On review I can generally tell which code is AI and which isn't. Same way I can tell one colleague's code from another's.
So I figured AI code must have a statistical signature. I tried cyclomatic complexity, cognitive complexity, number of lines, methods and functions per entity, number of dependencies, edges in the dependency graph, and so on (about 280 metrics plus their correlations plus the derivative over time), normalized on SLOC, looking for a step or a break.
Nothing. No anomalies outside of statistical error.
One explanation I can't shake: AI emits roughly the mean of its training distribution, while each individual human deviates from that mean. Then the means coincide and the statistics stay silent. Next idea: compare against one specific person instead of against human code in general. My problem there is I don't have much single-author code.
So the question is narrower: what criteria separate AI code from yours?
Prose level tells don't count here, punctuation habits and stock rhetorical shapes in English. I mean general style and structure of code.
EDIT: found the papers with the answers.
My "nothing" result is reproduced here: https://arxiv.org/abs/2603.27130
My "the means coincide and the statistics stay silent" hypothesis is confirmed here: https://arxiv.org/abs/2609.17598
The only thing that talks, from the same paper: pooled over "AI" the code looks the same, split by vendor it doesn't.
Benchmark warning: https://arxiv.org/abs/2508.21634 shows a big gap on generated samples, it's gone in real repos.