I had to make a boring infrastructure decision and could not find data on it, so I ran the experiment. Sharing it because two of the results surprised me and one of them is a genuine footgun. Shortest version: folders win and skills cost more The decision You want every colleague's Claude Code sessions to follow the same set of organisational rules. Two ways to deliver them: - Folder — rules live in the working directory (CLAUDE.md). They load in full at session start, every session. But only if you start in that directory. - Marketplace skill — the same text lives in a skill. Works from anywhere, and only the name and description sit in context until the model decides to load the body. Folder is guaranteed but positional. Skill is portable but conditional. Which one actually holds up? Setup One analytical question over a fixed dataset of project records, identical prompt in every arm. Twenty scorable rules: fourteen text rules, six computational, a mix of ALWAYS and NEVER, all picked so a model would not comply by accident. Three are baited — the prompt misspells the company name, the source data uses dd-mm-yyyy and calls its column deadline, while rules demand ISO dates and a different word. Arms: folder, skill, skill-with-forced-invocation, and a control with no rules at all. The control is not optional — without it you cannot tell "the rule worked" from "the model would have done that anyway". The trick that made scoring easy. The dataset is rigged so each computational rule shifts the requested total by a unique amount: −1, +64, +128, +256, +512, against a compliant total of 1,028. All subset sums are distinct, so one number tells you exactly which rules were applied. No free-text parsing, no LLM judge, no attribution guesswork. I wrote handwritten compliant and naive answers and validated the scorer against them before spending a single run. Then I varied the load: 20 rules, then 210, by adding realistic rules that do not apply to this task — at constant total length, so only composition changed. The last round used heterogeneous distractors across six formats, including 22 near-miss rules that contradict a core rule but are scoped elsewhere ("in e-mail use dd-mm-yyyy", "in offers use the full customer name", "in tickets the word deadline is fine").