Fullstack Developer Node.js

Yopeso has been developing a diverse range of software products, from large-scale applications to smaller solutions, for 20 years. We strive to maintain the highest professional standards and continuously challenge ourselves to develop high-quality, high-performance, and secure software solutions. Our approach is rooted in efficient collaboration among passionate professionals working in agile teams. About The Project We are building a sophisticated, open-source AI conversational platform that integrates multiple AI providers to deliver a rich, cutting-edge user experience. We are looking for an experienced Senior Full Stack Developer to join our core team as a B2B partner. While this is a full-stack role, your primary focus (80%) will be driving the Backend architecture (Node.js), while occasionally supporting the modern React frontend. If you are passionate about LLMs, prompt engineering, and building robust, scalable systems, this is the project for you. Contract: 100% B2B / Independent Contractor (You must be registered as an Autónomo or have a corporate entity in Spain). Location: Hybrid. Availability to be on-site at our Madrid office 2-3 days per week for architectural alignment, team syncs, and workshops. The rest of the time is remote. Language: Fluent English is required. Take ownership of the backend components, building a sophisticated AI chat platform. and complex chat functionalities. Design and optimize database structures using MongoDB and PostgreSQL. Ensure code quality through automated testing and participate in high-standard code reviews. Deploy and maintain containerized applications using Docker. Core Technical Stack (Required) Backend (Primary Focus): Node.js (v18+) with Express.js framework. Databases: MongoDB (with Mongoose), PostgreSQL, Redis (for caching & session management). Frontend: React.js (v18+) with functional components and hooks. TypeScript for type-safe development. State & Data: Tanstack Query (React Query), modern React state patterns, React Router. DevOps & Tools: Docker & Docker Compose. Git advanced workflows, ESLint, Prettier. Environment configuration and secrets management.