Fullstack (.NET + React) Developer

Instinctools; Polska; Hybride; PLN 180 000 - 240 000; Plein temps

Avantages offerts par ce poste

Business-critical work

Legacy + modern tech

Technical autonomy

Pragmatic AI adoption

Sustainable pace

International collaboration

Professional development

Résumé du poste

instinctools is seeking a Senior Fullstack (.NET + React) Developer to join a long-term project for a multinational tech company in the wireless sector. You will design, implement, and modernize backend services using C#/.NET across modern and legacy platforms, while contributing to frontend tasks with React when needed.

The role emphasizes production-quality engineering, architectural involvement, and AI-assisted development with occasional onsite sessions in Munich 1–2 times per year.

Qualifications

• 5+ years of professional experience in backend development with C#/.NET in enterprise production environments.
• Strong hands-on experience with modern .NET and .NET Framework.
• Practical experience with .NET 8 and Microsoft SQL Server.
• Experience working with legacy .NET applications, particularly .NET Framework 4.8, ASP.NET Web Forms, and IIS.
• Strong SQL Server skills, including database schema design, query optimization, and performance tuning.
• Solid experience designing and implementing REST APIs, including contract-first API development.
• Working proficiency with React and the ability to contribute to frontend tasks when required.
• Hands-on use of AI-assisted development tools and a willingness to apply them across backend, frontend, and testing activities.

Responsabilités

• Design, develop, and maintain backend services and APIs using C# and .NET.
• Work with both modern .NET 8 and legacy .NET Framework 4.8 applications, including ASP.NET Web Forms hosted on IIS.
• Design, implement, and optimize SQL Server schemas, queries, and data access, with a focus on performance and reliability.
• Own REST API design and contract-first implementation, collaborating with the team’s system architect and other engineers.
• Use AI-assisted development tools to implement features, investigate issues, generate or improve tests, and support frontend development.
• Review, test, and validate AI-generated code rather than relying on generated output without verification.
• Take ownership of tasks beyond your primary backend specialization when this helps the team deliver a complete solution.
• Contribute to frontend changes in React when needed, using AI tools to support implementation and testing.
• Participate in architectural discussions and contribute to the gradual modernization of existing systems.
• Coordinate technical dependencies across teams, share knowledge, and support the safe delivery of changes to production.
• Participate in sprint planning, PI planning, defect triage, and other team delivery activities.
• Collaborate with a distributed international team and participate in periodic onsite sessions in Munich, approximately 1–2 times per year.

Connaissances

C#

React

ASP.NET

SQL Server

REST APIs

AI-assisted development

Outils

IIS

Azure DevOps

Docker

Description du poste

*instinctools is a software development company that provides custom software solutions for businesses of all sizes. Our team works closely with clients to understand their specific needs and provide personalized solutions that meet their business requirements. Out team is looking for a Fullstack (.NET + React) Developer to join a long-term project for a multinational technology company operating in the wireless communications industry.

You will contribute to enterprise software that supports business-critical production operations. The systems require a high level of reliability, careful engineering decisions, and a clear understanding of how software changes affect real-world business processes.

The project combines established enterprise systems with modern technologies and ongoing modernization efforts. This creates an opportunity to work with both mature codebases and evolving architecture, contributing to technical decisions rather than limiting your work to routine maintenance.

The team values engineering quality, technical ownership, and pragmatic problem-solving. Developers have autonomy in choosing effective solutions within project requirements, with an emphasis on making changes safely and delivering reliable results.

Location: Europe.

What you will get hands-on with

Modern .NET development: Build and maintain backend services using .NET 8, C#, and Microsoft SQL Server.

Enterprise legacy modernization: Work with .NET Framework 4.8, ASP.NET Web Forms, and IIS as part of the regular development workflow, helping evolve established systems while maintaining their stability.

AI-assisted engineering: Use AI tools to accelerate development across backend, frontend, and testing tasks. Evaluate generated code, validate results, and take responsibility for the quality of the final solution.

Fullstack delivery with a backend focus: Lead backend implementation while using AI-assisted development to handle UI changes and testing when needed.

API design and architecture: Work with contract-first APIs, REST principles, and architectural decisions that affect multiple components.

Production-quality engineering: Apply careful testing, code review, quality controls, and safe release practices in a business-critical environment.

Cloud and delivery practices: Gain exposure to cloud-native approaches, containerization, and CI/CD practices where applicable.

What you will do

Design, develop, and maintain backend services and APIs using C# and .NET.

Work with both modern .NET 8 and legacy .NET Framework 4.8 applications, including ASP.NET Web Forms hosted on IIS.

Design, implement, and optimize SQL Server schemas, queries, and data access, with a focus on performance and reliability.

Own REST API design and contract-first implementation, collaborating with the team’s system architect and other engineers.

Use AI-assisted development tools to implement features, investigate issues, generate or improve tests, and support frontend development.

Review, test, and validate AI-generated code rather than relying on generated output without verification.

Take ownership of tasks beyond your primary backend specialization when this helps the team deliver a complete solution.

Contribute to frontend changes in React when needed, using AI tools to support implementation and testing.

Participate in architectural discussions and contribute to the gradual modernization of existing systems.

Coordinate technical dependencies across teams, share knowledge, and support the safe delivery of changes to production.

Participate in sprint planning, PI planning, defect triage, and other team delivery activities.

Collaborate with a distributed international team and participate in periodic onsite sessions in Munich, approximately 1–2 times per year.

What we expect from you

5+ years of professional experience in backend development with C#/.NET in enterprise production environments.

Strong hands‑on experience with modern .NET and .NET Framework.

Practical experience with .NET 8 and Microsoft SQL Server.

Experience working with legacy .NET applications, particularly .NET Framework 4.8, ASP.NET Web Forms, and IIS.

Strong SQL Server skills, including database schema design, query optimization, and performance tuning.

Solid experience designing and implementing REST APIs, including contract-first API development.

Working proficiency with React and the ability to contribute to frontend tasks when required.

Hands‑on use of AI‑assisted development tools and a willingness to apply them across backend, frontend, and testing activities.

Ability to critically review AI-generated code, identify potential issues, verify behavior, and maintain production‑quality standards.

Ownership mindset: willingness to take a task from requirements clarification through implementation, testing, and safe production delivery, including work outside your primary specialization.

Ability to work independently, make technical decisions, and coordinate dependencies with other engineers.

Strong problem‑solving skills and attention to detail, particularly when making changes to business‑critical systems.

• English: Upper–Intermediate (B2+) or higher.

Nice to have

Experience with AWS.

Familiarity with Azure DevOps and its CI/CD capabilities.

Experience with SonarQube or similar code quality and static analysis tools.

Experience with containerization and cloud‑native development practices.

Familiarity with SAFe, PI planning, or similar scaled Agile delivery frameworks.

Experience in manufacturing, industrial software, or other business‑critical enterprise domains.

Strong ownership and the ability to work independently.

Pragmatic engineering mindset: choosing solutions based on their value and suitability rather than novelty alone.

Willingness to take responsibility beyond a narrowly defined technical role.

Clear communication and effective collaboration across distributed teams.

Ability to assess risks and understand the potential impact of changes.

Openness to feedback, knowledge sharing, and continuous improvement.

A quality‑first approach without unnecessary pressure to deliver at the expense of reliability.

What we offer

Business‑critical engineering work. Contribute to enterprise systems where software reliability and the quality of engineering decisions have a tangible impact on business operations.

A combination of legacy and modern technologies. Work with established enterprise applications while contributing to their gradual modernization and architectural evolution.

Technical autonomy. Have the freedom to propose and implement effective solutions within the requirements and constraints of the project.

Pragmatic AI adoption. Use AI where it brings real value, with an emphasis on engineering judgment, validation, and maintainable code rather than using AI for its own sake.

A sustainable working pace. Focus on thoughtful implementation, sound technical decisions, and reliable results rather than artificial urgency.

International collaboration. Work with a distributed team and have opportunities to participate in onsite sessions in Munich.

Professional development. Access learning opportunities, seminars, conferences, and time to explore new technologies.

Join *instinctools to work on challenging enterprise software, combine modern development practices with legacy modernization, and take ownership of solutions that matter to real business operations.

Important Notice Regarding Your Personal Data Security

At *instinctools, we take the security of your personal data very seriously. Please be informed that all official communication, offers, and correspondence from *instinctools are conducted only through our verified company resources, including:

Important Reminder: *instinctools will NEVER ask for sensitive personal or financial information, including: - Bank account details - Credit/debit card numbers - Passwords or PINs

If you receive any suspicious requests claiming to be from *instinctools via unofficial channels (e.g., personal emails, social media DMs, or phone calls), please:

- Do not respond or share any information.

- Report it immediately toofficial contact.

Your trust and security are our top priorities. For any questions, always verify through our official channels.

Thank you for your attention and cooperation.