Your system prompt isn't instructions. It's data.

My system prompt had an example of a good Slack message in it. It opened with "Morning all, quick one:". The model started opening real Slack drafts with that exact phrase. Then it started saying "Morning." when I typed "hey", which is a small lie, because it cannot see a clock. So I added a rule telling it not to reuse examples from its own instructions. Three rebuilds. No change. Then I deleted the phrase. Fixed on the next build. That is when it clicked. The model does not read your system prompt as a list of instructions. It reads it as text that is likely to appear near its own output. Every finding below falls out of that one idea. The four rules I now write prompts by If a phrase must not appear in the output, it must not appear in the prompt. Banning it does not work. Deleting it does. Naming a bad example summons it. "Not the bank balance one" is an excellent way to get the bank balance one. Position beats wording. A rule buried mid-section gets read and traded away. The same words at the top of that section hold. Concrete beats principled. "Call fsync() before the rename" lands immediately. "Describe only the guarantee the code actually makes" does nothing. And the one that saved me the most time after it cost me the most time: verify on three seeds before you believe any of it. Here is the evidence for each. The setup Flash Onyx is the model line behind Flash , my local agent shell. There is no fine-tuning involved. Onyx is a base model plus a system prompt that has grown to roughly 680 lines, built into an Ollama tag with a small script: python3 models/build.py models/flash-onyx-2.5.Modelfile --size 31b-cloudbase -n Natuworkguy 2.5 is the version where I stopped editing that prompt by feel. The loop is not clever: edit the prompt, rebuild the tag, run a fixed set of prompts at pinned seeds, read the output, decide whether anything actually changed. Seeds are pinned so two runs are comparable. That is the entire method, and it is the difference between "t