Senior Fullstack Engineer (Java&Kotlin + JavaScript)

thecodest; Warszawa; On-site; PLN 223,000 - 290,000; Full time

Benefits offered by this job

Worksmile platform access

Growth-oriented environment

Job summary

The Codest is seeking an Engineer to lead integrations for a major European e-commerce client. You will design API contracts, implement partner integrations, and drive frontend work that consumes external services, bridging backend Java/Kotlin with React-based frontend.

This role emphasizes API-first design, secure auth (OAuth 2.0/OpenID Connect), and architectural leadership. English level B2+ and incident-response availability are required.

Qualifications

• Proven Java (and Kotlin) backend experience.
• Strong experience designing RESTful APIs and API-first design.
• Excellent frontend skills in JavaScript/TypeScript and React.
• Experience defining API contracts and leading technical discussions.
• English level B2+.
• Availability for Incident Response service.

Responsibilities

• API Design & Partner Integration: Designing robust API contracts, writing dev docs, and creating mock environments.
• Frontend Development: Building modern web interfaces that consume partner services.
• Technical Leadership: Bridging Java backend with frontend development for end-to-end solutions.

Skills

Java

Kotlin

RESTful APIs

OpenAPI/Swagger

JavaScript/TypeScript

React

OAuth 2.0/OpenID Connect

API contracts

English B2+

Incident Response

Tools

OpenAPI/Swagger

Job description

Hello World!

We are The Codest - International Tech Software Company with tech hubs in Poland delivering global IT solutions and projects. Our core values lie in “Customers and People First” approach that prioritises the needs of our customers and a collaborative environment for our employees, enabling us to deliver exceptional products and services.

Our expertise centers on web development, cloud engineering, DevOps and quality. After many years of developing our own product - Yieldbird, which was honored as a laureate of the prestigious Top25 Deloitte awards, we arrived at our mission: to help tech companies build impactful product and scale their IT teams through boosting IT delivery performance. Through our extensive experience with product development challenges, we have become experts in building digital products and scaling IT teams.

But our journey does not end here - we want to continue our growth. If you’re goal-driven and looking for new opportunities, join our team! What awaits you is an enriching and collaborative environment that fosters your growth at every step.

Project description:

Our client is one of the largest e‑commerce platforms in Europe, used by millions of users every day. The company connects sellers - from small businesses to global brands - with customers, offering a vast product selection, fast delivery, and a well‑crafted shopping experience.

The subject of the contract is the provision of professional services within the New Ventures Core Tech area. This area focuses on launching innovative services with external strategic partners. Under the operational model, external partners predominantly integrate by calling the Purchaser’s APIs.

Engineer in this project will act as a technical leader and the architectural anchor for these integrations.

The role:

• API Design & Partner Integration: Facilitating seamless onboarding of external partners by designing robust API contracts, writing high-quality developer documentation, and creating mock environments.

• Frontend Development: Driving the development of modern web interfaces (including checkout paths, landing pages, and discovery hubs) that consume the integrated partner services.

• Technical Leadership: Bridging Java backend design with modern frontend development to ensure cohesive, end‑to‑end technical implementation.

• Deep, hands‑on experience in Java (and Kotlin).

• Strong track record in designing, building, and scaling RESTful APIs (API‑first design, OpenAPI/Swagger).

• Excellent frontend skills: proficiency in moder JavaScript/TypeScript and a modern framework (React is highly preferred).

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

• Proven ability to define coding standards, lead technical discussions, and design flexible "API contracts" for external B2B partners.

• English level min. B2+

• Availability for the Incident Response service.

Nice to have:

• Experience dealing with data isolation and security standards

• Eagerness to explore and implement Agentic AI tools (e.g., Copilot, LangGraph) to accelerate engineering workflows.

Our offer:

• 20–26k PLN on a B2B contract

• Access to the Worksmile platform

• A work environment that values growth and innovation - you bring the ideas, and we want to hear them

Recruitment process:

• 30-minute screen with our recruiter, Justyna

• Technical Verification Meetings

• Stakeholder meeting

• Offer