We are a U.S.\-based SaaS startup looking for a Senior Full Stack MERN Developer to work directly with our hands\-on CTO. This is a senior role for someone who can not only ship features but also improve architecture, enforce clean code practices, and scale an MVP into a production\-grade SaaS platform. We are prioritizing candidates based in Argentina or Brazil to align with U.S. West Coast (Los Angeles) working hours. **What You’ll Do:** * Take ownership of an existing MERN MVP and evolve it into a scalable, production\-ready product * Refactor and improve messy or inefficient code for clarity, maintainability, and performance * Debug and resolve complex cross\-layer issues quickly (MongoDB, Node.js, React, CI/CD) * Design and optimize MongoDB schemas, indexes, Atlas Search, and triggers * Build event\-driven workflows with MongoDB triggers) * Integrate AI/LLM services (OpenAI, AWS Comprehend, embeddings, NLP features) into production apps * Support and improve Twilio\-based messaging and webhook integrations * Contribute to new product features end\-to\-end (backend \+ frontend) **Requirements:** * Expert in MERN stack: MongoDB, Express.js, React.js, Node.js * Deep MongoDB knowledge (indexes, performance, aggregation, triggers) * Strong backend/infra skills: Node.js async/memory, AWS (EC2, S3, Lambda), CI/CD pipelines * Experience integrating AI services into production apps * Excellent English communication (mandatory) * Availability during U.S. West Coast working hours (PST/PDT) * Based in Argentina or Brazil **Benefits:** * Direct collaboration with a technical founder/CTO * Long\-term role solving real production challenges (not just CRUD apps) * Remote role with U.S. time zone collaboration * Opportunity to grow into a lead role as the team expands