Validating the plan against a schema before any tool call is what makes an agent reproducible.

arXiv 2608.26197 stacked finite-state control, forced tool selection, output validation and bounded retries on two open-weight models, and got mixed results across all four model-task cells. Adding structured planning, where the plan is checked against a fixed schema before any tool fires, pushed three of four cells to a Reproducibility Rate and Determinism Index of 1.000 at N=100, with task success at 100%. Token cost fell in every cell. Latency split by model, one faster and one markedly slower, so that trade needs measuring rather than assuming. The retry and validation layers everyone reaches for first are not where determinism comes from. (arXiv)