Software that holds its invariants

Invariant Software is a software consultancy that builds correct, reliable systems — and we build them in OCaml, where whole classes of bugs are made unrepresentable rather than merely tested for.

An invariant is a property guaranteed to hold no matter what the rest of the system does. We named the company after it because that is the standard we hold our software to.

What we believe

Most software fails not because the problems are hard, but because invalid states are allowed. We design systems where illegal states cannot be expressed — so the compiler, not the on-call engineer at 3 a.m., catches the mistake.

• Correct by construction. Types and pure functions encode the rules, so violating them won't compile.
• Reliable in production. Boring deploys, observable systems, and code that fails loudly and early.
• Owned, not rented. Free and open-source foundations, no vendor lock-in — software you can actually keep.

What we do

We help individuals and organisations with all things software — from greenfield systems to rescuing ones that have grown fragile. See our services →

Built in the open

We use our own tools and contribute to the open-source software we rely on. Our first product, Chaufr, and the engineering decisions behind it have their own home.

Latest writing

• Hello, World 2026-06-14

Talented engineering can come from anywhere. Let's build something that lasts.