Are you available right now?

Usually within 2–4 weeks, and sometimes right away. I take on a small number of engagements at a time, so the fastest way to find out is to book a 20-minute call and bring your timeline.

How long is a typical engagement?

Anything from a couple of weeks to over a year. In-team work has run 15 months; builds run 6–16 weeks. There is no minimum term, and notice is agreed in the contract.

Do you only work with startups?

Mostly startups and small product teams, but not only. Small businesses digitising a manual process and research teams needing a bespoke tool hire me too. The shape is the same: I embed 3–5 days a week as a senior engineer, ship a discrete product or AI feature, or get a stalled build back on track.

Do you work on-site or remote?

I'm Cambridge-based and ship for clients across the UK and Europe. On-site days in Cambridge or London are easy; full remote works for most engagements.

How do you scope a project?

In-team engagements need no scoping: we agree days per week and a start date, and you extend or stop as the work dictates. For product and AI builds I start with a few paid discovery days, then give you a fixed price for the build.

IR35 applies to contractors who work through their own limited company. I contract as a UK sole trader, so it is out of scope: you engage me on ordinary self-employed contractor terms and I invoice you directly.

Yes — happy to sign yours, or use a short mutual NDA before sharing anything sensitive.

Isn't a generalist a risk?

For an early-stage product the bigger risk is the gaps between specialists — the API nobody owns, the deploy nobody automated, the compliance question nobody asked. I've spent a decade across every layer of the stack inside demanding engineering organisations, so "full stack" means the whole stack, not just both ends. And I'm explicit about what's not on the knife — native mobile, training ML models, large design work — where I'll bring in or recommend a specialist.

Jonas Mattes

A senior full-stack engineer for your team.

I join UK startups and small product teams 3–5 days a week and ship the whole stack: frontend, backend, data, infrastructure, and AI where it earns its place. Ex-Google, a decade in engineering, no agency margin.

From £500/day · Usually able to start within 2–4 weeks, sometimes right away · Cambridge, remote across the UK

Book a 20-minute call or email contact@jmattes.com

Track record

Previously, as an employee or via consulting.

GoogleEmployer
ZühlkeEmployer
BMWEmployer
AudiEmployer
SBBIndirect

“Jonas has been with Kipti from the start and built the product with us. He doesn't just think in tasks — he thinks in the product and the company, and brings as much strategically as he does technically.”

Jonas begleitet Kipti seit dem Anfang und hat das Produkt gemeinsam mit uns aufgebaut. Er denkt nicht nur in Tasks, sondern im Produkt und im Unternehmen mit – und bringt strategisch genauso viel ein wie technisch.

Björn Schriewer — Founder, Kipti

Kipti, edtech startup
Freelance
Tech lead, 12-week MVP then ongoing
Took an unshippable prototype to a production AI product: Next.js, Hono and Postgres, LLM features, and Kubernetes on German infrastructure. GDPR-compliant; three schools in active trial.
Read the case study →
Google
Employment
Software engineer (L3) · code review and IDE · 2022–2023
Owned and shipped data-driven improvements to Google's code review platform and its VS Code-based IDE integration. Used by 80,000+ developers; contributed to 25+ engineer-years of annual productivity savings.
SBB, via Zühlke
Employment
Software engineer · ticket terminals · 2020–2021
Rebuilt the interface used across 1,000+ terminals serving millions of passengers. Architected and owned the state machines for payment, ticket printing and transaction flows; engineered for 24/7 unattended operation and tested with disability advocacy groups.

Services

Which of these is you?

Your team needs a senior pair of hands.
3–5 days/week · From £500/day
I join as an in-team engineer 3–5 days a week, take ownership of real features from week one, and leave the codebase better than I found it.
Book a call →
You have a prototype that needs to become a product.
6–16 weeks · Fixed price
I take an idea or a stalled build to production end to end. We start with a few paid days of discovery so I can give you a fixed price.
Book a call →
You want AI in the product and nobody owns it.
2–6 weeks · Fixed price
I add an LLM layer that earns its place: chat, summarisation, pattern detection. Scoped after a short discovery.
Book a call →

Why one engineer

Everything the project needs, on one engineer.

Ten blades, one pair of hands. Each links to where it was used.

Frontend & UX
Interfaces people actually use — designed and built.
- React
- Next.js
- Tailwind
Kipti: teacher-first UI →
Backend & APIs
Type-safe from the database to the button.
- Node.js
- Hono
- oRPC
- tRPC
Kipti: rebuilt on solid foundations →
Data & databases
Schemas that survive the pivot.
- Postgres
- Drizzle
- Migrations
Process: start small →
AI features
LLM features that earn their place — and get switched off when they don't.
- LLM APIs
- Evals
- Prompt tooling
Kipti: AI summarisation and tooling →
Infrastructure
From Vercel to bare metal — and back.
- Kubernetes
- Docker
- Vercel
- Self-hosting
Kipti: GDPR-compliant infrastructure →
CI/CD
Every merge deploys. Every deploy is boring.
- GitHub Actions
- Preview environments
- Automated tests
Process: continuous deploys →
Security & compliance
GDPR as an engineering constraint, not a checkbox.
- Encryption
- Data residency
- Auth
Kipti: the compliance bar →
Performance
Fast by default, measured rather than assumed.
- Core Web Vitals
- Profiling
- Caching
Accessibility
Keyboard first, screen-reader tested, AA contrast.
- WCAG 2.2 AA
- Semantic HTML
This site: Lighthouse accessibility 100 →
Product thinking
Thinks in the product and the company, not in tickets.
- Discovery
- Scoping
- Roadmap
In Kipti's words →

Not on the knife Native mobile apps · Training ML models or data science · Large-scale brand and design-system work · 24/7 operations. For these I'll bring in or recommend a specialist.

How it works

From first call to first deploy.

A 20-minute call.
Your situation, whether I am the right fit, and when I can start. No pitch deck.
Start small.
No minimum term. Most engagements begin with a week or two and extend from there, with continuous deploys and real features owned from week one. For builds, a few paid discovery days come first and a fixed price follows.
Handover.
Documentation and a light retainer for the weeks after, so nothing depends on me.

Rates

From £500 a day.

In-team engagements: from £500/day, 3–5 days a week. No minimum term. Monthly invoicing.
Product and AI builds: fixed price after a short paid discovery.
No recruiter or agency fee. Remote-first; on-site days in Cambridge or London are easy.

Book a call

About

Cambridge-based, shipping for teams across the UK.

I'm Jonas, a senior full-stack TypeScript engineer with a decade in engineering. I was previously a software engineer at Google and shipped products at Zühlke for clients including SBB, and today I work in-team with startups and small product teams from Cambridge.

Read my full background →

FAQ

Common questions.

Are you available right now?: Usually within 2–4 weeks, and sometimes right away. I take on a small number of engagements at a time, so the fastest way to find out is to book a 20-minute call and bring your timeline.
How long is a typical engagement?: Anything from a couple of weeks to over a year. In-team work has run 15 months; builds run 6–16 weeks. There is no minimum term, and notice is agreed in the contract.
Do you only work with startups?: Mostly startups and small product teams, but not only. Small businesses digitising a manual process and research teams needing a bespoke tool hire me too. The shape is the same: I embed 3–5 days a week as a senior engineer, ship a discrete product or AI feature, or get a stalled build back on track.
Do you work on-site or remote?: I'm Cambridge-based and ship for clients across the UK and Europe. On-site days in Cambridge or London are easy; full remote works for most engagements.
How do you scope a project?: In-team engagements need no scoping: we agree days per week and a start date, and you extend or stop as the work dictates. For product and AI builds I start with a few paid discovery days, then give you a fixed price for the build.
What about IR35?: IR35 applies to contractors who work through their own limited company. I contract as a UK sole trader, so it is out of scope: you engage me on ordinary self-employed contractor terms and I invoice you directly.
Do you sign an NDA?: Yes — happy to sign yours, or use a short mutual NDA before sharing anything sensitive.
Isn't a generalist a risk?: For an early-stage product the bigger risk is the gaps between specialists — the API nobody owns, the deploy nobody automated, the compliance question nobody asked. I've spent a decade across every layer of the stack inside demanding engineering organisations, so "full stack" means the whole stack, not just both ends. And I'm explicit about what's not on the knife — native mobile, training ML models, large design work — where I'll bring in or recommend a specialist.

Book a call

Let's talk about what you're building.

Twenty minutes on your situation, whether I'm the right fit, and when I can start. Or open the calendar directly on Calendly.

Open Calendly or email contact@jmattes.com

A senior full-stack engineer for your team.

Kipti, edtech startup

Google

SBB, via Zühlke

Aviation-industry enterprise

Visana, via Zühlke

SBB web platform, via Zühlke

Which of these is you?

Your team needs a senior pair of hands.

You have a prototype that needs to become a product.

You want AI in the product and nobody owns it.

Everything the project needs, on one engineer.

From first call to first deploy.

A 20-minute call.

Start small.

Handover.

From £500 a day.

Cambridge-based, shipping for teams across the UK.

Common questions.

Let's talk about what you're building.