I Had a Working Agent at $3.41 a Run. Here's Where the Money Was Actually Going. (opens in new tab)

TL;DR Same task, same infrastructure, different harness: cost went from $3.41 to $0.03. Zero model changes. Prompt caching on the inner browser loop (~120 lines of wrapper code): −29% cost. The 8K of repeated system prompt + tools was being billed at full price 59× per run. Convergence rules in the prompt (three sentences): −54% cost on the desktop path, −75% wall time. The default behavior across Opus 4.8 and 4.6 is to keep searching. "After N items, STOP" is the most underrated sentence in ...