Reviews
5.0 average across 11 client reviews
My second great experience with Sem! 100% would work with him again.
Phenomenal experience working with Sem on very challenging front-end + algorithms work. Sem is a very talented computer scientist and software engineer, skilled, and with a strong "Will to finish" and tenacity needed to tackle very difficult tasks. Very well done, Sem! I am excited to work with you again in the immediate future.
I'm delighted to work with Sem. He is a very skilled engineer and very efficient. I highly recommend him.
Great! Sem explored a number of possible solutions and we got to the bottom of a difficult bug
Great communication & amazing technical skills
Very happy with Sem: good programming skills, smooth communication and very quick to reply and solve problems. Highly recommended
Highly recommend! It's always a pleasure to work with Sem, he is reliable, communicates well and delivers according to the requirements.
Everything is working exactly as per requirements, thank you!
Great to work with. Very responsive. Very knowledgeable. Went above and beyond to deliver a great app!
Sem is what you hope every upwork freelancer to be like. He communicates well, responds quickly and got the job done way before deadline. I would recommend him and will look to hire him again when similar projects pop up.
My second great experience with Sem! 100% would work with him again.
Phenomenal experience working with Sem on very challenging front-end + algorithms work. Sem is a very talented computer scientist and software engineer, skilled, and with a strong "Will to finish" and tenacity needed to tackle very difficult tasks. Very well done, Sem! I am excited to work with you again in the immediate future.
I'm delighted to work with Sem. He is a very skilled engineer and very efficient. I highly recommend him.
Great! Sem explored a number of possible solutions and we got to the bottom of a difficult bug
Great communication & amazing technical skills
Very happy with Sem: good programming skills, smooth communication and very quick to reply and solve problems. Highly recommended
Highly recommend! It's always a pleasure to work with Sem, he is reliable, communicates well and delivers according to the requirements.
Everything is working exactly as per requirements, thank you!
Great to work with. Very responsive. Very knowledgeable. Went above and beyond to deliver a great app!
Sem is what you hope every upwork freelancer to be like. He communicates well, responds quickly and got the job done way before deadline. I would recommend him and will look to hire him again when similar projects pop up.
My second great experience with Sem! 100% would work with him again.
Phenomenal experience working with Sem on very challenging front-end + algorithms work. Sem is a very talented computer scientist and software engineer, skilled, and with a strong "Will to finish" and tenacity needed to tackle very difficult tasks. Very well done, Sem! I am excited to work with you again in the immediate future.
I'm delighted to work with Sem. He is a very skilled engineer and very efficient. I highly recommend him.
Great! Sem explored a number of possible solutions and we got to the bottom of a difficult bug
Great communication & amazing technical skills
Very happy with Sem: good programming skills, smooth communication and very quick to reply and solve problems. Highly recommended
Highly recommend! It's always a pleasure to work with Sem, he is reliable, communicates well and delivers according to the requirements.
Everything is working exactly as per requirements, thank you!
Great to work with. Very responsive. Very knowledgeable. Went above and beyond to deliver a great app!
Sem is what you hope every upwork freelancer to be like. He communicates well, responds quickly and got the job done way before deadline. I would recommend him and will look to hire him again when similar projects pop up.
What I do
The engagements I take on. If your project sits at the seams, get in touch anyway.
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Custom application development
End-to-end design and implementation of bespoke applications across web, native iOS, and backend systems, including MVP builds for early-stage products.
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Web, API and backend development
Production websites, web apps, and APIs in React, Preact, Svelte, and vanilla web. Backends in Go and Python with REST or event-driven APIs.
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AI and LLM integration
LLM-powered features for existing apps and new products: agent workflows, RAG pipelines, and integrations with OpenAI, Anthropic, and similar providers.
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Legacy code modernization
Refactoring and modernizing older codebases. Framework upgrades, careful test-driven changes, and migrations from end-of-life platforms.
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Native iOS apps
Native iPhone and iPad apps in Swift and SwiftUI, including widgets, Live Activities, and App Store delivery.
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Browser extensions and integrations
Chrome extensions, Thunderbird add-ons, and integrations between browsers and external services.
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Point-of-sale and retail software
Custom POS systems, inventory and stock management tooling, and integrations with receipt printers and other store hardware.
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Performance-critical engineering
Algorithms, audio and video processing, codec work, P2P networking, and other engineering-depth projects where performance and correctness matter.
Selected work
Two builds in depth: what they do, and the engineering decisions behind them.
Questions, answered
A few things prospective clients ask before we start.
How does an engagement usually start?
With a short conversation, by email or video call, where you describe what you're trying to do. I'll come back with a rough idea of scope, approach, and a budget range. If we both want to move forward, we agree a small first milestone before committing to the rest.
Do you charge fixed price or hourly?
Both, depending on the work. For well-scoped pieces (a new feature, a migration, a clearly defined integration) I prefer a fixed price per milestone. For exploratory or maintenance work I bill hourly with a weekly cap so there are no surprises.
How long do projects usually take?
Most projects I take on run between two weeks and three months. Larger pieces are usually broken into several milestones rather than one big delivery, so we can re-plan as we go.
Will you sign an NDA?
Yes. A mutual NDA before sharing anything sensitive is fine, just send the version you want to use. I can also work under your master services agreement.
Can you work with my existing team or codebase?
Yes. A good chunk of my work involves joining an existing engineering team for a defined piece of work, or making careful, well-tested changes to a codebase someone else built. I'm comfortable with code review, pull-request workflows, and the usual collaboration tooling.
Where are you based and what timezones do you cover?
I'm based in the Netherlands (CET / CEST) and work remotely worldwide. I overlap comfortably with Europe and the eastern half of North America. For Pacific or Asia-Pacific clients, async-first works best, with one or two scheduled calls per week.
Contact
I'm Sem, a freelance full-stack developer based in Groningen, Netherlands. Since 2022 I've built web, iOS, and backend software for clients around the world. If you've got a project in mind, I'd be glad to hear about it.
- Email [email protected]
- Location Groningen, NL · worldwide