
• Leading technical discussions and supporting the team in engineering decisions.
• Working with distributed systems and complex system integrations.
• Participating in the on-call rotation.

Skills

Java

Kotlin

REST APIs

OpenAPI / Swagger

React

TypeScript

Distributed systems

OAuth 2.0 / OpenID Connect

API-first design

Architectural leadership

Development standards

Communication

English (B2)

Polish (B1)

Willingness to visit Warsaw office

Tools

Swagger/OpenAPI

Job description

About Xebia

Xebia is a global AI-first, digital transformation, and engineering partner. With over 25 years of experience and a team of 5,000 professionals across 16 countries, we help organizations design and build scalable products, platforms, and data-driven solutions. We specialize in Artificial Intelligence, Data and Cloud, Intelligent Automation, and Digital Products, combining deep technical expertise with a strong focus on engineering excellence and a people-first culture. In the CEE region, we’re a team of nearly 1,000 experts delivering modern applications, data platforms, and AI solutions for clients such as McLaren, Aviva, Deloitte, Spotify, Disney, ING, UPS, Tesco, Truecaller, AllSaints, Volotea, Schmitz Cargobull, Allegro, InPost, and many, many more. We work with leading technologies including AWS, Azure, GCP, Databricks, and Snowflake, and combine strong engineering culture with a consulting mindset and a continuous focus on growth and knowledge sharing.

Role

You will be: designing and developing APIs enabling integrations with external business partners, taking a technical lead in integration architecture and contributing to key architectural decisions, defining API contracts and following an API-first approach using OpenAPI / Swagger, developing backend services using Java and Kotlin, building modern web applications using React and TypeScript, defining and promoting software development standards and best practices, preparing and maintaining technical documentation, leading technical discussions and supporting the team in making sound engineering decisions, working with distributed systems and complex system integrations, participating in the on-call rotation.

Your profile

strong commercial experience with Java and Kotlin, proven experience designing, building, and maintaining REST APIs, hands‑on experience with API-first design and OpenAPI / Swagger, very good knowledge of React and TypeScript, solid understanding of distributed systems and system-to-system integrations, practical knowledge of OAuth 2.0 and OpenID Connect, practical experience using AI‑powered assistants (e.g. Claude Code, GitHub Copilot, Cursor) to improve productivity, quality, or decision‑making in software delivery, experience making architectural decisions and leading technical discussions, experience defining development standards and providing technical leadership within a team, strong communication and collaboration skills, English at min. B2 level, Polish at min. B1 level, willingness to visit the client's office in Warsaw 1–2 times per quarter. Work from the European Union region and a work permit are required.

Nice to have

experience with data security and regulatory compliance, particularly GDPR, experience with B2B integrations and working with external business partners, experience applying GenAI in a more structured way within the SDLC, including defined workflows, prompt patterns, or tool integrations embedded into daily work, interest in and familiarity with emerging AI‑driven practices (e.g. agent‑based workflows, automation patterns, AI‑augmented development), with a willingness to explore and experiment beyond standard approaches.

Recruitment Process

CV review – HR call – Technical Interview – Client Interview – Decision