On our 10-step InvoiceHub mock, max_chars=512 head+tail hit 100% success proxy — tail-only scored 13.16%

Author 10-step InvoiceHub mock multi-tool trajectory (9 tool observations + final-answer rule; seed 20260928, API $0). Swept per-observation max_chars ∈ {256,512,1024,2048,unlimited} × {head, tail, he