Principal Full Stack Engineer : AI First Development

Principal Full Stack Engineer – .NET / Angular / AWS Remote · Full-time About Lumen Dynamics Lumen Dynamics is a GPTW-certified, people-first company that partners with American software product companies to scale their development teams. We connect Brazil’s top 5% talent with innovative U.S. companies through a unique combination of staff augmentation and employer-of-record services. We are supporting a U.S.-based technology company building its next-generation SaaS platform from the ground up. We're looking for a Principal Full Stack Engineer to help define the platform architecture, establish engineering standards, and deliver secure, scalable, and high-performance applications using modern technologies and AI -assisted development. Challenge We're looking for a hands-on Principal Full Stack Engineer who is passionate about solving complex technical challenges while remaining deeply involved in coding. This role is ideal for an experienced technical leader who enjoys architecting scalable SaaS platforms, mentoring engineers, and driving engineering excellence through modern development practices. You'll work alongside Architects, Product Managers, UX Designers, and engineering teams to build a cloud-native platform from the ground up, leveraging AI -assisted development and a full Shift-Left engineering approach. You'll have the opportunity to influence architecture, development standards, engineering culture, and the adoption of cutting-edge AI technologies across the software development lifecycle. What You'll Do Design, develop, and deliver scalable multi-tenant SaaS applications using .NET , Angular, SQL Server/PostgreSQL, and AWS. Own end-to-end delivery of features, from domain modeling and backend services through frontend implementation, automated testing, deployment, and production support. Generate production-ready code using AI -assisted development tools such as GitHub Copilot, Cursor, Claude Code, and Replit-based workflows. Collaborate with Architects to define application architecture and validate technical approaches through Proof of Concepts (PoCs). Design systems using modern architectural patterns including: Domain-Driven Design (DDD) CQRS Event-Driven Architecture Backend-for-Frontend (BFF) API Gateway Build modern Angular applications using standalone components, Signals, RxJS, Reactive Forms, and reusable UI architectures. Design efficient database models and optimize SQL performance for high-volume SaaS applications. Implement scalable integration, messaging, and distributed system patterns. Troubleshoot complex production issues, perform root cause analysis, and continuously improve system reliability and performance. Mentor engineers through architecture guidance, technical leadership, code reviews, and knowledge sharing. Drive engineering best practices around quality, automation, security, observability, and AI -powered software development. Required Skills & Qualifications 10+ years of professional software engineering experience. Experience as a Principal Engineer, Lead Engineer, Software Architect, or equivalent technical leadership role. Expert-level proficiency with: C# .NET Entity Framework Angular SQL Server and/or PostgreSQL Strong experience designing and building: Multi-tenant SaaS applications Distributed systems REST APIs Event-driven architectures Deep expertise with: Domain-Driven Design (DDD) CQRS Secure and scalable application architecture Advanced knowledge of SQL optimization, data modeling, Redis, and OpenSearch/Elasticsearch. Experience with automated testing, CI/CD pipelines, Git, Agile methodologies, and modern DevOps practices. Experience with observability platforms and code quality tools such as SonarQube, Mend, Datadog, CloudWatch, or similar. Hands-on experience using AI -powered development tools such as GitHub Copilot, Cursor, Claude Code, or similar coding assistants. Strong leadership, mentoring, communication, and problem-solving skills. Bachelor's degree in Computer Science, Engineering, or equivalent professional experience. English proficiency at C1 level (written and verbal). Nice to Have Hands-on experience with AWS services including ECS/Fargate, Lambda, SQS/SNS, S3, and RDS. Experience with Infrastructure as Code using Terraform or AWS CDK. Experience with GraphQL, WebSockets, or Server-Sent Events (SSE). Experience with Playwright or SmartBear TestComplete. Familiarity with monitoring platforms such as Datadog, CloudWatch, or New Relic. Experience building AI -enabled applications using: Large Language Models (LLMs) Retrieval-Augmented Generation (RAG) Model Context Protocol (MCP) AI -powered application architectures Team & Environment Greenfield SaaS platform with significant architectural ownership. Highly collaborative engineering culture working alongside Architects, Product Managers, UX Designers, and senior engineers. Strong focus on engineering excellence, automation, AI -assisted development, and Shift-Left practices. Opportunity to influen