Meet Zügel – An MCP Server Giving Your AI Coding Agent An Architectural Conscience

Alexander von Zitzewitz Leave a comment

How Zügel keeps agent-written code inside your intended architecture — and helps you dig out of the debt you already have.

This article covers the version 26.3.4 (released on Aug 11th 2026) of Zügel.

AI coding agents have changed the economics of writing code. They have not changed the economics of structure. An agent will happily add the fifteenth dependency from your persistence layer back into your UI, import an internal class from a subsystem that was never meant to expose it, or close a dependency cycle that fuses two modules into one inseparable blob — all while the tests stay green. Architecture erosion used to happen at human speed. Now it happens at machine speed. Zügel – the German word for rein – is designed to stop that from happening. It puts you back in control and directs the coding agent to write architecturally and structurally sound code.

The traditional answer — periodic architecture reviews, static-analysis gates in CI — catches the damage after it is written. That is too late for agent workflows: by the time CI complains, the agent has already built three features on top of the illegal dependency.

Zügel moves the check to the moment that matters: before the agent writes the dependency. It is an MCP (Model Context Protocol) server that any MCP-capable agent — Claude Code, or anything else that speaks the protocol — connects to like any other tool provider. Under the hood it:

  • parses your sources itself (Java only Today with more languages coming soon),
  • maintains architecture rules based on Sonargraph’s architecture DSL (optional),
  • computes violations and dependency cycles from the resolved model,
  • tracks every change against baselines, so progress and regressions are visible per edit, and
  • answers precise dependency questions the agent would otherwise (badly) approximate with grep.

Zügel is a companion to Sonargraph, and can be used independently from it. Using it together with Sonargraph will still provide extra benefits like CI integration, dependency visualization and a powerful environment to design architecture rules based on the Sonargraph architecture DSL.

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Major Changes to our C/C++ System Model

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When we added C/C++ to Sonargraph our work was heavily influenced by John Lakos famous book “Large Scale C++ Design”. In this book Mr. Lakos presented a solid system to define the architecture of C++ systems based on components. A component in the most simple case is the combination of a header file and a source file, e.g. User.h and User.cpp. The header declares all elements that can be used from outside of the component, while the source file contains the implementation of the functionality. This unites the header and the source file into a logical component, that is better suited for dependency analysis.

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ArcTree Tutorial

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Free Trial and Licensing

ArcTree is a plugin for IntelliJ IDEA that visualizes code dependencies for Java and Kotlin projects in a unique and powerful way. It is a commercial product which needs a subscription to run after a free evaluation period. To test the plugin just download it from the JetBrains marketplace. When it is started the first time it will automatically acquire an evaluation license that allows you to test the plugin for free for a period of two weeks. If you cannot access the Internet from IntelliJ you will need a file-based evaluation license. To do so you must register on www.hello2morrow.com and click on the red “Try Now” button.

If you want to continue using it after that you can buy a subscription (under 100 USD per year) from our website www.hello2morrow.com. Otherwise just uninstall the plugin. The different license options are described in more detail later in this article.

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How to Break a Big Ball of Mud?

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Many non-trivial systems end up as a big ball of mud, not because developers are lazy or reckless, but because it is very hard to avoid that outcome without proper tooling. For example, if your architecture rules are spread by word of mouth or some articles in your company wiki, there is no way of knowing if the code actually conforms to any of your architecture rules. If rules are broken, most of the times developers are not aware of that. That will lead to the erosion of architectural boundaries (if they ever existed) and more and more cyclic dependency groups. In the beginning the cyclic groups start small, but they grow like cancer in your codebase. I actually did some research on that by tracking some open source projects over time. That research confirmed my assumption – if you do not address the problem of ever growing cyclic dependency groups things will only get worse over time, in some cases much worse.

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Spring Modulith & Sonargraph – Better Together

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We created Sonargraph with the vision in mind, that it would allow architects to formally specify an enforceable architectural model. Another goal was to provide exceptional dependency visualization capabilities, so that issues could be easily detected not only in a formal way, but also by just looking at a dependency graph. Sonargraph’s architecture DSL (domain specific language) solved the first problem, while our exploration view solved the second one in a very unique and scalable way. The DSL is quite powerful and easy to learn. For an introduction you could read “How to Organize your Code” on this very site.

But obviously we were not the only ones thinking about a way to formally define architectural rules. Spring Modulith turned out to be a very powerful and successful solution to define domain driven architectures for Spring-Boot applications. Spring Modulith follows a pretty simple hands-off approach that allows the checking of architectural boundaries with a minimum configuration approach.

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Designing a Metric to Detect Big Balls of Mud

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Almost everyone who worked in software development for a while has come in contact with the dreaded big ball of mud (BBoM). If you are not familiar with the term, it describes software systems that have lost their architectural cohesion and suffer from extreme coupling and large cyclic dependency groups. That makes it much harder to do any changes on those systems, because everything is literally connected to everything else. Therefor it requires developers spending almost all their time trying to understand code before they can risk doing changes. And even then, the chance of regression bugs stay pretty high. If a system reaches this state, doing changes becomes so expensive, that rewriting the system from scratch might be cheaper than maintaining the old system. Unfortunately, very often it is not possible to rewrite the system, because the users can’t wait for years for a system replacement. This puts many development organization in a very uncomfortable place.

A big ball of mud visualized by Sonargraph

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Changes to the Sonargraph Dashboard

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The Sonargraph dashboard was designed to give the user a quick overview about several important aspects of software quality. The most important aspects in our opinion are “Architecture” and “Structure”, which are displayed at the top of the dashboard. After some internal discussions we decided that we could improve the metrics in the “Structure” box. Those changes will be rolled out with Sonargraph version 15.2.0, which is expected to be released this week.

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Using Sonargraph-Enterprise to Track Key Quality Metrics

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Sonargraph-Enterprise is designed to allow stake-holders to track important software quality metrics over time. The idea is that this would enable the early detection of harmful trends so that issues can be fixed while they are still easy to fix. The metrics displayed by Sonargraph-Enterprise can be fully customized by the user. However, in this article I will explain the “default” profile that comes with a set of preselected metrics that cover important aspects of code health and quality.

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Setting up Sonargraph on your Project

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Over the years we implemented many different methods to set up Sonargraph for your project. Some of them are now quite outdated and are only kept in the tool for compatibility reasons. This article is meant to help you with your initial setup and chose the most effective method for your particular circumstances. There is one section for each language supported by Sonargraph.

General Recommendations

Sonargraph maintains a folder for each software system, that contains a description of your project, the locations of source folders (and class folders for Java), your architecture model and all other parts of a Sonargraph quality model. We call this folder the Sonargraph system folder. It only contains text files (mostly XML) and should ideally be placed in the root directory of your VCS (version control system) repository. You also should add this folder to your VCS, since the Sonargraph model will evolve together with your software system. This will allow you to share one Sonargraph model with all the developers working on your project.

Each Sonargraph software system contains at least one module. Modules usually correspond to the modules or sub-projects in your IDE.

How to initially create this folder for your project is the subject of this article. The folder should have the name of your software system and always ends with ‘.sonargraph’.

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