Python 3.15 changed what open() does on Windows when no encoding is named, from the cp1252 code page to UTF-8. Running both versions on Windows 11, none of the 159,706 assigned non-ASCII Unicode characters written by Python 3.15 came back right when Python 3.14 read it. 82% came back wrong without any error, and the rest raised. In the other direction Python 3.15 raised on every character Python 3.14 could write. R moved to UTF-8 on Windows in 2022, so every R to Python handoff tested that broke under 3.14 works under 3.15, and pandas never changed at all.

MathArena publishes every graded attempt, so its AIME 2026 leaderboard can be tested directly. Resampling the 30 problems, 25 of the 31 models with full coverage have a 95 percent rank interval that includes first place, and an exact paired test separates only one of the 29 imperfect models from a perfect score. Sixteen problems were solved by every model on every attempt, and five carry 80 percent of the spread. Intervals computed over problems rather than attempts run a median 1.7 times wider, because they answer a broader question.