"Compare against a baseline that already works well" is the line I'd want more agent write-ups to follow. A lot of "the model rewrote its own prompt and got better" results start from a prompt that never had a fair chance, so it's refreshing to read one where the baseline passed and the honest finding was about cost, not accuracy.

The step 6 example is the one I'd underline: fewer tool calls, but higher cost per successful task. Tool calls are easy to count in a trace, so they're the number people reach for, and here they pointed the wrong way. Reporting cost per success next to the cost of producing the revision keeps a "cheaper" version from hiding what it took to get there.

One small addition that could fit in step 5: save a plain diff of the baseline and the frozen replacement instructions next to the results. When a revision saves money, the diff usually shows why, for example a dropped check or a shorter tool list. That's what tells you whether the saving is likely to hold on a different workload.

Out of curiosity, did the model's replacement instructions mostly add rules or remove them?