AI Engineering — Field Notes

Field notes from building with teams of AI agents.

1. #032026-10-15AI ENGINEERING7 MIN The market was efficient after fees: an honest negative result Agent swarms tested about 1,500 hypotheses under pre-registered rules. Several edges turned out to be bugs. The market was efficient after fees.
2. #042026-10-15AI ENGINEERING6 MIN The prompt, not the model: what blind LLM-as-judge tests showed Blind LLM-as-judge tests showed the prompt, not the model, made an astrology app's text sound AI-written. The prompt rules and code checks around it.
3. #062026-10-15AI ENGINEERING7 MIN A score language for an AI composer Four bars of MIDI as raw JSON is about 1,600 lines, too fragile for an agent. So I designed a compact score language, a compiler and a Claude Code skill.
4. #072026-10-15AI ENGINEERING6 MIN Prompt caching cut a tool-use agent’s input cost by about 80% A tool-use agent resends its whole context on every turn. What prompt caching changes about that, and how I read a cut of about 80% in input cost.