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

Seargin Ltd; Warszawa; Hybrid; PLN 180,000 - 270,000; Full time

Benefits offered by this job

B2B Contract

MultiSport

Private Medical Care

International Projects

Job summary

Seargin Ltd in Warsaw is seeking aSenior Full-Stack Engineer to design and implement scalable architectures across .NET and React stacks. You will translate clear specifications into working code using AI-assisted tools and validate code through rigorous testing and security checks.

You will collaborate with PMs and BAs, participate in client workshops, and ensure fast, predictable delivery within a dynamic, international firm. Strong Polish and English communication are essential.

Qualifications

• 5+ years of commercial full-stack engineering experience with independent responsibility for project architecture.
• Very good knowledge of .NET and C#, including Web API and Entity Framework Core.
• React with TypeScript for modern frontend development.
• Proven AI-assisted code generation and refactoring workflow with explainable validation.
• Experience with spec-driven or test-driven development and solid test coverage.
• Good knowledge of REST APIs, SQL Server or PostgreSQL, Git, and CI/CD.
• Ability to work independently and own delivery in a small team.

Responsibilities

• Design and maintain scalable architecture across .NET backend and React frontend.
• Translate specs into working code using AI-assisted tools.
• Review and validate code with testing, security, and readability checks.
• Collaborate with PMs, BAs, and client workshops; participate in demos.
• Coordinate with DevOps on CI/CD and environments; partner with QA on testing.

Skills

Full-Stack

.NET & C#

React TS

AI-assisted Development

Spec-Driven TD

REST APIs

Relational DBs

Delivery Ownership

Polish & English

Tools

Git

CI/CD

Azure

Job description

• Design simple, predictable, and maintainable application architecture across .NET backend and React frontend
• Specification-Driven Development
  • Translate clear specifications and requirements into working code using AI-assisted development tools
• Code Quality & Validation
  • Review and validate both personal and AI-generated code through testing, edge-case analysis, security checks, and readability reviews
• Testing & Coverage
  • Apply rigorous testing practices and ensure implementation provides reliable coverage against defined requirements
• Business Collaboration
  • Work closely with PMs and BAs to clarify specifications and participate in client workshops and demonstrations
• Collaborate with DevOps on CI/CD and environments and with QA on testing activities and acceptance criteria
• Spec-Driven Delivery
  • Work in short iterations with clear specifications, fast implementation, regular client demos, and predictable progress
• Documentation & Handover
  • Maintain clear documentation and repeatable solutions to support smooth project handover and future development

Requirements

• Full-Stack Experience
  • 5+ years of commercial full-stack engineering experience with independent responsibility for project architecture
• Very good knowledge of .NET and C#, including Web API and Entity Framework Core
• React & TypeScript
  • Very good knowledge of React with TypeScript for modern frontend development
• AI-Assisted Development
  • Proven personal workflow for AI-assisted code generation and refactoring with the ability to explain how AI-generated output is validated
• Spec-Driven & Test-Driven Development
  • Hands-on experience with spec-driven or test-driven development and the ability to work from specifications with solid test coverage
• Development Best Practices
  • Good knowledge of REST APIs, relational databases such as SQL Server or PostgreSQL, Git, and CI/CD
• Delivery Ownership
  • Ability to work independently and take full ownership of delivery within a small team and tight timelines
• Business Communication
  • Strong Polish and English communication skills with confidence participating in discussions with enterprise clients

What we offer

• B2B Contract
  • Employment based on a B2B contract
• Stable and Dynamic International Firm
  • Opportunity to work in a stable, dynamically developing international company
• Engaging Projects and Latest IT
  • Chance to participate in interesting projects and work with the latest information technologies
• Competitive Rates
  • Attractive remuneration rates offeres
• Renowned International Projects
  • Involvement in the most prestigious international project
• MultiSport and Private Medical Care
  • MultiSport and Private Medical Care

Nice to have

• Experience working directly with enterprise clients or in a Forward Deployed Engineer-type role
• Understanding of data modelling for reporting and collaboration with Power BI teams
• Experience with Azure, including App Service and Azure SQL, or another cloud platform
• Experience working in a consulting or project-based environment

Work with us