Lars Feldeisen · Freelance full-stack developer & IT consultant
I build web applications, backends and REST APIs with TypeScript, Node.js, Next.js and Go. I also integrate identity management with Keycloak and AI into existing systems. For clients in Germany and worldwide.
Building blocks form an application. Interfaces connect systems. AI agents support research, implementation and reviews. The personal code review stays with Lars Feldeisen. Closing: Feldeisen Software & IT. Good engineering. A direct line. The music contains no spoken information.
Available for projects from November 2026 · 15+ years of software and IT experience · Your direct contact
01 / Services
I build web applications with React and Next.js, backends with Node.js, Express and Go, and REST APIs – for new projects and existing systems.
Learn more: Full-stack development with TypeScript, Node.js & GoI integrate IAM systems, develop Java extensions and provide Keycloak support and hosting.
Learn more: Keycloak & identity managementI build tools with and for AI: agents that transform data, interfaces for AI agents and integrations of LLMs and vision models – with traceable checks.
Learn more: AI integration, agents & AI toolsI clarify architecture and dependencies and guide the technical implementation.
Learn more: IT consulting & technical planningFor smaller projects too: I look after WordPress and Drupal, handle CMS administration, updates and maintenance and write custom extensions in PHP.
Learn more: CMS maintenance & PHP development02 / Tech stack
Languages, frameworks and platforms from my projects.
03 / Experience
Backends, web applications and integrations from my professional work.
A backend built on Parse Server and Express: REST API, authentication with Keycloak and a search system in which AI makes decisions.
Professional experience · employed, client anonymised
View case study: Backend with Node.js, Express and Parse ServerA Next.js application in TypeScript for supporting several hundred Matrix Synapse servers: manage users, rooms and servers in one web interface.
Professional experience · employed
View case study: Managing Matrix Synapse with Next.jsI use AI to analyse large logs and codebases faster and to build applications more quickly. I also build tools with and for AI – such as agents that transform data. Concept, architecture and technical decisions remain with me.
The time saved starts with careful preparation: I clarify the task, prepare relevant information and plan verifiable steps. With technical know-how I choose suitable tools, models and the necessary context. This avoids unnecessary token usage, failed attempts and rework.
Static view. Start the sequence if you are interested.
A classifier recognises the request and chooses the right route: search for information, help in the system or handoff to a person. Uncertain or unsupported requests need follow-up questions or human help.
searchsystem_helpsupport_humanSearch and system help use relevant information to draft an answer. Human support leads straight to a handoff. Unclear requests need follow-up questions or human help.
I translate your requirements into a technical structure: components, interfaces and clearly scoped tasks. Within these specifications, AI and agents support me in writing code, research and reviews. I bring the results together and check them in the context of the application.
I direct the tasks. In parallel where dependencies allow.
Changes needed? Back to concept & implementation.
Technical responsibility stays with me: I assess AI suggestions against the requirements and the planned architecture. Mistakes and unsuitable changes are reworked. I can also set up this workflow with your development team.
Describe your project. The assistant recognises known requests in German and English and shows matching services or next steps. For open project questions, an enabled AI service can add a supplementary answer. Uncertain requests lead to a follow-up question.
The live test requires JavaScript. You can also discuss your project with me directly.
“technologies” lists detected mentions, not confirmed requirements. The demo assigns one main area.
Tools I work with
I work with Mistral, Claude and OpenAI. Which services and models fit depends on your task, the costs and your requirements for data protection and data processing.
Local & self-hosted
I also work with locally run models such as Devstral and Qwen – on your own hardware or in a self-hosted environment. Together we clarify model choice, hardware and integrations, looking at the data flows of the entire system, including connected tools.
DevstralQwenOther suitable models
Mistral is a trademark of Mistral AI, Claude of Anthropic and OpenAI of OpenAI.
05 / Working together
I'm an independent software developer and IT consultant based in Marl-Polsum, Germany. For more than 15 years I've worked with software and the IT behind it – from Linux and Windows to backends with Node.js and Go and web applications with React and Next.js. I work independently or as part of existing development teams.
You talk to me directly: from technical planning to implementation and ongoing support. We agree on scope, schedule and availability personally. I work with clients in Germany and worldwide.
We clarify your goal, existing systems and dependencies. From this I develop the technical concept, the architecture and concrete implementation steps.
I write and integrate the code, supported by AI as a tool within my specifications. I check changes for function, security and maintainability.
We discuss the result and the next steps. Further development, handover and ongoing support are agreed to suit your project.
Beyond client work
My own software projects give me room to experiment and think further.
My side projectsExisting software, a website or a new idea: we discuss what kind of support fits your project.
Let's talk
Briefly describe your project: the task, technologies, desired timeframe and scope. I'll get back to you to discuss the next steps.