Further Optimizing the Vision-Only Harness: the Notes Rule

A short follow-up to I replicated the vision-only Pokémon run Anthropic showcased on the Fable 5 launch page. Rereading the harness code, I found the notes rule was still doing design work on the model's behalf — so I changed it, and compared the effect across two runs, one on each side of the change. The code and every log quoted below are in the project repo. Rereading the code: the notes rule was still constraining The principle of this harness is thinness: each turn the model gets one screenshot plus its own notes, and its only tool is pressing buttons (1 turn = 1 screenshot + 1 decision; that's what turn means in the logs). Notes are the single non-visual mechanism allowed — a context window cannot hold thousands of turns, so something has to survive compression. I had always thought of the notes as "the model's own thing," but going back through the code showed otherwise: in the system prompt and the tool description, I had prescribed a fixed checklist — recor