Backend Developer .NET / ASP.NET Web API

Description

Job Summary: We are seeking a Senior Backend Developer specialized in ASP.NET Web API to join a development team involved in the full lifecycle of applications, building well-designed, maintainable, and scalable software. Key Highlights: 1. Solid, technology-driven company with its own product. 2. Participation in the entire software lifecycle. 3. Opportunity to design, build, improve, and maintain backend solutions. Mystery Project is looking for a Senior Backend Developer specialized in ASP.NET Web API on behalf of one of its clients, to join the IT department of an industrial company located in Biscay. This is a solid, technology-driven company with its own product, where software development plays a key role in the evolution of its processes, internal systems, and digital solutions. We seek a candidate with hands-on backend experience, ability to design robust APIs, strong technical judgment, and enthusiasm to work in a stable, collaborative, and quality-oriented development environment. What will be your mission? You will join the development team and participate in the full application lifecycle: from requirements gathering and technical design through development, testing, documentation, deployment, and support of implemented solutions. This position is ideal for someone who enjoys building well-designed, maintainable, and scalable software using Microsoft technologies while collaborating with other technical and functional teams. What will you do daily? Your main responsibilities will include: Designing, developing, and maintaining REST APIs with ASP.NET. Participating in the technical definition of new features together with product owners and internal teams. Collaborating with frontend profiles to define integrations, dependencies, and communication between applications. Designing backend architecture and business logic. Participating in database design and evolution. Applying best practices in development, testing, and documentation. Working with SOLID principles, dependency injection, and clean code. Performing troubleshooting, incident analysis, and continuous improvement of existing APIs. Participating in application publishing, deployment, and evolution processes. Collaborating in a technically demanding, solution-oriented work environment. Key Technologies and Knowledge We seek experience in environments such as: ASP.NET / ASP.NET Core. ASP.NET Core 6\.0 or later versions. REST API development. Entity Framework. Backend testing. SOLID principles. Dependency injection. Databases. Backend architecture. Application integration. Backend application deployment. Additionally, experience or knowledge in the following areas will be considered favorably: Microservices architectures. Cloud environments. Azure. IIS. HTML, JavaScript, and CSS. Technical English. Industrial, manufacturing, or machine-tool environments. What Are We Looking For? We would like to hire a candidate with: A degree in Technical Engineering, Computer Engineering, Mathematics, Physics, or a related field. Minimum 5 years of experience developing applications with ASP.NET. Solid experience in REST API development. Experience working with Entity Framework. Experience in backend testing. Knowledge and application of SOLID principles. Ability to work autonomously with sound technical judgment. Strong communication skills with both technical and non-technical teams. Initiative, responsibility, and quality orientation. Residence in Biscay or availability to work in a hybrid mode in the Zaldibar area. The position operates in a hybrid model: 3 days per week onsite. 2 days per week remote. The workplace is located in the Zaldibar area, Biscay. Working Hours Standard schedule is Monday to Friday, full-time: From 8:00 AM to 1:00 PM. From 1:30 PM to 4:30 PM. What Do We Offer? You will join an established industrial company with a stable IT department and technology projects tied to product, processes, and internal digital transformation. Additionally, you will find: Permanent contract. Stable, long-term project. Hybrid work model. Technically advanced environment with high responsibility. Participation in the full software lifecycle. Work with Microsoft technologies and backend development. Collaborative team focused on best practices. Opportunity to contribute technical judgment and enhance existing solutions. A serious, stable, and forward-looking professional environment. Why Might This Interest You? Because this is not merely about coding isolated tasks. It is an opportunity to join a company with its own product, stability, and genuine need to continue evolving technologically — an environment where you can design, build, improve, and maintain backend solutions with direct organizational impact. If you have experience with ASP.NET, enjoy developing well-structured REST APIs, and are seeking a stable technical project in Biscay, we want to meet you. To better evaluate your application When applying, it would help us if you could briefly indicate: How many years of experience you have developing applications with ASP.NET. Whether you have worked with ASP.NET Core 6\.0 or later versions. How many years of experience you have developing REST APIs. Your experience applying SOLID principles and dependency injection. Whether you reside in Biscay or are available to work hybrid in Zaldibar. A relevant project where you developed REST APIs with ASP.NET Core, specifying your role, technologies used, and main functionalities. Your annual gross salary expectation. At Mystery Project, we will handle your application with absolute confidentiality. Our client will only learn about your profile if there is a genuine match with the position.