Short answer
You keep an AI trading agent from using future data by accident by turning the idea into a repeatable decision rule, attaching realistic turnover and risk constraints, and checking whether the workflow still holds up once the flattering assumptions are removed.
In point-in-time data and agent safety, the useful version of this workflow is the one that survives a clear benchmark, realistic execution assumptions, and a portfolio context that does not quietly change the rules after the backtest is done.