Build a Self-Improving Agent Harness in an Afternoon

You cannot retrain Claude. You cannot fine-tune GPT. The weights belong to Anthropic and OpenAI, and no amount of clever prompting changes a single parameter. That fact makes a lot of engineers feel stuck — like the only path to a better agent runs through a training run they will never get to make. It doesn't. The model is fixed. The code you wrap around it is not. So I built a harness that improves itself — and this post walks through exactly what it does, how it tests itself, what it got…