Full Stack Developer

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. The Setup & Work Arrangement - Location: Hybrid. We require 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. What You Will Do - Take ownership of the backend components, building a sophisticated AI chat platform. - Implement new AI provider integrations (OpenAI, Claude, etc.) and complex chat functionalities. - Design and optimize database

structures using MongoDB and PostgreSQL. - Implement and maintain real-time features using WebSocket connections. - 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). - RESTful API design, JWT, and session-based authentication. Frontend: - React.js (v18+) with functional components and hooks. - TypeScript for type-safe development. - Tooling & Styling: Vite, TailwindCSS, Radix UI / Headless UI. - 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. Nice-to-Haves (Bonus Points) - Hands-on experience with OpenAI API, Anthropic Claude, or similar LLM APIs. - Understanding of prompt engineering and AI model integration. - Experience with the LangChain framework. - RAG (Retrieval-Augmented Generation) implementation experience. Why Partner With Us? - Work on a modern, open-source AI product. - Direct collaboration with product and design stakeholders in a fast-paced, innovative environment. - Long-term B2B collaboration with competitive market rates.

Yopeso has been developing a diverse range of software products, from large-scale applications to smaller solutions, for 20 years. With a growing team of over 300 employees across five locations, we are dedicated to fostering a culture of growth, transparency, and professionalism. At Yopeso, we value authenticity, curiosity, and ambition. These values drive us to build strong connections within our community and with our partners, ensuring trust, integrity, and transparency in all our business practices. 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. Guided by curiosity and ambition, we strive to create products that are meaningful and impactful, while remaining true to our authentic selves. About the Project We are building a sophisticated, open-source AI