Fullstack Engineer (.NET / React) – AI-Native Spec-Driven Delivery

Seargin; Warszawa; Híbrido; PLN 150.000 - 230.000; Jornada completa

Descripción de la vacante

Seargin in Warsaw seeks a Senior Fullstack Engineer to design and build an AI-native, spec-driven Checkout app for a major FMCG client. You will own backend (.NET/C#) and frontend (React/TypeScript) architecture, collaborate with PMs and client teams, and ensure robust quality with AI-assisted tooling.

The role is contract, full-time, hybrid from Warsaw, with a strong emphasis on delivery ownership, testing, and scalable code that lasts beyond the project.

Formación

• Senior level: 5+ years of commercial experience as a full-stack engineer.
• Strong backend .NET/C# (Web API, EF Core) and frontend React (TypeScript).
• Proven AI-assisted workflow for code generation and validation.
• Experience with spec-driven / test-driven development and solid test coverage.
• Good practices: REST APIs, SQL Server / PostgreSQL, Git, CI/CD.
• Independence and ownership of delivery in tight timelines.
• Fluent English; Polish language skills are a plus.

Responsabilidades

• Designing application architecture (backend .NET, frontend React) and keeping it simple and maintainable.
• Turning specifications into working code using AI tools and owning output quality.
• Rigorous review and validation of code, including testing, edge cases, and security.
• Collaborating with PMs/BAs to clarify specs and participating in client demos.
• Working with DevOps and QA to ensure CI/CD pipelines and acceptance criteria.
• Maintaining documentation and ensuring smooth handover after engagement ends.

Descripción del empleo

As a dynamic multinational tech company operating in 50 countries, we drive innovation and create projects that shape the future and greatly enhance the quality of life. You will find our solutions in the space industry, supporting scientists in the development of cancer drugs, and implementing innovative technological solutions for industrial clients worldwide. These are just some of the areas in which we operate!

About the role

Currently, for a new Seargin project we are looking for Fullstack Engineer (.NET / React) – AI-Native Spec-Driven Delivery to design and build Checkout app for our big client from FMCG industry.

Employment Type: contract

FTE: full-time

Working mode: hybrid from Warsaw (flexible)

Responsibilities:

• Designing application architecture (backend .NET, frontend React) — keeping it simple, predictable, and maintainable beyond the end of the project.
• Turning specifications and requirements into working code using AI tools, while taking full ownership of the quality of the outcome.
• Rigorous review and validation of code (both your own and AI-generated): testing, edge cases, security, and readability.
• Working closely with PMs/BAs to clarify specifications and participating in workshops and demos with the client.
• Collaborating with DevOps (CI/CD, environments) and QA (testing, acceptance criteria).
• Maintaining documentation and repeatability so that the project can be smoothly handed over and further developed after the engagement ends.

How We Work:

• Model: Spec-driven — clear specifications first, followed by fast code generation and rigorous validation.
• Rhythm: Short iterations, regular client demos, and predictable progress.
• Culture: Quality and client relationships are in our DNA — the code needs to work not only for the demo, but a year from now as well.

Requirements:

• Senior level: 5+ years of commercial experience as a full-stack engineer, including independently leading project architecture.
• Very good knowledge of .NET / C# on the backend (Web API, EF Core).
• Very good knowledge of React (TypeScript) on the frontend.
• A proven, personal AI-assisted workflow for code generation and refactoring, along with the ability to explain how you validate AI-generated output.
• Hands-on experience with spec-driven / test-driven development — able to work from specifications and provide solid test coverage.
• Good knowledge of best practices: REST APIs, relational databases (SQL Server / PostgreSQL), Git, CI/CD.
• Independence and ownership of delivery — small team, tight timelines.
• Strong business communication skills (Polish and English) — participation in discussions with enterprise clients.

Nice to Have:

• Experience working directly with enterprise clients or in a role similar to a Forward Deployed Engineer.
• Understanding of data modelling for reporting, facilitating collaboration with the Power BI team responsible for reporting.
• Experience with Azure (App Service, Azure SQL) or another cloud platform.
• Experience in a consulting / project-based environment.

To learn more about Seargin, please visit our web page: www.seargin.com