In our projects we have been using ArchUnit for years to write unit tests that fail when the code no longer follows the architecture we chose. Over the last year, we’ve written more and more of these tests. What changed? Our audience grew. These tests no longer just help fellow developers stick to the architecture; they help our coding agents too.

Coding agents are getting quite good at making larger changes to existing codebases. Give an agent a task, some context and a test suite, and it often finds its way surprisingly well. But one type of context is much harder to provide: the architecture we intended. That is exactly the gap ArchUnit fills.

In this blog post I explain why ArchUnit becomes more valuable when working with coding agents, and I show some rules that actually help to guide them. The list is far from complete, but I hope it inspires you to capture your own architecture in rules, whether you write them yourself or let your agent do it.