ab-v3 left T prompts at ~22k tokens (vs ~6-7k for B). The 18kb
elements-cookbook teaches per-tool arg shapes, which is what
composite multi-tool chains genuinely need; single-tool obvious
prompts can route correctly from just the decision tree + PREFER
list alone.
buildSystemPrompt now takes opts.difficulty. T+obvious strips the
cookbook (saves ~18kb on the 47/52 ab-v3 obvious prompts);
T+composite, T+optional, and undefined keep both halves. B variant
unchanged — still strips both, so the A/B comparison stays clean.
Verified by 8 new build-prompt.test.ts cases including a 10kb floor
on the obvious-vs-composite delta. ab-v4 will measure whether the
diet hurts arg compliance on weak models; per-domain split is the
Phase 2 fallback if obvious-T garbage rate creeps up.
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