The `epsActual` That Wasn't: 15% of an LLM Backtest's Trades Were Decided on Data That Didn't Exist Yet (opens in new tab)

We were backtesting an LLM-driven earnings signal against a field called epsActual — the kind of field everyone treats as ground truth. It isn't. About 41.4% of those "actual" values were different from what the vendor had first reported. About 15.3% differed enough to flip a tradeable decision. When we re-ran the backtest using only the values that actually existed at each decision date, the strategy kept ~73% of its returns and ~82% of its Sharpe. The rest was look-ahead bias — and it rode ...