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Sebastian Karasiewicz

Sebastian Karasiewicz

Senior Full-Stack Engineer | Cloud-Based Systems | Scalable Web Platforms | Team Leadership

Senior Full-Stack Engineer with over 12 years of experience delivering scalable, cloud-native applications across finance, SaaS, e-commerce, and real estate. Specialized in modern web architectures using React, Next.js, and TypeScript, with strong backend expertise in Python, Go, and Java. Daily practitioner of AI-driven development, leveraging Claude and other AI tools for enhanced productivity and code quality. Experienced in AWS serverless infrastructure, API development, and leading teams to build solutions that balance performance, maintainability, and business impact.

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Technical Skills

TypeScriptJavaScriptReactNext.jsNode.jsPythonDjangoFlaskGoRubyRailsRESTful APISQLNoSQLPostgreSQLMySQLMongoDBAWSAWS LambdaS3SNSSESAPI GatewayAWS ServicesGCPGoogle App EngineCloud DatastoreCloud StorageAzureVercelDockerGitGitHubE2EUnit TestingSEOTailwindshadcn/uiAlgoliaContentful

Work Experience

  • Senior Full-Stack Engineer

    Diageo

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    • Migrated landing pages for brands like Yeni Raki and Johnnie Walker to a modern, SEO-optimized architecture using React and Next.js
    • Unified frontend architecture across multiple brand websites
    • Led the testing strategy with robust unit and E2E test coverage
    • Contributed to internal tooling and frameworks
    • Tech stack: React, Next.js, Tailwind, Vercel, Contentful, Algolia

  • Senior Lead Software Engineer

    SoftKraft

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    • Switchboard: Led the frontend team for an enterprise-grade network cost optimization tool
    • VideoTranscoding: Built a serverless video processing pipeline on AWS
    • CAAS: Developed an event-driven microservice in Go for transforming and routing communications
    • MLSWorldwide: Created a backend API and ingestion engine for a global property aggregator
    • Tech stack: React, Python, Django, Go, Docker, AWS (SES, SQS, SNS, Lambda, API Gateway, S3)

  • Senior Full-Stack Engineer

    Sofomo

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    • JUUL: Extended a Rails-based system for selling e-cigarettes, focused on feature development and stability
    • Voyajoy: Developed APIs for listing synchronization across Airbnb, Booking.com, FlipKey
    • Strings: Built a social platform combining features of Facebook and Twitter (X)
    • Tech stack: Python (Django, Flask), React, Ruby on Rails, PostgreSQL, InfluxDB, Docker, Azure Cloud

  • Software Development Manager

    PLUSGPS Q4B Sp. z o.o. Sp. k.

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    • StaffSupervisor: Developed backend for monitoring telemarketer activities with Go + React
    • Beecron: Delivered beacon-driven indoor navigation backend
    • TachoOnline: Built APIs for driver work-time tracking based on tachograph data
    • Tech stack: Go, Google App Engine, Cloud Datastore, Cloud Storage, Docker

  • Backend Java Developer

    Coderion

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    • IdeaFaktor: Built and maintained a factoring platform for financial institutions
    • Tech stack: Java, Apache Wicket, Hibernate, PostgreSQL

  • Backend Developer

    Selleo

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    • PipelineDeals: Contributed to a CRM platform for sales teams
    • Tech stack: Ruby on Rails, RSpec, MySQL, CoffeeScript, Backbone.js, SASS

  • Backend Developer

    Merlin.X

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    • Merlin X: Worked on backend feature development and system upgrades for a major B2B travel reservation system
    • Tech stack: PHP, JavaScript, jQuery, MySQL

Get In Touch

Email

sebastian@karasiewicz.dev
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Phone

+48 535 303 183
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Languages

Polish – Native

English – C1

Japanese – Beginner