Last June, METR caught o3 reward hacking on its RE-Bench and HCAST benchmarks. In a particularly humorous case, o3, when tasked with optimizing a kernel, decided to “shrink the notion of time as seen by the scorer”.

The development of Humanity's Last Exam involved “over 1,000 subject-matter experts” and $500,000 in prizes. However, after its release, researchers at FutureHouse discovered “about 30% of chemistry/biology answers are likely wrong”.

LiveCodeBench Pro is a competitive programming benchmark developed by “a group of medalists in international algorithmic contests”. Their paper describes issues with the benchmark's predecessor:

Benchmarks like LiveCodeBench [35] offer coding problems, but suffer from inconsistent environments, weak test cases vulnerable to false positives, unbalanced difficulty distributions, and the inability to isolate the effects of search contamination.

However, the authors assure us that their own test cases are of high quality:

Many problems in our benchmark originate from Codeforces, which uses the Polygon problem-setting platform. Each problem is then rigorously vetted by a team of expert testers—typically drawn from the community's top 1%, and overseen by at least one coordinator, usually among the top 0.1%. These specialists verify both the soundness and originality of every problem, ensuring it has never appeared [...]

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Outline:

(02:38) Terminal-Bench 2 Audit

(05:59) Why does this matter?

(07:15) What to do about it

(09:22) Appendix: More benchmark issues

The original text contained 7 footnotes which were omitted from this narration.

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First published:
January 24th, 2026

Source:
https://www.lesswrong.com/posts/HzjssjeQqhf3kRw9r/every-benchmark-is-broken

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Narrated by TYPE III AUDIO.

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Images from the article:

Code comment about a performance trick to speed up time measurement.

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“Every Benchmark is Broken” by Jonathan Gabor

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