Senior Fullstack Developer (.NET, React, Claude) w Kraków, Polska

We’re hiring a hands-on Software Engineers to help us build two brand-new products. This is a total greenfield setup: new architecture, new codebases, new delivery approach, built as separate systems in .NET + React, with a strong focus on AI-first engineering. Day to day, you’ll use tools like Claude Code and work with a framework that leverages AI agents to speed up delivery, improve quality, and automate repetitive development work. You’ll join early, which means real influence: shaping architecture and technical direction, helping define quality standards, testing approach, deployments, and how the team works. The setup is closer to a startup-style product team than a rigid corporate project, while still having dedicated Engineering Manager, Product Manager, and Tech Lead to support delivery. What you’ll do: Build backend services and RESTful APIs using C#/.NET (.NET Core / .NET) Develop and evolve web frontends in React (experience layer / UI consuming APIs) Contribute to solution design in a service-oriented / microservices architecture Use Claude Code in daily work: prompt-driven coding, refactoring, test generation, and building/iterating on coding agents Help establish and improve engineering practices: testing strategy, code standards, CI/CD, quality automation Implement and enhance observability (logging, metrics, tracing) and API monitoring Work with cloud-native services (Azure/AWS/GCP) and containerised deployments (e.g., Kubernetes) Collaborate in an Agile setup: refinement, estimation, code reviews, continuous improvement What we’re looking for: minimum 5 years of professional software development experience Strong commercial experience with C#/.NET and building REST APIs Proven delivery of web applications / SaaS products Solid engineering fundamentals (data structures, algorithms, design patterns) Experience with cloud architectures (Azure, AWS or GCP) and cloud-native services Understanding of distributed systems concepts (scalability, reliability, concurrency) Strong problem-solving skills, clear communication, and a collaborative mindset Genuine interest in combining classic software engineering with practical AI tooling (Claude / agents) Nice to have: Experience with API gateways, identity/authentication, messaging (Kafka/SQS/RabbitMQ) Workflow orchestration / job scheduling / search-related experience Container orchestration knowledge (Kubernetes/Nomad/Mesos) Strong front-end fundamentals beyond React (HTML/CSS/JS best practices) Experience leading technical initiatives or acting as a go-to engineer on key topics