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Hi, I’m Marcel Kai. I’m a Senior Full Stack Engineer with over 10 years of experience building scalable SaaS and cloud-native applications. I design multi-tenant architectures, secure authentication, and monetization workflows with Stripe, while delivering AI-powered capabilities that enhance user engagement and business automation. Throughout my career I’ve worked across the stack with Next.js, React, Node.js, Express, PostgreSQL, and MongoDB, shipping production-grade systems and leading end-to-end product development in Agile settings. I thrive on collaboration with product, design, and backend teams to turn complex requirements into robust, maintainable software, and I continuously improve observability, deployment pipelines, and reliability.…Hi, I’m Marcel Kai. I’m a Senior Full Stack Engineer with over 10 years of experience building scalable SaaS and cloud-native applications. I design multi-tenant architectures, secure authentication, and monetization workflows with Stripe, while delivering AI-powered capabilities that enhance user engagement and business automation. Throughout my career I’ve worked across the stack with Next.js, React, Node.js, Express, PostgreSQL, and MongoDB, shipping production-grade systems and leading end-to-end product development in Agile settings. I thrive on collaboration with product, design, and backend teams to turn complex requirements into robust, maintainable software, and I continuously improve observability, deployment pipelines, and reliability.

Marcel Kai

Full Stack Developer, Back-End Developer, Front-End Developer, +2

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Hi, I’m Marcel Kai. I’m a Senior Full Stack Engineer with over 10 years of experience building scalable SaaS and cloud-native applications. I design multi-tenant architectures, secure authentication, and monetization workflows with Stripe, while delivering AI-powered capabilities that enhance user engagement and business automation. Throughout my career I’ve worked across the stack with Next.js, React, Node.js, Express, PostgreSQL, and MongoDB, shipping production-grade systems and leading end-to-end product development in Agile settings. I thrive on collaboration with product, design, and backend teams to turn complex requirements into robust, maintainable software, and I continuously improve observability, deployment pipelines, and reliability.…Hi, I’m Marcel Kai. I’m a Senior Full Stack Engineer with over 10 years of experience building scalable SaaS and cloud-native applications. I design multi-tenant architectures, secure authentication, and monetization workflows with Stripe, while delivering AI-powered capabilities that enhance user engagement and business automation. Throughout my career I’ve worked across the stack with Next.js, React, Node.js, Express, PostgreSQL, and MongoDB, shipping production-grade systems and leading end-to-end product development in Agile settings. I thrive on collaboration with product, design, and backend teams to turn complex requirements into robust, maintainable software, and I continuously improve observability, deployment pipelines, and reliability.

Available to hire

Hi, I’m Marcel Kai. I’m a Senior Full Stack Engineer with over 10 years of experience building scalable SaaS and cloud-native applications. I design multi-tenant architectures, secure authentication, and monetization workflows with Stripe, while delivering AI-powered capabilities that enhance user engagement and business automation.

Throughout my career I’ve worked across the stack with Next.js, React, Node.js, Express, PostgreSQL, and MongoDB, shipping production-grade systems and leading end-to-end product development in Agile settings. I thrive on collaboration with product, design, and backend teams to turn complex requirements into robust, maintainable software, and I continuously improve observability, deployment pipelines, and reliability.

Skills

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Expert

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Intermediate

Work Experience

Senior Full Stack Engineer at Comarch

May 1, 2023 - Present

Led end-to-end architecture design and execution of multi-tenant SaaS platforms, establishing scalable foundations for growing user bases. Defined and implemented Next.js and TypeScript frontend architectures with Tailwind CSS, delivering responsive, accessible UI across complex SaaS workflows. Engineered Node.js/Express REST APIs with secure authentication and RBAC across distributed services. Integrated Stripe subscription billing workflows, including plan lifecycle, webhook processing, and customer portal automation. Built AI-powered capabilities leveraging OpenAI and Anthropic APIs to automate workflows and provide contextual assistance. Optimized PostgreSQL schemas for data isolation and high concurrency. Implemented CI/CD pipelines with Vercel and Railway for automated deployments and zero-downtime releases. Collaborated with product, design, and backend teams in Agile environments and enhanced observability through structured logging and monitoring.

Senior Full Stack Engineer at Siemens

January 1, 2019 - March 1, 2023

Architected scalable SaaS solutions using Node.js, Express, and modern frontend frameworks; implemented secure multi-role authentication (JWT, OAuth) across enterprise applications; built reusable frontend components with Next.js/React and contributed to design systems. Developed RESTful APIs for complex workflows and third-party integrations, and implemented Stripe billing workflows for recurring revenue. Optimized PostgreSQL and MongoDB data models for performance and integrity, dockerized cloud applications, and established CI/CD automation. Contributed in Agile teams and improved observability with centralized logging, dashboards, and proactive performance tuning.

Full Stack Engineer at Sii Poland

December 1, 2016 - December 1, 2018

Developed backend services for enterprise financial systems using Node.js and RESTful APIs; implemented authentication and authorization to support strict user and admin separation; designed and optimized PostgreSQL schemas for high-integrity financial data; built internal dashboards and reporting tools; integrated external financial and enterprise APIs; participated in Agile cycles and contributed to reliability and performance improvements.

Junior Full Stack Engineer at mBank

September 1, 2015 - December 1, 2016

Contributed to enterprise software for automation, monitoring, and operational workflow management; built backend modules in JavaScript, supported data processing logic, and worked with SQL databases to ensure consistency and reliability; collaborated with senior engineers in Agile environments to deliver feature enhancements and maintain production stability.