Senior Backend Developer - Python and Java

Legal AI is only as good as the knowledge behind it. At Lefebvre Sdu, we've been building that knowledge for over 200 years. Day in, day out. Law by law. Word by word. As the Dutch heart of Lefebvre Group, the European leader in legal and tax knowledge, we're also developing our own platform: GenIA-L, where lawyers and tax advisors don't just get an answer, but an answer they're willing to attach their name to. More than 75,000 professionals use it daily, generating over 100 million euros in ARR across Europe. We're looking for a Senior Backend Developer to help build that. HOW YOU'LL MOVE US FORWARD Our Knowledge Team is the layer between the content and the AI. We pull in millions of documents from internal databases, public sources, and the web, standardize them, and make them usable for the AI agents that distill answers from them. The real challenge isn't storage but selection. Out of those millions of documents, exactly one paragraph needs to surface the moment a lawyer asks a question. Surface the wrong one, and the agent gives an answer that sounds plausible but is wrong. In our field, that's not a bug: it's a liability issue. What you build determines whether the answer holds up. There's work to be done, and it's the kind of work you could talk about for a whole evening. The pipeline has been running for six years, built in Scala and Java, and parts of it deserve a new foundation. Reranking is the biggest need: determining which passage, out of a mountain of legal documents, actually answers the question being asked. Not just which one contains the matching words. That's the problem we’re solving. The team is strong on the scientific side, in retrieval, and in understanding what a model can and can't handle. What we're looking to add is engineering firepower: someone who can turn ideas into systems that deliver. Not to rebuild everything over six months just because it was once set up differently, but to see where the real gains are and start there. You build and improve the pipeline that turns content into knowledge our AI agents can use.You work on reranking quality, so the agent gets the right information, not just the most information.You help shape the approach to retrieval and knowledge extraction, bringing in what you've seen work elsewhere.You work in Python for everything new, and in Java where the existing codebase calls for it.You make technical decisions, even when no one is looking over your shoulder. WHAT WE'RE LOOKING FOR You're an engineer who looks beyond the ticket. You want to understand why something is being built and what it delivers for the customer, and you're not afraid to have an opinion about it. You don't need a manager to make a technical call, and you speak up when you think something should be done differently. Experience with Python and Java. Everything newly built is developed in Python, closely aligned with the AI teams who also work in it. But a large part of our current codebase runs on Java, so you need to be comfortable working with that too.Experience with Elasticsearch. That's where it's determined which information the agent gets to see.Experience with Scala or Kotlin is a plus. The ingestion layer runs largely on Scala.You come from SaaS, a product-driven organization, or a scale-up, and you thrive on autonomy rather than oversight. WHAT YOU'RE SIGNING UP FOR At Lefebvre Sdu, you'll work in an organization that's in the middle of a transformation and takes the public market seriously. That's the direction, and we're making the journey together. You'll get the mandate, the space, and the colleagues to make it happen. You'll join a team with a Product Owner, three frontend developers, a UX designer, and a Scrum Master who also works across multiple teams. You'll have room to shape your role and help determine how we work, with a clear shared goal: building the AI-first platform. You'll bring the backend experience the team needs to take real end-to-end ownership, and you'll report to the Lead Frontend Developer. You'll take part in the team's regular scrum rituals (dailies, refinements, retros, planning) and have one-on-ones with your manager. Through the team, you'll also be an important point of contact for application administrators and customer support who need technical assistance, so you should enjoy breaking down technical problems and communicating about them with less technical colleagues. A place where you have influence from day one: on the approach, the processes, and the direction.Colleagues who deliberately chose Lefebvre Sdu over big tech jobs because here, they're building something that doesn't exist yet.Base salary between €5,500 and €6,450 per month, based on a 36-hour week, since that's full-time for us. Plus a lot of extra allowances (see below).A bonus tied to company performance: a fixed year-end bonus of 4% and profit sharing of up to 6%.23 vacation days and a personal choice budget of 12%, of which 8% is holiday pay and 4% is freely allocable. Want more vacation? Just buy it.A bonus of up to 10% of your salary, based on company performance and individual performance.A solid pension plan through PGB, including supplementary partner insurance.Collective insurance plans.A travel allowance of €0.25 per kilometer or a public transport reimbursement, plus an attractive bike plan. For employees with a commute of less than 10 km or more than 60 km, we have an adjusted arrangement.Two days a week in the office in The Hague, within walking distance of Voorburg station. The rest of the week, wherever you work best.Room to grow: in your craft, in your role, in the organization. WANT TO KNOW MORE? We'd love to tell you all about the team and the role. Send us your CV and you'll hear from us within a few business days. Have a few questions first? Message or call Elize at 0641186590. A reference check and a VOG (certificate of good behaviour) are part of the selection process.