4xxi
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Services

Three ways to start working with us, and the engineering behind them: AI, backend, web, mobile and infrastructure.

01Three ways to start

Fixed price. Measured results. Then we keep going.

Start small and see the work before you commit. The audit fee is credited in full toward a project signed within 60 days.

Start here2 weeks

AI opportunity audit

$5–10kfixed

We look at your product and your operations — development, support, marketing, back office — and tell you where AI will actually pay off, where it won’t, and which corners are safe to cut.

  • Where AI helps, and where it doesn’t
  • What to automate first, what to leave alone
  • Your existing AI reviewed, if you have one
  • Measured report + roadmap

Full money-back guarantee. The fee is credited in full toward any project signed within 60 days.

Start with an audit →
Prove it5 business days

Prototype sprint

$20kfixed

A working, deployable prototype on your data, with a fixed scope agreed up front.

  • Runs on your documents and systems
  • Deployable, not a demo
  • Fixed scope and price
Plan a sprint →
Keep goingOngoing

Ongoing engineering

Hourlyrate on request

Senior engineers only. No juniors, no handoffs. Quoted in hours, not seats.

  • The people you meet build it
  • Founder involved
  • Can join your existing team and codebase
Book a call → (opens in a new tab)

Kickoff typically within 3 business days of signing.

The founder is also available as a fractional CTO or AI advisor.About 4xxi →

02What we build

The engineering behind it.

AI is our focus now. The backend, web, mobile and infrastructure work we have shipped since 2008 is what makes it hold up in production.

Illustration of a circuit board in the shape of a brain

LLMs · RAG · NLP · Evaluation

AI and machine learning

We automate tasks with natural language processing and language models, and build AI that answers from your own content. From data preprocessing to model training and optimisation, we integrate and deploy AI and ML in production systems, and measure them before anyone relies on them.

  • Model training and optimisation. We train and fine-tune AI and ML models to the performance the task needs.
  • Deployment and integration. We integrate AI and ML solutions into existing systems and deploy them for production use.
  • Measured before launch. Answers come from your documents, with references assigned by retrieval. We test on real cases from your business before launch.
Illustration of a developer working at a screen full of code

Symfony · PHP · APIs

Backend development

Symfony is our backend framework of choice. We use it for everything from APIs and dashboards to high-performance, scalable architectures.

  • Versatile development. We develop APIs, non-interactive websites, dashboards, marketing websites and proofs of concept with Symfony.
  • Robust architecture. We design scalable and maintainable backend architectures with Symfony.
Illustration of a laptop and a monitor showing a web application

React.js · Redux · Tailwind UI

Frontend development

React.js is our preferred framework for complex and scalable web applications. We build clear user interfaces with efficient state management, using UI component libraries such as Material-UI or Tailwind UI.

  • Preferred choice. React.js is our go-to framework for frontend development in all our web projects.
  • Long-standing expertise. Our team has used React.js since its initial release.
  • State management. Efficient state management with Redux or related libraries.
Illustration of smartphone screens from a mobile app

Kotlin · Swift · Flutter

Mobile development

We build native (Kotlin, Swift) and cross-platform (Flutter) mobile apps, with attention to UI and UX, offline functionality, push notifications and app performance.

  • Native and cross-platform. Native apps in Kotlin and Swift, and cross-platform apps in Flutter, for a range of platforms and devices.
  • UI and UX. Our team focuses on intuitive interfaces and smooth user experiences.
Illustration of a glass cube with the DevOps infinity loop

AWS · Azure · Kubernetes · Docker

DevOps and infrastructure

We prioritise scalable, vendor-lock-free infrastructure built on open-source tools such as Docker, Kubernetes and Helm, on AWS, Azure, a private cloud or your own servers. We design, automate, monitor and secure it, and set up CI/CD pipelines for development and deployment.

  • Scalable solutions. Vendor-lock-free setups with a focus on open source, on AWS, Azure and Kubernetes.
  • Infrastructure and automation. We design and set up infrastructure, implement CI/CD and containerise existing applications.
  • Security and compliance. We implement robust security measures and ensure compliance with industry standards.
03How we work with AI

How we make AI you can rely on.

  1. 01

    Robust where it must be, probabilistic where it can be

    LLMs are probabilistic. We don’t let them be the only judge. Deterministic checks — scores, rules, assertions — sit alongside model-based evaluation.

    deterministic_check() && llm_judge()
  2. 02

    Research, applied

    Semantic chunking, multi-criteria reranking, calibrated LLM-as-judge: the methods come from current papers, not from tutorials.

    method ← paper, not tutorial
  3. 03

    Measured before launch, and after

    Before anyone relies on an AI feature, we run it against real cases from your business. Baselines, regression suites and cost budgets are part of the delivery.

    baseline → change → regressions[]

Read how we work with AI →

04Questions

What buyers ask us first.

Not here? Ask on a call, or write to [email protected].

What does it cost?

The AI opportunity audit is $5–10k, fixed, and takes 2 weeks. The prototype sprint is $20k, fixed, for 5 business days. Ongoing engineering is billed hourly; we share the rate on a call.

How does the prototype sprint work?

We agree a fixed scope up front, then build a working, deployable prototype on your documents and systems in 5 business days, for a fixed $20k.

What if the audit isn’t useful?

The audit has a full money-back guarantee. If you sign a project with us within 60 days, the fee is credited in full toward it.

Who does the work?

Senior engineers only, with no juniors and no handoffs. There is no sales layer: the engineers you meet on the first call build your project, and the founder works on it too.

Where does our data go?

Your data stays yours. EU/UK-based team, on-premises or private-cloud deployment, no training on your data.

Can you work with our existing team and codebase?

Yes, and we do it often. We join existing teams to add the expertise they’re missing — AI, architecture, delivery — and work in your codebase, your process and your tools.

Have a product that needs AI that works?

Book a call (opens in a new tab)Start with an audit