Senior Fullstack Engineer: Kotlin & React, API-First

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

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.