I architect correctness-critical systems — and then I build them.

Fractional CTO + implementation for companies that need reliable systems and can't afford to get the architecture wrong.

Book a conversation

Who I work with

Financial technology and IoT

Where a bug in production means real money lost or devices behaving dangerously. You need systems that are correct by construction, not correct by accident.
Non-technical founders

You need someone who makes technical decisions with authority, not someone who waits to be told what to build.
Teams drowning in technical debt

Your system grew faster than your architecture. Incidents are increasing. You need someone to untangle the mess and build a path forward.

Start with a Technical Architecture Audit

A one-week deep dive into your codebase, infrastructure, and technical decisions. You get a written report covering:

Architectural risks and single points of failure
Technical debt hotspots costing you velocity
Reliability gaps that will bite you at scale
A prioritized roadmap with concrete next steps

€5,000. Fixed scope. No ongoing commitment required.

Most audits surface significant savings in wasted engineering time — making the ROI obvious before any further engagement.

Discuss an audit

Selected work

Architected and built core platform components for a reinsurance technology company streamlining the global reinsurance market. The system handles complex multi-party deal workflows where data integrity and correctness are non-negotiable. Read about solving a production deadlock →

Led technical architecture decisions for a construction technology IoT platform, delivering production systems where device reliability and data correctness are critical at scale. Read the full story → · How I improved firmware throughput 7x →

Designed a new architecture for Cabal, Haskell's core package manager used by every Haskell project. Wrote the formal proposal, built consensus among the maintainers, and got it accepted by the core team →

How we work together

Advisory retainer

Strategic technical oversight. Architecture reviews, technical hiring guidance, vendor evaluation. A few hours per week.

Operational retainer

Embedded in your team 1–2 days per week. I make technical decisions and implement them. Full decision-making authority.

Project engagement

Fixed-scope builds. I architect and deliver the system end to end. You get a working product, not a specification.

All engagements start with a conversation about your situation. I'll tell you honestly whether I can help.

About

I'm Jappie Klooster. I run a consultancy that makes technical decisions for companies and then executes on them. Not advice — decisions and delivery.

I build with technologies chosen for correctness and long-term maintainability, including Haskell, Nix, and formal verification tools. But the technology is an implementation detail — what matters is that your system works, scales, and doesn't wake anyone up at 3am.

For more technical writing and case studies, visit my blog.

Let's talk about your situation

I take on 2–3 new engagements per year. If you're building something where correctness matters, get in touch.