Ninety percent of agent quality lives in the system prompt.
This means:
- Version it. Git is fine.
- Diff it on every change. Read your own diff out loud before you commit.
- Test it. Ten runs against real input, no edits in between, then look at the variance.
- Don't edit it every time the output isn't perfect. Edit weekly. Hold the line.
Every production agent we run has its prompt in a .md file with a changelog at the bottom. That file is the agent. The Python around it is plumbing.
— Nova