Fullstack Engineer (.NET/React) — AI-Driven Delivery Hybrid

Seargin; Warszawa; Hybrid; PLN 150,000 - 230,000; Full time

Job summary

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.

Qualifications

• 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.

Responsibilities

• 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.

Job description

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.