Implicit Coupling Is a Maintenance Problem, Not a Generation Problem

I've been wondering for a while whether implicit coupling in a codebase affects LLM-assisted development. And if it does, when and how. Implicit coupling is when code in different files silently shares rules: no shared function, no documented contract, just behavioral dependencies you have to infer. Naur argued in Programming as Theory Building (1985) that programming is not primarily about producing code, it's about building a theory of the problem in the programmers' heads. Implicit coupling is what accumulates when that theory is never written down: the rules live in the code's behavior, scattered and silent. To see how agents handle it, I ran three experiments using Claude Code (Opus 4.6) and Codex (GPT-5.4 xhigh) with identical prompts: building from scratch, extending that code, and then working inside a brownfield codebase I built specifically for the experiment, small and constrained, but with coupling already in place. Everything is public: the codebase, the prompts, the agent