Open to work
Hi, I’m Julien Karst Tech Lead & Developer
I build and lead web and mobile products used by millions. Founder or CTO? Let’s ship yours.
Worked withHBO Max · TNT Sports · Eurosport · Discovery+ · BT Sport · Warner Bros · Meetic
Available for new projectsNancy · Paris
Open source used by
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Selected work
Work I’ve shipped
Five engagements from 2019 to 2025: four as Tech Lead in streaming and sports media, one mobile app redesign.
All projects
TNT Sports
Tech Lead — Website
Year 2025 · Web platform
Next.jsReact.jsMonorepoYear 2025
Discovery+
Tech Lead — Web & Mobile
Year 2023 · Web platform
React.jsTDDGithub actionsYear 2023
Eurosport
Tech Lead — React & React Native
Year 2019 · Web platform
React.jsReduxReact-NativeYear 2019
Meetic Affiny
Mobile — React Native (iOS & Android)
Year 2019 · Mobile app
React-NativeiOSAndroidYear 2019
About
- Location
- Nancy · Paris
- Availability
- Available for new projects
Open to work
I’m a hands-on Tech Lead based in Nancy, France.
I lead by making teams stronger: reviews that teach, pairing when it helps, code anyone can pick up after I leave. I’m freelance, so you work with me directly, with no agency in between.
Outside client work, I maintain two open-source GitHub Actions, team-labeler-action and pull-request-name-linter, and build and run my own products: Outfiles.io, a PDF generation API, and ProspectIm, a real-estate prospecting SaaS. I know what it takes to keep software running after launch.
Key numbers
- 10+
Years of experience
- 120+
Projects delivered
- 50+
Engineers mentored
Services
What I do
Build, advise or review, on a new product or an existing one. Open a row for the details.
All servicesReact & Next.js: the TNT Sports site
- TypeScript
- Performance & Core Web Vitals
- SEO & accessibility
iOS & Android: the HBO Max apps
- React Native
- Native iOS in Swift
- App Store & Play Store releases
Node.js APIs and data models on AWS
- REST & GraphQL
- Postgres & MongoDB
- Monitoring & logging
A senior opinion before you commit
- Architecture decision records
- Technical roadmap
- Delivery workflow & CI
The real state of your code, in writing
- Code quality & maintainability
- Security & dependency review
- Performance bottlenecks
Designed by the person who builds it
- Wireframes & prototypes
- Design systems
- Accessibility (WCAG)
Process
How we work together
Each step ends with something you can hold me to. No surprises on scope, timing or price.
- Step 1
Discovery
One call and a look at what exists. You get a written brief: goals, constraints, scope, a first estimate.
- Step 2
Planning
Architecture, milestones and price agreed in writing, plus what I need from your side.
- Step 3
Build
Short cycles: working software every week and a demo at each milestone. No black box.
- Step 4
Delivery
Launch with tests, monitoring and docs in place, and a handover session for your team. I stay reachable after.
Technologies I work with
- React
- Next.js
- TypeScript
- React Native
- iOS
- Swift
- Android
- Node.js
- GraphQL
- PostgreSQL
- MongoDB
- AWS
- Tailwind CSS
- Git
- AI
Writing
Working notes
Short notes written during projects. I publish when I have something to say.
All notesFAQ
Before you write
What to check before you hire a freelance Tech Lead.
Julien KarstAvailable for new projects
Tell me what you’re building.
Two lines on the product and where it stands are enough. I reply within one business day.
Or write directly
Both. I set the architecture and write code alongside the team. That is how I worked as Tech Lead on HBO Max, Discovery+ and TNT Sports.
Remote works well for me, on CET hours. On site for kick-offs, workshops or when the team needs it: easy in Paris, where I work regularly; elsewhere, we plan it together.
In your repository, with your tools and your rituals. I start from how your team already works and change only what slows it down.
Not much. A call to start, then a short check-in at the pace you choose, and quick answers when I’m blocked. You won’t have to manage me.
My current availability is in the contact panel. Send your ideal start date and how long you need me, and I’ll reply with dates I can commit to.