Software engineer

I build web products end to end, with most of my depth in the frontend: live data, state that has to survive a refresh, and screens people make real decisions on. On the backend I have worked in C#, Elixir and Node. I studied mathematics at KTH and live in Lausanne.

carl.cronsioe@gmail.com

Work

  1. Frontend Developer, RA2 Tech

    2024–2026 · Montreux, Switzerland

    Lending and trading products for digital assets, with an AI assistant. Users moved real money through these screens.

    • Built the frontend for an AI chat assistant, streaming the model's answers into a responsive interface.
    • Wrote a transaction tracker that keeps the state of ongoing transactions through page refreshes, account changes and several actions at the same time. Now used in two products.
    • Built a live liquidity depth chart for traders, plus token sale and staking pages, deployed on four blockchain networks.

    React 19, TypeScript, Vite, TanStack Router and Query, Tailwind, Vitest, Storybook, Playwright

  2. Frontend Developer, Kambi

    2019–2024 · Stockholm, Sweden

    B2B sportsbook platform for more than 50 operators in 60 jurisdictions, processing 500 million transactions a month.

    • Built and optimised the live event page for the largest client, where WebSocket updates and rendering performance decided the experience for hundreds of thousands of concurrent users.
    • Built three applications for betting shops on one shared component set: self-service terminals, clerk tools for the counter and digital signage for in-venue screens.
    • Wrote the live odds rendering over WebSockets for signage screens that ran unattended for hours and had to stay in sync.

    React, TypeScript, WebSockets, REST, Jest, AWS

  3. Fullstack Developer, Time Care

    2017–2019 · Stockholm, Sweden

    Scheduling software for hospitals and home care providers.

    • Built backend services in C# and ASP.NET with Microsoft SQL Server.
    • Built frontend features in React and Backbone.js for the scheduling and personnel management interfaces.

    C#, ASP.NET, Microsoft SQL Server, React, Backbone.js

Own products

  • Langu

    Source

    Turns any YouTube video or podcast into a language lesson, with karaoke-style transcripts and spaced repetition. Claude generates translations and lesson content, ElevenLabs provides speech.

    Elixir, Phoenix LiveView, PostgreSQL, Fly.io, Claude API, ElevenLabs

  • Communication trainer

    Roleplay practice with an AI partner. The partner's answers change depending on how you communicate, and every message gets feedback. The reply streams in while feedback on the previous message is still being generated.

    React 19, TanStack Start, Cloudflare Workers, Convex, Claude API

Skills

Frontend
React, TypeScript, TanStack (Router, Query, Table, Virtual, Start), Tailwind, Vite, Webpack, Storybook
Testing
Vitest, Jest, React Testing Library, Playwright
Backend and data
C# and ASP.NET, Elixir and Phoenix, Node.js, PostgreSQL, Microsoft SQL Server, REST, WebSockets
Cloud and delivery
AWS (S3, CloudFront, ECS), Cloudflare Workers, Fly.io, GitHub Actions
AI
Claude API integration, streaming chat interfaces, AI-assisted development with Claude Code
Languages
Swedish native, English C2, French B1

Education

  • MSc in Applied and Computational Mathematics2015–2017
    KTH Royal Institute of Technology, Stockholm
  • BSc in Mathematical Statistics2012–2015
    KTH Royal Institute of Technology, Stockholm

Contact

I am looking for a frontend or fullstack role in Switzerland and would relocate within the country. Swedish citizen, Swiss B permit, available immediately.

carl.cronsioe@gmail.com