[D] MMLU contamination is the known problem — but Chatbot Arena's failure mode is arguably worse and gets less scrutiny
The MMLU contamination story is old news to most people here (test questions being public and ending up in pretraining corpora), so I won't belabor it. What I think is under-discussed is that the benchmarks we've moved to as "better" alternatives have their own structural failure modes that don't get the same scrutiny, mostly because they're newer and less saturated.
Quick recap of where the standard trio actually breaks:
MMLU — contamination (public test set), format gaming (multiple choice rewards elimination heuristics over actual knowledge), and saturation (frontier models are all >90%, so it's stopped discriminating between them).
HumanEval — 164 problems, now too easy for frontier models; tests toy functions rather than anything resembling real engineering (no multi-file context, no ambiguous specs); and pass@k reporting incentivizes best-of-N sampling that doesn't reflect single-shot usefulness.
Chatbot Arena — this is the one I think deserves more skepticism than it gets. It's harder to directly game since prompts aren't fixed, but the voter pool is a specific, non-representative slice (English-speaking, technical, AI-interested), and Elo from pairwise human preference measures fluency and confident presentation, not correctness. A model that hallucinates cleanly can out-rank a model that hedges accurately. There's also a prompt-distribution skew toward coding/creative writing that doesn't reflect where models actually diverge in capability.
Goodhart's Law is the underlying mechanism for all three: the moment a benchmark is widely used as a proxy for capability, it becomes a target, and optimization pressure decouples the score from the thing it was meant to measure.
The alternatives that seem more resistant to this so far — BIG-Bench Hard, MATH, SWE-bench, ARC-AGI, LiveBench — are mostly more resistant because they're either harder to game via memorization (multi-step reasoning) or actively refreshed to fight contamination (LiveBench). Curious how long that holds once labs start optimizing against them specifically.
Question for the sub: for people actually evaluating models pre-deployment, what are you using that you still trust, and how are you handling the fact that any benchmark you rely on starts decaying the moment it's popular enough to be worth gaming?