Your Backtest Can Pass Every Unit Test and Still Know the Future
A two-timestamp schema and a future-corruption test can catch silent look-ahead leakage in financial feature pipelines. A backtest does not need an obvious bug to cheat. The feature calculations can be correct. The train/test split can be chronological. Every unit test can pass. Yet the model may still be reading financial data that was revised, restated, or simply unavailable when the historical…
A two-timestamp schema and a future-corruption test can reveal silent look-ahead leakage in financial feature pipelines. This leak occurs when a model reads financial data that was revised or unavailable at the time of the historical decision, even if the backtest appears bug-free. It is crucial to distinguish between event time (when the fact becomes true) and knowledge time (when the system can actually use that fact).
A naïve backtest often joins datasets only on event time, missing the critical availability timestamp. The correct approach is to add an "available_at" timestamp, which is the later of the publication and ingestion times, indicating when the system could legitimately use the value. Every fact should have its own availability timestamp, and versions should be appended instead of overwriting when new data arrives.
The SEC company-facts API serves as an example, providing multiple timestamps such as reporting dates, accession numbers, and filing dates. A production pipeline should capture the highest-precision dissemination timestamp from its source. The key is to build features with an "as-of" boundary, ensuring no candidate row crosses the information boundary.
Pandas' merge_asof can be used for backward-looking joins, but the join key should represent availability, not just the date the observation describes. To prevent look-ahead leakage, a test can be added that poisons the future by calculating features at a historical cutoff and then corrupting every source value that became available after that cutoff. If the output changes, it indicates a path for backward information leakage.
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