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Senior Full Stack Developer (Node.js Focus)

Spain, Madrid B2B · Job Posted May 04, 2026
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Job Description

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.

Job Responsibility

  • 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

Requirements

  • Node.js (v18+) with Express.js framework
  • MongoDB (with Mongoose)
  • PostgreSQL
  • Redis (for caching & session management)
  • RESTful API design
  • JWT
  • session-based authentication
  • React.js (v18+) with functional components and hooks
  • TypeScript for type-safe development
  • Vite
  • TailwindCSS
  • Radix UI / Headless UI
  • Tanstack Query (React Query)
  • modern React state patterns
  • React Router
  • Docker & Docker Compose
  • Git advanced workflows
  • ESLint
  • Prettier
  • Environment configuration and secrets management
  • Fluent English is required
  • You must be registered as an Autónomo or have a corporate entity in Spain

Nice to have

  • 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

What we offer

  • 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

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