Full Stack Product Engineer – Fintech (AI-first, high ownership)

Hub; København; On-site; DKK 700,000 - 1,100,000; Full time

Job summary

Flex Funding in Copenhagen is seeking a strong full stack engineer to help scale a fintech platform with AI at its core. You will join a small, high-performing team and own end-to-end development across the stack.

You will modernise the monolith, deliver new features, and collaborate closely with product, compliance, and external partners in a fast-paced startup environment.

Qualifications

• Solid full stack engineering fundamentals.
• Fluent in using AI in development workflow.
• Ability to review AI-generated code to maintain quality.

Responsibilities

• Build end-to-end solutions from idea to production-ready systems.
• Modernise an existing monolith while delivering new features.
• Apply AI across development, testing, and documentation.
• Own reliability, security, performance, and maintainability.
• Work with financial workflows and external integrations.
• Collaborate with product, business, and partners.

Skills

Full stack engineering

AI in development

Code review

Proactive

Tools

C#

ASP.NET Core

TypeScript

Vue / Nuxt

Azure SQL

Azure CI/CD

REST / GraphQL

Job description

Do you want to build the next generation of a well-functioning fintech platform - and help set the standard for how we build with AI?

Do you want to build a real fintech platform used by businesses every day - and help redefine how a small, high-performing team builds with AI?

Flex Funding is a Scandinavian crowdlending platform helping SMEs access flexible financing across Denmark and Norway, backed by 4,000+ investors. We are expanding into Sweden and building new financing products - while evolving how we design, build, and scale our platform.

We are looking for a strong full stack engineer with solid engineering foundations and the appetite to amplify their skills with AI. Someone who takes ownership, enjoys working across the stack, and wants to improve both the product and how we build it.

The role

This is not a ticket-driven role.

You will work alongside one other full stack developer, forming a two-developer team with broad responsibility for the platform. You will make technical decisions, challenge each other's thinking, and hold each other to a high standard.

We work closely together in the office, often at the whiteboard to explore ideas and solve problems. You'll also work with our external DevOps agency on our Azure infrastructure.

You will:

• Build end-to-end solutions: from idea and prototype to tested, production-ready systems used by real customers.

• Help modernise an existing monolithic platform, improving its structure and maintainability while continuing to deliver new features.

• Use AI across development, testing, documentation, and refactoring - and help make these practices more effective.

• Own technical solutions beyond the initial implementation, including reliability, security, performance, and maintainability.

• Work with financial workflows, data, and third-party integrations where correctness matters.

• Collaborate closely with product, business, compliance, and external partners.

This is a startup-style environment inside an established fintech: high autonomy, fast decisions, and visible impact. The scope is broad, and you should enjoy taking on unfamiliar problems and learning as you go.

How we build with AI

We use AI extensively and expect you to be fluent in using it as part of your development process.

Our principle is simple: strong engineering fundamentals, amplified by AI. You should know how to give AI useful context, break down problems, assess its suggestions, and turn its output into reliable software.

You remain responsible for the code you ship. That means understanding it, challenging assumptions, and validating behaviour, security, and maintainability - whether you wrote it yourself or developed it with AI.

As a financial company, we hold a high bar for quality. We follow a test-driven development process and expect thoughtful testing and review to be part of how you work. Faster development must come with confidence in what reaches production.

We also want someone curious about building AI capabilities into our applications, with the same care for quality and reliability.

Current tech and platform

• Backend: C#, ASP.NET Core / .NET, REST / GraphQL

• Frontend: TypeScript, Vue / Nuxt, component-based UI development

• Database: Azure SQL

• Infrastructure: Azure and CI/CD pipelines, supported by our external DevOps agency

• Platform: an existing monolith with API integrations and financial workflows, which we are actively modernising

You will work with what is already there, understand the trade-offs, and help us improve it step by step.

What we're looking for

We are looking for someone who builds, takes responsibility, and gets things done - with the foundations to do it well.

You:

• Have solid full stack engineering fundamentals and can explain the reasoning behind your technical choices.

• Use AI tools confidently in your development workflow and actively explore how they can improve your work.

• Can critically review and validate code, including AI-generated code, against high quality standards.

• Take ownership beyond implementation - you care about whether the solution works in practice and remains maintainable.

• Are comfortable working across frontend, backend, databases, and integrations.

• Are proactive: you don't wait for perfect specs, and you know when to ask questions, challenge assumptions, or move forward.

• Have an open mindset, enjoy learning, and are willing to work on unfamiliar parts of the platform.

• Communicate clearly with technical and non-technical colleagues and enjoy close, in-person collaboration.

You should be comfortable with the responsibility and variety that come with being one of two developers. You will have real influence, and we will rely on your judgement.

It's a strong plus if you have

• Experience with C#, .NET, TypeScript, and Vue / Nuxt.

• A good understanding of SOLID principles and component-based design, and how to apply them pragmatically.

• Strong database practices, including data modelling, query optimisation, pagination, and indexing.

• Experience with complex financial integrations, such as bank connections or payment providers like Danske Bank or Mastercard.

• Experience in fintech, lending, payments, or other regulated environments.

• Worked on modernising a monolithic platform while keeping a live product running.

• Built products in a small team or startup environment.

Language

Danish is required for this role due to our day-to-day collaboration and regulatory context.

Why join Flex Funding

• A live platform creating real value for businesses every day.

• A small, experienced team with high trust, close collaboration, and low bureaucracy.

• Real ownership and influence on architecture, product, and AI adoption.

• The opportunity to build new products and modernise an established platform.

• A growing fintech expanding across the Nordics.

Hiring process

1. A call with our CPO, the hiring manager, to discuss the role, your experience, and what you are looking for.

2. A technical conversation with our other developer about engineering fundamentals, technical decisions, and how you work with AI.

3. A final conversation with our CEO/CFO.

A project you can walk us through is a bonus. If you have one, we would love to see how you approached it, the choices you made, and how you validated the result.

Practical

• Full-time

• Location: Copenhagen, CPH Fintech Lab

• Start: as soon as possible